Amino acid nutrition and metabolism in domestic cats and dogs

Peng Li; Guoyao Wu

doi:10.1186/s40104-022-00827-8

Amino acid nutrition and metabolism in domestic cats and dogs

Li, Peng; Wu, Guoyao 2023-02-21 00:00:00 Domestic cats and dogs are carnivores that have evolved differentially in the nutrition and metabolism of amino acids. This article highlights both proteinogenic and nonproteinogenic amino acids. Dogs inadequately synthesize citrul- line (the precursor of arginine) from glutamine, glutamate, and proline in the small intestine. Although most breeds of dogs have potential for adequately converting cysteine into taurine in the liver, a small proportion (1.3%–2.5%) of the Newfoundland dogs fed commercially available balanced diets exhibit a deficiency of taurine possibly due to gene mutations. Certain breeds of dogs (e.g., golden retrievers) are more prone to taurine deficiency possibly due to lower hepatic activities of cysteine dioxygenase and cysteine sulfinate decarboxylase. De novo synthesis of arginine and taurine is very limited in cats. Thus, concentrations of both taurine and arginine in feline milk are the greatest among domestic mammals. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for many amino acids (e.g., arginine, taurine, cysteine, and tyrosine), and are less sensitive to amino acid imbalances and antagonisms. Throughout adulthood, cats and dogs may lose 34% and 21% of their lean body mass, respectively. Adequate intakes of high-quality protein (i.e., 32% and 40% animal protein in diets of aging dogs and cats, respectively; dry matter basis) are recommended to alleviate aging-associated reductions in the mass and func- tion of skeletal muscles and bones. Pet-food grade animal-sourced foodstuffs are excellent sources of both proteino - genic amino acids and taurine for cats and dogs, and can help to optimize their growth, development, and health. Keywords Animal-sourced foodstuffs, Cats, Dogs, Health, Metabolism, Nutrition gradually between 2013 and 2022 by 677% and 147%, Introduction respectively [3]. In the United States, 25.4% and 38.4% of The domestic dog (Canis familiaris ) and the domes- households owned cats and dogs, respectively, in 2018, tic cat (Felis catus) have been human companions for as companions or family members [4]. Most petfoods at least 12,000 and 9000 years, respectively [1, 2]. These are commercially manufactured, although some peo- animals contribute to the mental health and well-being ple choose to prepare meals for their own pets by using of children, adolescents, and adults, and have become animal- and plant-sourced ingredients. Thus, the global increasingly popular in many countries and worldwide petfood industry has grown substantially in recent years. over the past decades (Table 1). For example, the num- The compound annual growth rate of the global petfood bers of domestic cats and dogs in China have increased market is expected to be 4.6% between 2020 and 2027 (monetary value, US $124.9 billion by 2027) [5]. The dog is a domesticated descendant of the grey wolf *Correspondence: Guoyao Wu (an obligate carnivore), and was from the taxonomical g-wu@tamu.edu order Carnivora over 15,000 years ago [6]. The cat, which North American Renderers Association, Alexandria, Virginia 22314, USA was also from the order Carnivora, is the only domesti- Department of Animal Science, Texas A&M University, College Station, TX 77843, USA cated species in the family Felidae [7]. The feline domes - tication occurred approximately 10,000 years ago [7]. To © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 2 of 21 Table 1 Numbers of domestic dogs and cats as pets worldwide that are distinct from omnivorous mammals such as pigs, 6 a (× 10 ) rats, and humans [10, 14, 15]. The National Research Council (NRC) [16] recognizes that the dog is a carni- World or country Year Dogs Cats vore anatomically but has many metabolic characteris- World 2018 471 373 tics of omnivores, including the conversion of β-carotene 2020 510 400 to vitamin A, tryptophan to niacin, cysteine to taurine, Brazil 2010 34 18 and linoleic acid to arachidonic acid. Dogs also differ 2018 52 22 from cats in many of these aspects. For example, unlike 2020 56 23 cats that require much more dietary protein (expressed Canada 2012 6.5 8.0 as % of diet) than provided in grains and vegetables, most 2018 7.6 8.1 breeds of dogs can thrive on taurine-free vegetarian diets 2020 7.7 8.1 that are properly balanced and sufficient in non-tau - China 2013 55 22 rine nutrients through supplementation with those either 2017 85 69 absent from plants or inadequately synthesized de novo 2018 94 87 [16]. Regardless of their sources of food, adequate knowl- 2022 136 171 edge of nutrient metabolism and requirements by cats Europe 2010 74 85 and dogs is crucial to ensure their optimal growth, devel- 2018 85 104 opment, and health. The major objective of this article is 2020 90 110 to highlight the nutrition and metabolism of amino acids Japan 2013 10.9 9.7 (AAs) in cats and dogs. Unless indicated, AAs except for 2018 8.9 9.6 glycine and taurine refer to the L-isomers herein. 2020 8.5 9.6 India 2014 13 1.2 Digestibilities of amino acids in the diets of cats 2018 19 1.7 and dogs 2020 22 1.9 Determination of AA digestibilities in cats and dogs USA 2011 70 74 usually requires a 5-d period of adaptation and a 5-d 2018 77 58 period of sample collection [16]. To date, the fecal col- 2020 85 65 lection method has been largely used to measure the a apparent total tract digestibilities of AAs in cats, and Data for the USA are taken from the American Veterinary Medical Association at https:// www. avma. org/ resou rces- tools/ repor ts- stati stics/ us- pet- owner ship- some studies have involved sample collection at the stati stics. Data for other regions of the world are taken from Statista at https:// end of the ileum in dogs [17–20]. Data on total tract www. stati sta. com/ stati stics digestibilities of amino acids are not useful for formu- lating animal diets, but comparison between apparent ileal and total tract AA digestibilities may help to assess date, about 450 dog breeds [8] and 60 cat breeds [9] the extent of metabolism of individual AAs in the large with differences in shape, size, and color are recognized intestine [15]. Although there are substantial amounts globally. Puberty or sexual maturity occurs in domes- of endogenous AAs in the small intestine of both cats tic cats and dogs at 6–12 and about 6 months, respec- and dogs (Table 2) due to gastrointestinal secretions tively, whereas the life span of cats and dogs is 12–18 and such as sloughed mucosa and digestive enzymes [21– 10–16 years, respectively, depending on size, breed, and 24], there are no data on the true ileal digestibilities of nutritional status. Both cats and dogs have: (a) a rela- AAs and other nutrients in these animals due to both tively shorter digestive tract, longer canine teeth, and technical and ethical challenges. Nevertheless, stand- a tighter digitation of molars than omnivorous mam- ardized ileal digestibilities of individual AAs based mals such as humans and pigs, (b) a very low activity of on corrections for endogenous AA flow in the ileum salivary α-amylase, (c) a limited ability to synthesize de have been reported for dogs [15]. According to pub- novo arginine and vitamin D or to convert α-linolenic lished studies [25, 26], standardized ileal digestibili- acid to 5,8,11,14,17,20-docosahexaenoic acid, and (d) ties of AAs in dogs are similar to, or lower than, those instinct preferences for meat to plant products [10, 11]. for pigs (Table 3). Compared with dogs, cats generally Based on their anatomical, metabolic, and natural feed- have lower apparent digestibilities of AAs for lower- ing characteristics, dogs (facultative carnivores) and cats quality proteins due to a shorter small intestine relative (obligate carnivores) are classified as carnivores in clas - to body weight (BW) but have similar values for high- sic animal nutrition [12] and veterinary medicine [13] quality proteins (e.g., those with 90% or higher appar- textbooks, but these animals have evolved to have some ent digestibilities) [27]. unique feeding behaviors and metabolic characteristics Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 3 of 21 Table 2 Endogenous excretion of amino acids and nitrogen at the terminal ileum of adult cats, dogs, rats, and pigs a b a c AminoAdult dogsAdult catsAdult rats Pigs acid Protein-free diet Enzyme- Protein Enzyme- Protein-free Enzyme- 15-kg 75-kg hydrolyzed free diet hydrolyzed diet hydrolyzed BW BW casein casein casein μg/g of dry matter intake Ala 515 951 666 1380 269 474 437 454 Arg 370 728 540 948 159 328 480 435 Asp + Asn 960 2428 1283 2725 660 1095 755 640 Cys – – 452 853 – – – 136 Glu + Gln 1089 5993 1427 4240 696 1513 786 735 Gly 600 1001 665 1298 513 435 1660 1053 His 268 700 397 897 136 218 231 166 Ile 362 1191 398 1205 249 575 231 265 Leu 560 1180 884 1823 378 706 399 459 Lys 441 866 570 1101 193 390 312 427 Met 119 323 164 411 75 157 – 65 Phe 465 624 632 1015 171 316 237 265 Pro 643 1814 820 1913 442 691 3557 3104 Ser 925 3386 1013 2734 407 887 549 345 Taurine – – 2091 299 – – – – Thr 1168 2015 1235 2127 450 659 571 423 Tyr 444 648 599 1046 180 361 181 213 Val 536 1448 696 1687 273 599 321 387 mg/g of dry matter intake AA N 1.27 3.33 1.9 3.6 0.71 1.23 1.63 1.40 Total N 2.27 4.12 2.4 3.6 1.91 1.63 2.16 – “–” data are not available Hendriks et al. [21]. Adult dogs and rats were fed either a protein-free diet or a diet containing 23% enzyme-hydrolyzed casein Hendriks et al. [22]. Adult cats were fed either a protein-free diet or a diet containing 14% enzyme-hydrolyzed casein The values were determined in 15-kg growing pigs [23] and 75-kg growing pigs [24] fed a protein-free diet The cecectomized rooster model has been used destroys trypsin inhibitors, but prolonged heating to assess the digestibilities of AAs in foodstuffs for decreases the digestibilities of crude protein (CP) and cats and dogs [28]. Based on this assay, a very low AAs (e.g., Lys and Arg) due to the Maillard reaction [16]. true digestibility of glycine (e.g., only 22%) has been Cooking can result in improved digestibilities of AAs in reported for lamb and brown rice, compared with the dogs [30] and of starch in cats [31]. Increasing the dietary values of > 80% for most AAs [28], possibly due to an level of soluble fiber reduces CP digestibility in both dogs inaccurate measurement of endogenous glycine flow in [32, 33] and cats [31]. In contrast, dietary supplementa- the ileum. In addition, results from a study with a swine tion with yucca schidigera (60–120 mg/kg) enhances the model indicated that the apparent ileal digestibility of digestibility and absorption of AAs in these animals, cysteine and proline (i.e., 30%) in meat and bone meal thereby reducing the odor of pet stools [34]. Extrusion of as compared with the values of > 69% for most AAs [29] meat and bone meal via high temperature and pressure was likely due to analytical problems, as it is a techni- (135 °C, 3 bar, 20 min) increases the digestibility of dietary cal challenge to determine these two AAs by using ion- CP in adult cats but had no effect in adult dogs [27], indi - exchange chromatography. Caution over data accuracy cating species differences in nutrient digestion. and test animal models should be taken when interpret- The apparent ileal and total tract digestibilities of CP in ing and using literature data on AA digestibilities for diets containing both plant ingredients (primarily wheat feeding companion animals. and corn grains) and meat (lamb meal, poultry meal, or Many of the factors that influence apparent AA digest - fish meal) were 74% and 84%, respectively, in adult dogs ibilities in dogs have similar effects in cats. In manufac - [35]. The apparent ileal digestibilities of individual AAs turing pet foods, the heating of plant-sourced foodstuffs in this canine diet (%) were: Ala, 80.5; Arg, 85.1; Asp + Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 4 of 21 Table 3 Apparent ileal, total tract, and standardized digestibilities of amino acids in adult dogs, cats, and pigs, % a c NutrientsAdult dogs Apparent digestibility Pigs (total tract) by adult cats Apparent digestibility Standardized Hair Hair Apparent Standardized ileal removed included ileal ileal digestibility Ileal Total tract digestibility digestibility (fecal) Dry matter 75.1 81.2 – 80.1 72.0 – – Organic matter 79.4 85.3 – – – – – Crude protein 76.2 81.9 81.4 82.4 73.4 80.7 87.8 Lipids 96.5 92.4 – 95.3 93.4 – – Carbohydrates 88.9 95.5 – – – – – Ala 77.7 81.6 80.8 86.8 78.6 78.9 85.9 Arg 87.3 91.7 89.6 91.4 85.3 91.0 94.9 Asp + Asn 67.0 82.0 71.1 82.1 70.7 82.8 88.0 Cys 56.5 75.3 – 75.4 42.2 79.9 87.6 Glu + Gln 81.6 87.1 84.3 85.5 76.9 87.1 90.8 Gly 73.1 83.7 76.7 90.0 82.9 74.3 89.1 His 61.4 78.7 66.1 – – 87.5 91.3 Ile 77.7 80.6 81.6 – – 81.9 87.5 Leu 79.4 84.9 82.3 85.1 76.1 85.0 89.1 Lys 76.8 81.6 80.0 – – 83.8 88.9 Met 82.6 83.8 85.1 – – 86.4 90.1 Phe 80.3 84.6 84.7 – – 85.2 89.4 Pro 78.8 88.2 82.6 – – 80.5 104.2 Ser 69.0 82.6 78.1 84.9 72.9 84.7 90.6 Thr 62.0 79.0 75.1 83.5 72.5 78.5 86.2 Tyr 77.2 82.8 82.5 – – 85.1 90.5 Val 72.3 79.6 76.9 – – 79.9 86.0 AA nitrogen 76.7 84.6 80.2 85.7 76.5 83.1 90.0 AA Amino acid, “–” data are not available Hendriks et al. [15]. Adult female dogs with a mean body weight of 25.3 kg, a mean age of 3 years, and a mean food intake of 389.4 g/d (15.4 g food/kg body weight/d) were fed commercial dry canine foods containing 24.3%–32.7% crude protein (dry matter basis). Apparent ileal digestibility = (AA intake – AA in ileal digesta)/AA intake × 100; Apparent total tract (fecal) digestibility = (AA intake – AA in feces)/AA intake × 100; Standardized ileal digestibility = [AA intake – (AA in ileal digesta – basal endogenous AA flow in ileum)]/AA intake × 100 Kim et al. [25]. Adult domestic cats (7 males and 7 females) with a mean body weight of 4.5 kg and a mean age of 3.3 years were fed a dry extruded diet (chicken- based, grain-free commercial feed with 2% refined cellulose and 3% sugar beet pulp) containing 33% CP. Hair of cats was removed from or included in feces for determining the apparent digestibilities of nutrients Pigs (92-kg body weight) were fed a corn- and soybean meal-based diet [26] dogs fed plant (extruded wheat, corn meal, soybean meal, Asn, 52.0; Cys, 54.5; Glu + Gln, 82.2; Gly, 76.2; His, 76.0; and beet pulp)- and animal (fishmeal, poultry meal, meat 4-hydroxyproline, 79.5; Ile, 78.7; Leu, 80.2; Lys, 78.9; meal, and meat & bone meal)-based diets containing 30% Met, 82.0; Phe, 81.9; Pro, 81.5; Ser, 70.5; Thr, 69.7; Tyr, CP, 19% lipids, 29%–34% starch, and 6%–13% fiber [37], 77.4; and Val, 77.8 in the mixed breed Alaskan Husky and for adult dogs fed commercial dry foods (Table 3). [35]. 4-Hydroxyproline is converted into glycine via the Comparisons between the apparent ileal and total tract 4-hydroxyproline oxidase pathway in animal tissues digestibilities of methionine and lysine in dogs indicate [36]. For comparison, the apparent total tract digestibili- that, in contrast to pigs [38], (a) a substantial amount of ties of individual AAs in this canine diet (%) were: Ala, protein metabolites is absorbed by the canine large intes- 87.3; Arg, 90.2; Asp + Asn, 78.3; Cys, 71.0; Glu + Gln, tine and (b) the microbes of the canine large intestine do 88.9; Gly, 88.3; His, 86.9; 4-hydroxyproline, 94.4; Ile, 85.1; not have a net synthesis of methionine and lysine. These Leu, 86.8; Lys, 83.8; Met, 86.1; Phe, 86.4; Pro, 89.9; Ser, findings suggest an important difference in gut microbial 82.3; Thr, 82.6; Tyr, 84.5; and Val, 84.5 in these dogs [35]. metabolism among animal species. Similar results were reported for adult German shepherd Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 5 of 21 Digestibilities of AAs are influenced by the breed Metabolism of AAs by cats and dogs and age of dogs, as well as diet and the method of its The metabolism of most AAs by cats and dogs is similar preparation. For example, at 11, 21, 35, and 60 weeks to that of other mammals [43]. In support of this view, of age, apparent CP digestibilities were greater in the plasma concentrations of most AAs from differ - large breeds than small breeds of dogs [39] and were ent research groups [47–50] are similar between adult increased with age, possibly due to a greater activ- cats and dogs, except for Asn, Asp, Citrulline, Glu, Gly, ity of intestinal microbes. The apparent digestibil- His and Pro (that are lower in dogs than in cats) and for ity of CP was reduced by 5% in 11-week-old puppies lysine (that is higher in dogs than in cats) (Table 4). Inter- compared with 2- to 4-year-old adult dogs, but no estingly, the plasma concentrations of Arg, citrulline, and differences were detected between 0.5- and 2-year- ornithine in both cats and dogs were lower than those old dogs [40]. Likewise, no difference in CP digest- for pigs [51, 52] (Table 4), suggesting differences in the ibility was noted between 2- and 17-year-old beagles whole-body metabolism of AAs among the three animal [41]. Dietary supplementation with β-mannanase species. (the enzyme hydrolyzing polysaccharides made from The qualitative dietary requirements of dogs for most D-mannose) enhanced the apparent digestibilities of AAs are similar to those for omnivores (e.g., humans and CP in dogs fed a diet containing a large amount of pigs) [16, 53]. However, in contrast to most breeds of plant-sourced protein ingredients, but had no effect dogs, cats have a very limited ability to synthesize taurine in dogs fed a diet containing a large amount of ani- and arginine [14]. Because taurine is present in animal mal-sourced protein ingredients [42]. Compared with products but absent from plants [54], cats must be pro- the extruded diet, slight cooking enhanced CP digest- vided with at least a portion of animal-sourced foods or ibility in dogs [30]. In adult dogs, increasing the con- the same essential nutrients from synthetic supplements tent of dietary CP from 18% to 42% did not affect the [43]. This is consistent with the much greater concen - standardized ileal digestibilities of AAs [17], indicat- trations of both taurine and arginine in the milk of cats ing a high ability of these animals in digesting dietary [55–58] as compared with ruminants and pigs (Table 5) protein. Likewise, the inclusion of 7.5% fiber in diets [59–61]. Thus, there are peculiar differences in the did not adversely affect the apparent ileal digestibili- requirements of certain AAs, such as taurine (essential ties of CP and AAs in adult dogs [30], further sup- for tissue integrity) [62] and arginine (essential for main- porting the notion that these animals can adapt well taining the urea cycle in an active state) [63] between cats to an appropriate proportion of plant-sourced ingre- and dogs. Likewise, the concentrations of many AAs in dients in their diets [43]. plasma differ between cats and dogs offered diets high Hair, which is lost from the skin, should be removed in carbohydrate, high in fat, or high in protein [64]. Cats from feces for accurately measuring nutrient digestibil- and dogs chose a different mix of food, which is consist - ity in cats [25]. Harper and Turner [44] reported that ent with cats needing a higher protein concentration in 19-week-old cats had higher apparent CP digestibilities food than dogs [64]. In these two animal species, the syn- than younger kittens. In adult cats, the apparent digest- thesis of glucose from AAs in the liver and kidneys plays ibilities of CP were 91%–94%, 87%–88%, or 89%–90%, an important role in maintaining glucose homeostasis respectively, in a meat (a mixture of beef and mutton)- [43]. When diets do not provide sufficient starch, glyco - based diet containing 22% cornstarch, 14.3% corn, or gen or glucose, dogs must synthesize glucose from glu- 14.3% wheat grains, and were not affected by fine grind - cogenic AAs in their liver and kidneys [65]. In contrast to ing or cooking [31]. Likewise, the apparent digestibili- modern breeds of dogs that consume both animal- and ties of CP by healthy adult cats did not differ between plant-sourced foods, a natural food (i.e., meat) for cats two diets containing 36%–55% high-quality proteins contains only a small amount of glycogen (primarily from (a high proportion of meat) and 37%–56% low-quality muscle and liver; < 5%, DM basis) and no starch [14]. proteins (a low proportion of meat) (i.e., 89%–90% ver- u Th s, in cats, gluconeogenesis from AAs plays an essen - sus 88%–91%, respectively) [45], but most likely did tial role in the provision of glucose to the brain, red blood not reflect the true digestibilities of the proteins due cells, and immunocytes, and therefore their survival [12]. to microbial AA metabolism in the large intestine. However, the apparent digestibility of dietary CP was Endogenous nitrogen excretion decreased in cats fed raw corn starch and raw potato In adult dogs (BW ranging from 2.8 to 51 kg) fed a pro- starch compared with cooked foods [46]. These results tein-free, semi-purified diet, the outputs of endogenous indicate effects of age and dietary composition on the urinary nitrogen, metabolic fecal nitrogen [nitrogen digestion of dietary protein and microbial AA metabo- originating from the sloughed gastrointestinal epithelium lism in the gut. and bacteria, as well as other endogenous sources (e.g., Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 6 of 21 Table 4 Concentrations of amino acids (AAs) in the plasma of dogs, cats, and pigs AAs Dogs Cats Pigs < 2 wk 2 wk to 2 12 to 60 > 60 mo Young Adults 4 d 2.5 mo 18 mo c e f g of age mo of age mo of age of age kittens (n = 120)of ageof ageof age (n = 7) (n = 7) (n = 7) (n = 8) (n = 8) (n = 10) (n = 8) (n = 8) Ala 315 ± 46 296 ± 180 400 ± 129 332 ± 162 603 ± 62 462 ± 160 1049 ± 478 596 ± 91 352 ± 58 Arg 278 ± 80 245 ± 101 103 ± 29 104 ± 36 – 95 ± 38 163 ± 63 159 ± 45 165 ± 19 Asn 121 ± 43 121 ± 48 52 ± 16 47 ± 10 – 91 ± 25 118 ± 28 62 ± 8 66 ± 10 Asp 19 ± 9 11 ± 3 trace trace – 28 ± 12 25 ± 9 13 ± 6 12 ± 5 Cit 88 ± 39 107 ± 45 34 ± 10 38 ± 12 – 18 ± 6 89 ± 25 64 ± 6 60 ± 8 h g Cys 42 ± 38 56 ± 36 32 ± 22 42 ± 26 – 26 ± 9 165 ± 33 173 ± 12 170 ± 23 Gln 564 ± 174 461 ± 139 593 ± 175 658 ± 213 – 664 ± 134 469 ± 120 513 ± 79 491 ± 59 Glu 112 ± 82 73 ± 32 35 ± 8 38 ± 12 – 73 ± 38 156 ± 63 172 ± 40 92 ± 15 Gly 283 ± 85 364 ± 107 181 ± 61 171 ± 42 – 398 ± 279 824 ± 237 664 ± 62 688 ± 81 His 140 ± 45 85 ± 28 66 ± 13 65 ± 15 – 116 ± 24 114 ± 47 103 ± 20 97 ± 16 Ile 83 ± 24 70 ± 23 67 ± 21 63 ± 18 51 ± 8 63 ± 29 159 ± 57 112 ± 28 104 ± 15 Leu 212 ± 39 147 ± 69 142 ± 34 135 ± 48 91 ± 14 146 ± 49 186 ± 51 246 ± 42 196 ± 18 Lys 275 ± 68 195 ± 121 149 ± 46 191 ± 55 – 108 ± 61 223 ± 82 88 ± 48 103 ± 35 Met 52 ± 33 73 ± 20 46 ± 15 52 ± 16 116 ± 51 64 ± 28 100 ± 51 35 ± 6 47 ± 6 Orn 85 ± 31 43 ± 16 19 ± 9 17 ± 7 – 21 ± 12 107 ± 38 85 ± 17 82 ± 20 Phe 60 ± 13 64 ± 25 61 ± 14 63 ± 18 72 ± 11 70 ± 15 117 ± 35 87 ± 11 89 ± 14 Pro 389 ± 106 290 ± 105 145 ± 61 114 ± 15 – 258 ± 76 628 ± 373 395 ± 59 382 ± 50 Ser 241 ± 40 262 ± 91 123 ± 28 138 ± 31 – 179 ± 85 267 ± 123 153 ± 45 157 ± 38 Taurine – – – 77 ± 24 – 118 ± 55 169 ± 57 56 ± 17 60 ± 21 Thr 523 ± 245 250 ± 151 196 ± 63 157 ± 50 168 ± 51 173 ± 54 369 ± 145 89 ± 45 93 ± 32 Trp 66 ± 27 55 ± 15 69 ± 27 54 ± 17 – 60 ± 17 38 ± 19 47 ± 8 45 ± 9 Tyr 88 ± 31 54 ± 24 48 ± 7 43 ± 8 36 ± 8 57 ± 15 242 ± 73 105 ± 17 112 ± 24 Val 248 ± 42 200 ± 83 199 ± 43 199 ± 63 131 ± 34 164 ± 62 350 ± 114 280 ± 37 216 ± 31 Cit Citrulline, mo months, Orn Ornithine, Tau Taurine, wk weeks, “–” data were not available Values are means ± SD Blazer-Yost and Jezyk [47] for all amino acids except taurine Delaney et al. [48]. Adult cats (n = 131) had a median age of 5.3 years (ranging from 2 to 14 years) Hargrove et al. [49]. Kittens (1 to 2 kg body weight) were fed a casein-, soy protein-, and AA mix-based diet Heinze et al. [50] Flynn and Wu [51] Wu et al. [52] Wu G (unpublished work). Adult gilts were fed, twice daily at 1% of body weight per meal, a corn- and soybean meal-based diet containing 12.2% CP [53]. Blood samples were obtained from the jugular vein at 2 h after feeding to prepare plasma for AA analysis [52] cysteine + ½ cystine saliva, mucus, bile, and pancreatic and intestinal secre- The endogenous excretions of total, urea, ammonia, tions)], and total endogenous nitrogen were 210, 63, and and creatinine (a metabolite of creatine) nitrogen for cats 0.75 273 mg/kg BW /d, respectively [66]. This is equivalent fed the protein-free diet were 360, 243, 27.6, and 14.4 mg/ 0.75 0.75 to the catabolism of 1.71 g protein/kg B W /d. There kg BW /d, respectively [67]. Similarly, Earle [68] was no significant effect of either sex or BW on the meas - reported that adult cats could maintain nitrogen balance ured variables expressed per metabolic BW, but endog- or had minimal endogenous nitrogen loss at 1.4–1.7 g enous urinary nitrogen output was positively correlated protein/kg BW/d. These values are greater than those for with BW loss during the 14-d feeding period [66]. The dogs and pigs (Table 2). For comparison, the rates of uri- 0.75 maintenance requirement of adult dogs for CP is 82 g/kg nary nitrogen excretion (mg nitrogen/kg BW /d) when of the diet containing 4.0 kcal ME/g diet, and the NRC- fed a nitrogen-free diet were: human, 62; marmoset, 110; recommended allowance for CP is 100 g/kg of the diet rat, 128; pig, 163; dog, 210; and cat, 360 [16]. Accord- containing 4.0 kcal ME/g (DM basis) [16]. ingly, adult cats require 2 to 3 times more dietary protein Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 7 of 21 Table 5 Concentrations of total amino acids (free plus peptide-bound) in the mature milk of cats, dogs, cows, goats, and pigs a b b c f Nutrient Dogs Cats Cows Goats Pigs (n = 16) (n = 4) (n = 4) (n = 30) (n = 10) Water, g/kg milk 773 790 877 870 799 Dry matter, g/kg milk 227 210 123 130 201 Crude protein, g/kg milk 75 75 33 35 48 Total amino acids, g/L milk Ala 2.50 2.80 ± 0.5 1.08 ± 0.03 1.18 1.97 ± 0.06 Arg 2.93 4.85 ± 0.5 1.14 ± 0.03 1.36 1.43 ± 0.08 Asp + Asn 9.53 6.51 ± 2.0 2.35 ± 0.17 2.51 5.12 ± 0.12 Cys 2.48 0.91 ± 0.5 0.30 ± 0.03 0.31 0.72 ± 0.05 Glu + Gln 8.90 15.8 ± 0.5 6.99 ± 0.07 6.95 9.44 ± 0.37 Gly 1.56 0.76 ± 0.5 0.61 ± 0.03 0.56 1.12 ± 0.07 His 1.35 2.04 ± 0.5 0.81 ± 0.03 1.23 0.92 ± 0.05 4-Hydroxyproline – – 0.48 ± 0.04 – 0.82 ± 0.04 Ile 1.97 3.26 ± 0.5 1.58 ± 0.03 1.61 2.28 ± 0.10 Leu 5.48 8.93 ± 0.5 3.33 ± 0.03 3.41 4.46 ± 0.17 Lys 3.17 4.32 ± 0.5 2.89 ± 0.07 3.43 4.08 ± 0.15 Met 1.41 2.42 ± 0.5 0.87 ± 0.03 0.78 1.04 ± 0.04 Phe 3.53 2.27 ± 0.5 1.68 ± 0.03 1.76 2.03 ± 0.11 Pro 3.62 7.12 ± 1.0 3.36 ± 0.13 3.11 5.59 ± 0.26 Ser 3.26 3.33 ± 0.5 1.88 ± 0.03 1.53 2.35 ± 0.11 Thr 3.44 3.48 ± 0.5 1.41 ± 0.03 1.39 2.29 ± 0.14 Trp 0.26 – 0.43 ± 0.02 – 0.66 ± 0.02 Tyr 3.57 3.41 ± 0.5 1.58 ± 0.03 1.63 1.94 ± 0.05 Val 3.80 3.56 ± 0.5 1.75 ± 0.03 2.10 2.54 ± 0.09 d d f e f Taurine 0.33 ± 0.14 0.36 ± 0.04 0.007 ± 0.001 0.098 0.19 ± 0.013 Adapted from Rezaei et al. [55] for the content of water, dry matter, and crude protein in milk. Values for total amino acids were calculated on the basis of their intact molecular weights, and are expressed as either means or means ± SEM when data are available Ferrando et al. [56] for proteinogenic amino acids Davis et al. [57] for proteinogenic amino acids Ceballos et al. [58] for proteinogenic amino acids Rassin et al. [59] Prosser [60] Wu [61] than adult dogs and herbivores (e.g., cows, sheep, and of AAs in a tissue-specific manner [61]. This must be horses) [14]. The maintenance requirement and the rec - taken into consideration when determining AA require- ommended allowance of dietary CP by the adult cat are ments of cats. 160 and 200 g/kg diet containing 4.0 kcal ME/g diet (DM basis), respectively [16]. This is equivalent to the mini - Metabolism of arginine mum maintenance requirement of adult cats for dietary There have been many studies of arginine nutrition in protein energy (16% of dietary ME). For comparison, dogs since the pioneering work of Rose and Rice in 1939 a dietary intake of protein energy accounting for 3.5%– [72]. Dogs can synthesize arginine from dietary glu- 4.5% of dietary ME is sufficient to maintain BW, nitrogen tamine/glutamate and possibly dietary proline, as well balance, and carcass nitrogen content in adult rats [69, as arterial glutamine via the intestinal-renal axis [63]. In 70]. The obligatory loss of nitrogen in cats appears to be adult dogs [73, 74], as in many other adult mammals (e.g., similar when they are fed a nitrogen-free diet or are food humans, pigs, rats, and sheep) [61], the small intestine deprived [16]. Interestingly, nitrogen balance in adult cats synthesizes and releases citrulline, which is taken up by fed a low-protein diet may be maintained when lean body extraintestinal tissues (primarily the kidneys) for arginine mass is reduced [71], possible due to reduced oxidation synthesis. The small intestine and other organs of dogs Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 8 of 21 express arginase for the hydrolysis of arginine to urea and not ornithine, can restore growth in young cats fed an ornithine [75]. arginine-free diet [80]. Such results can be explained by In adult dogs, the activity of arginase (expressed on the the findings that dietary or arterial blood ornithine is not basis of tissue protein) is similar between the duodenum used for the intestinal synthesis of citrulline in cats [63], and jejunum, with values for the ileum being 24%–37% as reported for other mammals including dogs [73] and of those for the upper parts of the small intestine [75]. pigs [53]. This is due to both the preferential metabolism Unlike pigs [61], the small intestine of postabsorptive of dietary ornithine into proline by enterocytes and the dogs does not release arginine [73], likely due to either lack of uptake of arterial blood ornithine by the gut [63]. low activities of argininosuccinate synthase and lyase for We suggest that higher protein requirements by cats than arginine synthesis or the further hydrolysis of arginine by dogs may result, in part, from a much lower ability to arginase in enterocytes. Thus, the homeostasis of argi - synthesize arginine in cats. The sensitivity of mammals to nine in the body depends on the rates of its endogenous dietary arginine deficiency is cats > dogs > rats [14]. synthesis and catabolism. Growing and adult dogs cannot synthesize sufficient arginine to meet functional needs Metabolism of aspartate, glutamate, and glutamine (e.g., ammonia detoxification via the urea cycle) beyond In canine and feline nutrition, aspartate, glutamate, and maintaining nitrogen balance [43]. Thus, a dietary level glutamine are among the traditionally classified nutri - of 0.4% and 0.28% arginine is needed for the maximum tionally nonessential AAs (NEAAs), but this has been growth of young dogs and the hepatic ureagenesis in disputed [81]. Mammals, including dogs, use dietary adult dogs, respectively, if other AAs are sufficient [43]. aspartate, glutamate, and glutamine as well as arterial This indicates that both mature and immature dogs have glutamine as the major metabolic fuels in their small an inadequate or limited ability to synthesize arginine de intestine [82], but cannot adequately synthesize aspar- novo. Syndromes of arginine deficiency in dogs include tate, glutamate, and glutamine [75]. Both aspartate and decreased food intake, hyperammonemia, severe emesis, glutamate are essential for intestinal metabolism, but are frothing at the mouth, and muscle tremors, and can be not taken up by the small intestine from the arterial prevented by dietary supplementation with arginine or blood [61]. Thus, these two AAs are required in cat and citrulline [16]. Dietary or arterial blood ornithine is not dog diets. used for arginine synthesis and cannot correct arginine There is a large database on the metabolism of the deficiency symptoms in dogs [43]. Interestingly, dog’s glutamine family of AAs in the small intestine [75] and milk contains much more arginine than the milk of herbi- kidneys [83] of dogs. Dietary aspartate, glutamate, and vores (e.g., cows) and omnivores (e.g., humans and pigs) glutamine are extensively degraded in the mucosa of the [76] to ensure that canine neonates receive adequate argi- canine small intestine as metabolic fuels [82]. Glutamine, nine for survival and growth. When fed a milk-replacer but not aspartate and glutamate, in the arterial blood, is diet containing inadequate arginine, dog puppies develop taken up by the canine small intestine for metabolism cataract [77]. Dietary arginine deficiency also occurs in [73]. The small intestine accounts for ~ 30% of the arte- human infants (causing hyperammonemia and death) rial blood glutamine utilized by healthy adult dogs, and and adults [reducing nitric oxide (NO) synthesis, sperm release ammonia, alanine, proline, and citrulline but little production, and fetal growth], and in rats (impairing or no ornithine and arginine [84–86]. This involves the growth and spermatogenesis) [61]. conversion of glutamine into alanine, proline and citrul- Cats have a very limited ability to synthesize citrulline line via a series of enzymes including glutaminase, glu- and arginine de novo because of the low activities of pyr- tamate transaminases, P5C synthase, and P5C reductase roline-5-carboxylate (P5C) synthase and ornithine ami- [61]. In addition, glutamine is the major source of gluta- notransferase [78]. The latter also limits the formation mate for gluconeogenesis and ammoniagenesis for the of citrulline from proline via the proline oxidase path- regulation of acid-base balance in the canine kidneys way. There is evidence for the synthesis of arginine from [87]. The utilization of arterial glutamine by the small citrulline and the catabolism of arginine via arginase in intestine of dogs is increased by ~ 80% during treadmill feline renal tubules [79]. In cats, when dietary intake of exercise due to elevated concentrations of glucagon, lead- arginine is insufficient, food ingestion is reduced, fol - ing to a 17% decrease in plasma glutamine concentration lowed by hyperammonemia (occurring within 1–3 h after [88]. Likewise, an intraluminal infusion of glucose, which feeding) due to impaired ureagenesis in the liver, vom- stimulate the release of glucagon from the pancreas, can iting, neurological signs, severe emesis, ataxia, tetanic enhance the uptake of arterial blood glutamine and the spasms, and death [80]. Dietary supplementation with release of ammonia, alanine, glutamate, and citrulline citrulline or ornithine to cats can prevent hyperammone- by the small intestine of dogs [89]. In response to meta- mia due to arginine deficiency. However, citrulline, but bolic acidosis, the uptake of glutamine by the canine Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 9 of 21 kidneys is markedly increased to meet the demand for At present, little is known about BCAA metabolism renal ammoniagenesis. Interestingly, in contrast to rats in the small intestine and other tissues of cats. However, [82], the extraction of glutamine by the small intestine of increasing the dietary protein content from 15% to 65% dogs is increased during progressive fasting (up to 4 d) of dietary ME increases the whole-body oxidation of leu- via unknown biochemical mechanisms [90], indicating cine and urea production in adult cats by about 3 times, 13 15 another species difference between dogs and omnivo - as measured with [1- C]leucine and [ N ]urea [95]. This rous mammals in the regulation of intestinal glutamine finding indicates that cats can adapt well to dietary AA metabolism in response to food deprivation. intake through modulating AA oxidation. In contrast Like dogs, the small intestine of cats takes up arte- to dogs, cats, like other Felidae species, use leucine and rial blood glutamine and releases ammonia [82]. In the valine to synthesize isovalthine and isobuteine, respec- fasted state, the feline small intestine extracts ~ 20% of tively, with hitherto unknown physiological function glutamine from the arterial blood [86]. Little is known [96, 97]. We are not aware of studies to compare tissue- about the metabolism of other AAs in the small intestine specific or whole-body BCAA metabolism among animal of cats. Besides published data on arginine synthesis from species (including cats). citrulline and the catabolism of arginine via arginase in renal tubules [79], there are no reports on the metabo- Metabolism of methionine, cysteine, and taurine lism of other AAs in the feline kidneys. We are not aware In the liver of dogs, methionine is catabolized to cysteine of studies to compare the tissue-specific- or whole-body and then to taurine, but neither taurine nor cysteine is metabolism of aspartate, glutamate, and glutamine converted to methionine [14]. The rate of oxidation of among animal species (including cats). cysteine to taurine depends on the dietary intakes of sul- fur-AAs. Methionine is often the first or the second (after Metabolism of branched-chain AAs (BCAAs) lysine) most limiting AA in plant-based diets for dogs. Dietary BCAAs (~ 30%) are extracted by the small Dietary cysteine can replace up to 50% dietary methio- intestine of fed dogs in first-pass metabolism, with 55% nine in these animals [43]. and 45% of the utilized leucine entering the transami- Most breeds of dogs can synthesize sufficient tau - nation and protein synthesis pathways, respectively rine when fed a methionine- and cysteine-adequate [91]. As reported for rats, the liver of dogs does not diet [43]. However, an inadequate intake of methionine degrade BCAAs [92] due to the near absence of BCAA and cysteine in diets [e.g., plant (e.g., peas, lentils, and transaminase in hepatocytes. Rates of BCAA uptake by rice)-based foods with no or insufficient taurine] may extrahepatic tissues determine the availability of these contribute to the development of dilated cardiomyo- nutrients for metabolic utilization. In fasted dogs (20 kg pathy characterized by thin heart muscle and enlarged BW; a food deprivation period of ~ 36 h), skeletal mus- chambers in some breeds of dogs [98, 99]. For example, cle takes up leucine from the arterial blood at the rate of a small proportion (1.3%–2.5%) of Newfoundland dogs 0.89 μmol/kg BW/min [93]. Based on the uptake of total fed commercially available diets that were considered to BCAAs (0.143 μmol/kg BW/min) by the skeletal muscle be complete and balanced in nutrition have a deficiency of 12 h-fasted pigs (20–25 kg BW) [94] and the ratio of of taurine [100] due to reduced taurine synthesis pos- leucine:isoleucine:valine (0.32:0.23:0.45) in the plasma sibly as a result of gene mutations [101]. When fed pro- of 12-h fasted pigs [75], it can be estimated that the skel- tein-restricted diets, certain breeds of dogs (e.g., golden etal muscle of pigs fasted for ~ 12 h takes up leucine from retrievers) are more prone to taurine deficiency and the the arterial blood at the rate of 0.046 μmol/kg BW/min. development of dilated cardiomyopathy even when fed It remains to be determined whether the reported large meat-based diets due to a combination of factors, includ- discrepancy in leucine uptake by skeletal muscle between ing complex interactions among dietary, metabolic, and dogs and pigs results from differences in animal species, genetic factors [102]. This disorder may result from low age, nutritional state, and research methodology. In dogs activities of cysteine dioxygenase and cysteine sulfinate and other mammals, most of the diet-derived BCAAs decarboxylase, as well as a limited availability of cysteine. bypass the liver and are used by extra-hepatic tissues An ability of some breeds of dogs to form taurine does (mainly skeletal muscle) for the synthesis of alanine and not necessarily mean that they do not require dietary glutamine in the presence of α-ketoglutarate and ammo- taurine for optimum health. Only in the breeds of dogs nia [92, 93]. The ammonia is derived from the blood as that possess sufficient enzymes for taurine synthesis well as the intramuscular catabolism of purines and AAs can the adequate provision of methionine plus cysteine [61]. In the post-absorptive state, alanine and glutamine in their diets prevent metabolic diseases such as dilated account for about 50% of the AAs released from the skel- cardiomyopathy. Although concentrations of taurine in etal muscle of dogs, pigs, rats, and humans [61]. plasma and skeletal muscle reflect its availability in these Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 10 of 21 animals, those in the whole blood may not be a sensitive no production of ammonia by the gut [107]. Less than indictor of taurine depletion caused by a low intake of 3% of glycine absorbed by the canine jejunum intralu- bioavailable sulfur AAs in dogs, especially in large dogs minally infused with 10 mmol/L glycine is released as [99]. serine [107], suggesting either a low activity of serine Methionine is generally the most limiting AA for cats hydroxymethyltransferase or an insufficient availability 5 10 fed a meat-based conventional diet [16]. The feline liver of N ,N -methylenetetrahydrofolate as a methyl group can convert methionine into cysteine. However, young donor in the intestinal tissues. In postabsorptive dogs, and adult cats have a limited ability to synthesize taurine there is no release of glycine or serine by the small intes- from cysteine due to low activities of cysteine dioxyge- tine [73], indicating the lack of their synthesis under this nase and cysteinesulfinic acid decarboxylase; therefore, nutritional condition. they have a requirement for dietary taurine [43]. In these Aromatic AAs are utilized via multiple metabolic path- animals, taurine deficiency results in dilated cardiomyo - ways in animals, including dogs [108–110]. Interestingly, pathy, heart failure, central retinal degeneration, blind- dogs require at least twice as much phenylalanine plus ness, deafness, and poor reproduction [103]. Thus, all tyrosine for maximal black hair color (adequate eumela- foods for cats must include sufficient taurine. nin in hair) as for growth [108, 110]. Furthermore, sup- Cats synthesize felinine, isovalthine, and isobuteine plementation with tryptophan (0.145% of diet, the from cysteine plus acetyl-CoA, cysteine plus isovaleryl- precursor of serotonin) can reduce territorial aggression CoA (a metabolite of leucine), and cysteine plus in dogs fed a low (19%)-CP diet (with the basal trypto- isobutyryl-CoA (a metabolite of valine) as unique sulfur- phan content of 0.18%) but has no effect in dogs fed a containing AAs [96, 97]. In intact adult male cats, the high (31%)-CP diet (with the basal tryptophan content of rates of urinary excretion of felinine and isovalthine are 0.24%) [111]. In addition, Pereira [112] found that dietary 122 and 1.8 μmol/kg BW/d, respectively [96]. At present, supplementation with tryptophan (12.5 mg/kg BW/d) there are no quantitative data on the urinary excretion of reduced bark and stare behavior in multi-housed dogs. isobuteine by cats. The direct source of cysteine for these At present, we are not aware of studies regarding proline synthetic pathways is glutathione. Felinine, isovalthine, catabolism in the intestine and other tissues of dogs. and isobuteine may serve as pheromones in cats for the Little is known about the catabolism of glycine, pro- purpose of territorial marking, intra-species commu- line, serine, threonine, phenylalanine, tyrosine, or tryp- nications, and chemical signals to attract females [104]. tophan in the feline intestine. However, these AAs are Indeed, males produce 239% more felinine than females degraded in the liver of cats [16]. As for dogs, the mainte- (122 vs. 36 μmol/kg BW/d) [105]. In addition, it is pos- nance of adequate eumelanin in the hair of cats requires sible that the production of unique cysteine metabolites twice as much phenylalanine plus tyrosine as for whole- (non-toxic, non-reactive, and relatively stable) helps body growth [109]. As a precursor of serotonin (a neu- to prevent excessive formation of toxic and acidic sub- rotransmitter and antioxidant), tryptophan along with stances (e.g., H S, SO , and H SO ) in cats [61]. α-casozepine (a bioactive peptide originating from the 2 2 2 4 Cysteine is the most abundant AA (accounting for S1 casein protein in cow’s milk) relieved anxiety while ~ 16% of total protein) in the hair of cats and dogs [43]. alleviating stress and aggression in cats [113]. Dietary During the growth period, these animals have greater supplementation with tryptophan (12.5 mg/kg BW/d) dietary requirements for cysteine than other mammals reduced vocalization, agonistic behavior, exploring, with fewer hair in the skin (e.g., pigs and humans) [61]. scratching, and agonistic interactions in multi-housed Although sulfur AA restriction has been reported to cats [114]. Interestingly, unlike omnivores, hepatic tryp- improve health (e.g., delayed aging and longer lifespans) tophan 2,3-dioxygenase [the enzyme oxidizing trypto- in adult rodents by altering the intestinal microbiome phan to N-formylkynurenine (the immediate precursor profile [106], we are not aware of such studies with cats of kynurenine)] is not induced by glucocorticoids in cats or dogs. [14]. This indicates another species difference in the regulation of AA metabolism between cats and other Metabolism of other AAs mammals. The mucosa of the canine intestine does not degrade threonine (the major AA in mucins), phenylalanine, AA imbalances and antagonisms tyrosine, or tryptophan, and its ability to catabolize or AA imbalances (improper ratios of AAs) occur in dogs interconvert glycine and serine is very limited [85, 107], fed commercial plant-based diets containing either a but these AAs are degraded in the canine liver [108]. small amount of or no animal-sourced ingredients (e.g., u Th s, when the canine small intestine is intraluminally meat) [98, 102]. This is largely because most plant pro - infused with each of these AAs (10 mmol/L), there is teins (particularly those in cereals) are deficient in some Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 11 of 21 EAAs (particularly lysine, tryptophan, threonine, methio- of lysine from 1.1% to 3.6%, 6.1%, and 8.6% in a diet con- nine, and cysteine), as well as glycine and proline (the first taining 1.3% arginine and 4.09 kcal ME/g diet does not and second most abundant AAs in the animal body) [54]. result in lysine-arginine antagonism in adult cats [119]. For AAs with similar chemical structures (e.g., BCAAs) Further increasing the dietary content of lysine to 11.1% or net electric charges (e.g., arginine and lysine), their and 13.1% gradually reduced the food intake of adult cats improper ratios often result in antagonisms because they by 30%–35% without affecting health [119]. Such a severe share the same transporters in the cell membrane for imbalance between lysine and arginine would be detri- absorption and uptake, the same enzymes for catabolism, mental for omnivores and herbivores [61]. and/or the same inhibitors of their respective pathways [61]. Effects of dietary AA imbalances in dogs are more Metabolic adaptation to low or high protein intakes similar to those in rats than in cats [14]. For example, in Most omnivores (e.g., rats and pigs) and herbivores (e.g., response to diets deficient in one or two EAAs, the food sheep and cattle) can adapt to: (1) low-protein diets by intake of dogs is not decreased as rapidly and severely increasing food intake and reducing AA catabolism, and as that for cats. Inclusion of adequate animal-sourced (2) high-protein diets by initially reducing food intake foodstuffs or dietary supplementation with deficient AAs for 1–5 d (depending on dietary protein levels), followed may aid in preventing and correcting AA imbalances and by up-regulating the expression and/or activities of AA- antagonisms in canine nutrition. catabolic enzymes [61]. For example, rats can down- or Based on changes in hepatic activities of AA-cata- up-regulate AA-catabolic and urea-cycle enzymes in bolic and urea-cycle enzymes as well as blood and urine response to a low or high intake of AAs [14]. In contrast, nitrogenous metabolites, cats are less sensitive to imbal- low protein intake did not affect whole-body protein deg - ances of most AAs (except for arginine, methionine, and radation in the postabsorptive state, and increasing die- lysine) in diets than dogs, rats, chicks, and pigs [14]. tary protein intake from 32 g CP/Mcal ME (low intake) This is because, compared with many omnivores, cats to 63 g CP/Mcal ME (medium intake) and to 148 g CP/ have a lower ability to sense protein-free or protein- Mcal ME (high intake) did not affect the rate of leucine deficient (e.g., 6% protein) diets and the diets containing oxidation in adult dogs under these nutritional condi- excess protein (e.g., 63% protein) or most AAs, even if tions [120]. Thus, dogs may not be as efficient as rats they exhibit protein malnutrition, lose weight, and grow in metabolic adaptation to low or high protein intakes. poorly [115, 116]. However, cats select for or against Experimental evidence shows that dogs can tolerate at some proteins (e.g., casein and soy protein) [115] and least 30%–32% dietary protein [111, 121]. the solution of some AAs [116], possibly based on their Adult cats have a 100% greater requirement for dietary chemical properties including taste. For example, in con- protein than do adult dogs, but dietary requirements for trast to omnivorous mammals (e.g., rats and pigs), cats some EAAs do not appear to differ appreciably between do not select a diet containing adequate (0.6%) threonine these two animal species [16]. This may be explained by versus a threonine-free diet [116]. In addition, only a higher requirements of cats for NEAAs to fulfill meta - mild AA antagonism is exhibited by cats when fed a diet bolic needs than do dogs [61]. First, dietary NEAAs (par- with isoleucine or valine as the limiting AA in the basal ticularly aspartate, glutamate, and glutamine) may be the diet, and these animals do not avoid a diet containing a major energy sources for the feline small intestine [61]. highly excessive amount of leucine (e.g., 10% of leucine in u Th s, based on the dietary AA intake, cats oxidize more diet) [117]. Thus, cats are less sensitive than omnivores to NEAAs than EAAs [122]. Second, there are metabolic leucine-isoleucine and valine antagonisms. Furthermore, demands for the increased use of BCAAs to synthesize a dietary deficiency of arginine greatly reduces food aspartate, glutamate, and glutamine in extra-intestinal intake by cats [80], likely because of neurological disor- tissues (including skeletal muscle, white adipose tissue, ders induced by hyperammonemia and reduced nitric- and heart) in cats as in other carnivorous mammals [61]. oxide availability [61]. Interestingly, Rogers et al. [116] Aspartate, glutamate, and glutamine in plasma may be reported that when adult cats were offered diets contain - the primary nutrients for ATP production in the feline ing either 0 or 0.2% methionine, they initially ate only the liver, skeletal muscle, and kidneys, as reported for car- methionine-containing diet, but beginning on d 3, they nivorous fish [123]. Third, most NEAAs are used as glu - consumed an increasing amount of the methionine-free cogenic substrates in the feline liver and kidneys because diet, and on d 6 selected the same amount of each diet. the natural diet (i.e., meat) of cats provides only a small It is unknown why cats select for methionine, but this amount of glucose [124]. When cats are fed diets (e.g., may be because of its bitter taste, just like leucine. Finally, some commercial foods) containing sufficient digest - although cats select for 0.5 to 50 mmol/L lysine (prepared ible carbohydrates, there is less gluconeogenesis from in saline) over saline [118], increasing the dietary content dietary AAs when compared with diets with insufficient Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 12 of 21 digestible carbohydrates. Finally, when the content of one to reduced muscle protein synthesis, increased prote- of 10 EAAs (Arg, Lys, His, Ile, Leu, Met, Phe, Thr, Trp, olysis, and augmented lipogenesis [61] while develop- and Val; provided at 1.8 to 3.3 times NRC requirements ing sarcopenia (defined as a loss of ≥3% lean body mass [16]) is decreased to one-half that present in the basal over 3 years) [128]. This can be illustrated by studies with diet, there is no decrease in the weight gain of young cats, Labrador retrievers (males plus females) that were fed a but the presence of one or more NEAAs is required for growth diet (containing 27.5% CP and 15.0 kJ ME/g diet) their maximal growth [14]. between 8 weeks and 3.25 years of age) and an adult diet Cats respond to nitrogen-free intake by decreasing (containing 21.2% CP and 14.8 kJ ME/g diet) between whole-body AA oxidation, leading to reduced excretion 3.25 and 14 years of age; those dogs lost 21% lean body of urinary total nitrogen, urea, and ammonia [22]. Thus, mass between 8 and 13 years of age and had a median life increasing the dietary content of protein from 0 to 4%, span of 11.2 years [129]. Because a 15% reduction in lean 7%, 10%, or 13% augmented the excretion of the nitrog- body mass of animals (including dogs) impairs organic enous metabolites in a dose-dependent manner [67]. and physiological functions, and a > 30% reduction may These animals can down-regulate AA catabolism and be fatal [130], it is imperative to mitigate sarcopenia in reduce urinary nitrogen excretion when protein intake is aging dogs. To alleviate insulin resistance with aging, pro- below their requirement (20% CP). Interestingly, regard- tein requirements should be increased by about 50% in ing AA metabolism, cats differ from omnivores (e.g., rats, older dogs compared with young adults [130, 131]. Pro- pigs, and chickens) and herbivores (e.g., sheep) that often tein restriction for healthy older dogs can be detrimen- exhibit 3- to 4-fold greater activities of hepatic AA-cat- tal to their health (particularly regarding the mass and abolic enzymes in response to a high-protein diet [125]. function of skeletal muscles and bones) and, therefore, For example, the activities of aminotransferases and urea- should be avoided in feeding practice. To meet the die- cycle enzymes in the liver do not differ in adult cats fed a tary requirements of dogs for high-quality protein, ani- high (70%)-CP diet and a low (17.5%)-CP diet [125]. It is mal-sourced foodstuffs [which provide proper ratios and likely that (1) tissues of cats express high basal levels of adequate amounts of proteinogenic AAs as well as func- enzymes for AA catabolism and urea-cycle enzymes; (2) tional nutrients (e.g., taurine, 4-hydroxyproline, creatine, when fed a low-protein diet, cats are unable to downreg- and carnosine)] can be useful ingredients for canine diets ulate these enzymes, but actual metabolic fluxes through [54]. In support of this view, increasing dietary CP intake the enzymes in vivo are decreased under conditions of from 16% to 32% enhanced whole-body protein synthesis such a nutritional state (e.g., reduced concentrations of in both young adult (2 years of age) and aging (8 years of AAs and cofactors in cells) to meet a need for conserv- age) dogs [132]. ing AAs; and (3) the activities of these enzymes are suf- Like dogs, cats lose lean body mass with age. For exam- ficient to degrade excessive AAs and remove excessive ple, aging cats lose 34% lean body mass over an 8-year ammonia as urea in healthy cats fed high-protein diets. period between 7 and 15 years of age [128]. As for older In support of this view, Russell et al. [126] reported that dogs, older cats need adequate high-quality protein (i.e., compared with adult cats fed a moderate-CP (35% of 40% animal protein in diet; DM basis) to alleviate aging- dietary ME) diet, feeding a high-CP (52% of ME) diet for associated reductions in the mass and function of skel- 50 d increased protein oxidation by 47% but decreased etal muscles and bones [71]. Such diets should also help fat oxidation by 37%. Likewise, increasing the dietary CP to improve the anti-oxidative and immune functions of level from 9.1% to 59.6% increased protein oxidation by senior cats. Consistent with this notion, adult cats (neu- 507% in adult cats (Table 6). Thus, cats are metabolically tered males) needed 1.5 g protein/kg BW/d (i.e., 2.1 g/kg 0.75 more capable of adapting to high protein intake than pre- BW /d) to maintain nitrogen balance but required 5.2 g 0.75 viously realized based on enzyme activity data [125]. Cats protein/kg BW/d (i.e., 7.8 g/kg BW /d) to maintain lean can tolerate at least 60% dietary protein [122]. In animals body mass [71]. This value is equivalent to 40% protein (including cats), AA oxidation increases to remove excess in the diet and greatly exceeds current NRC [16] recom- AAs [61]. mendations for dietary protein requirement of adult cats. Protein-restricted diets for healthy adult cats must be Loss of muscle protein with aging avoided in feeding practice. To meet the dietary require- According to the current NRC [16], the requirement of ments of adult cats and dogs for high-quality protein, non-pregnant and non-lactating adult dogs for dietary animal-sourced foodstuffs can be used to manufacture CP is similar to that of adult pigs but is only 50% of that diets for these animals [54]. of adult cats. Without nutritional intervention, aging Dietary AA intake by cats and dogs, like other ani- dogs usually lose a substantial amount of lean body mass mals, depends on dietary AA content and food con- and gain white adipose tissue [127, 128], possibly due sumption [61]. Multiple factors should be considered in Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 13 of 21 Table 6 Increasing dietary protein intake increases the oxidation of protein in adult cats Variable Diets containing different levels of protein Low Adequate Moderate High protein protein protein protein Body weight, kg 3.84 3.99 4.2 4.17 Dietary energy and nutrient content and food intake Protein content, % (as-fed basis) 9.1 17.2 32.7 59.6 Protein content, % of metabolizable energy 7.5 14.2 27.1 49.6 Fat content, % (as-fed basis) 20.9 21.0 20.9 20.9 Carbohydrate, % (as-fed basis) 65.1 57.0 41.0 13.5 Metabolizable energy, MJ/kg dry matter 20.3 20.3 20.2 20.1 Food intake, g dry matter/d 41.5 58.5 54.0 57.8 Nitrogen intake, excretion, and balance Nitrogen intake, g/d 0.60 1.61 2.82 5.52 Total urinary nitrogen, g/d 0.76 1.16 2.29 4.58 Urinary ammonia nitrogen, mg/d 117 152 157 198 Urinary creatinine nitrogen, mg/d 48.7 47.1 56.5 54.5 Fecal nitrogen, g/d 0.09 0.12 0.15 0.17 Nitrogen balance, g/d −0.25 0.33 0.39 0.76 Dietary intake and the oxidation of nutrients Protein intake, g/d 3.8 10.0 17.6 34.5 Protein oxidation, g/d 5.7 8.6 17.1 34.6 Fat intake, g/d 8.7 12.3 11.3 12.1 Fat oxidation, g/d 12.1 10.7 13.9 9.5 Carbohydrate intake, g/d 27.1 33.3 22.2 7.8 Carbohydrate oxidation, g/d 21.6 27.1 20.3 9.2 Concentrations of amino acids in plasma Total amino acids, μmol/L 1827 2579 4461 5363 Leucine + Isoleucine + Valine, μmol/L 154 221 564 1286 Urinary excretion of amino acids EAAs (excluding taurine) and NEAAs, μmol/d 418 563 458 527 EAAs/NEAAs, mol/mol 5.4 5.7 2.6 1.7 Felinine , μmol/d 388 458 1009 919 EAAs Nutritionally essential amino acids, NEAAs Nutritionally nonessential amino acids Adapted from Green et al. [122] Two males and two females formulating diets, including endogenous AA synthesis, growth) is minimal in the first 6 weeks of gestation and the digestibility and bioavailability of dietary nutrients, increases by 25% during the last 3 weeks of gestation the presence of antinutritive factors in foodstuffs, the fer - [134]. In cats, maternal weight gain (including both the maternal body fat gain and the conceptus growth) occurs mentability and quantity of dietary fiber, and interactions linearly during gestation and is 43% of their pre-preg among food constituents [33, 61, 133]. Furthermore, - requirements of cats and dogs for dietary AAs (including nancy BW [134]. In cats and dogs, 82% and 90% of fetal sulfur AAs) may critically depend on the catabolism of growth occurs in the last 3 weeks of gestation, respec- these nutrients by the intestinal microbiota [75, 99]. tively [134]. A deficiency of maternal dietary taurine (≤ 0.05%) causes early embryonic resorptions and fetal Requirements of cats and dogs for dietary amino defects in cats and their diets must contain > 0.05% tau- acids during pregnancy and lactation rine for optimum pregnancy outcomes [136]. At present, Average pregnancy length in cats and dogs appears to little is known about the dietary requirements of preg- similar (65 and 63 d, respectively), but there are differ - nant dogs for taurine. ences in the patterns of both maternal and fetal weight Lactation lasts approximately 7–8 weeks in cats and gains between these two species [134, 135]. In dogs, dogs, with peak milk production around weeks 3 to 4 maternal weight gain (almost exclusively the conceptus [134]. There are differences in maternal weight change Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 14 of 21 after parturition between cats and dogs. Specifically, and proline) via the intestinal-renal axis [63]. Of par- after giving births, the bitch generally returns to her ticular note, most dietary glutamine (~ 70%) and glu- pre-breeding BW immediately after delivery [134]. In tamate (~ 95%) as well as ~ 40% of dietary proline are contrast, the queen losses BW (mainly fats) gradually to extracted by the mammalian small intestine during reach her pre-breeding weight at 24 d post-partum and their first pass into the portal vein [75] and, thus, most her BW at 5 to 7 weeks post-partum is about 95% of her of these three AAs in the body are derived from endog- pre-breeding weight [137]. As in other mammals, dietary enous syntheses. Interestingly, glutamine is the only deficiencies of AAs impair milk production by lactating AA in the arterial blood that is taken up by the small cats and dogs [16]. intestine for citrulline and arginine production; there- To date, dietary requirements of pregnant and lactating fore, it is of nutritional and physiological importance cats and dogs for proteinogenic AAs have not been well to convert BCAAs (the source of the amino group and defined. It has been assumed that dietary CP require - amide nitrogen) into glutamine in extra-hepatic tissues ments for the growth of young cats and dogs would (primarily skeletal muscle) [61]. Because BCAAs are meet their requirements for gestation and lactation [16]. not formed de novo in all animals, these EAAs must However, embryos of mammals, including cats and dogs, be provided from high-quality and high-quantity pro- are highly sensitive to ammonia toxicity and, therefore, tein. Even the same ingredient may not supply the same maternal intakes of dietary CP and AAs should not be amount of nutrients depending on the method of food excessive [138, 139]. In gestating and lactating dogs, the processing, and some of diet-derived small peptides requirement for dietary CP is 260 g/kg diet containing can exert signaling and regulatory functions in the 4.0 kcal ME/g of the diet without dietary carbohydrate intestine and extraintestinal tissues [141]. Despite an or 200 g/kg diet containing 4.0 kcal ME/g of the diet with endogenous synthesis of arginine, both young and adult dietary digestible carbohydrate [16]. The NRC [16] rec - dogs must ingest adequate arginine in diets to main- ommends that the dietary requirements of dogs or cats tain its vital physiological functions beyond nitrogen for CP and AAs be the same during late pregnancy and balance [43], as noted previously. Interestingly, hydro- peak lactation, but it is likely that such estimates do not lyzed feather is a rich source of arginine (5.83%, as-fed reflect the true requirements of the animals because of basis) [54]. Inclusion of hydrolyzed feather meal in dry marked differences in physiological states (pregnancy or wet foods for dogs can meet their high requirements versus lactation) and products (conceptus versus milk). for arginine. In addition, even when fed a diet contain- In addition, the NRC [16] recommends substantial ing sufficient methionine and cysteine, some breeds of increases in the dietary contents of CP and most pro- dogs have a limited ability to synthesize taurine due to teinogenic AAs except methionine, cysteine and tryp- genetic mutations as noted previously and, therefore, tophan for adult dogs during late gestation and peak must be provided with adequate dietary taurine (e.g., lactation compared with non-pregnant and non-lactating 0.4% of dietary DM) [100]. counterparts (Table 7). Disappointingly, the NRC [16] did As noted previously, cats have a very limited ability to not explain its recommended 3%, 6%, and 14% decreases, synthesize both arginine and taurine and, therefore, must respectively, in the dietary content of cysteine, methio- consume diets containing these two AAs to ensure nor- nine and tryptophan for dogs during both late gestation mal blood flow, the proper digestion of dietary lipids and and peak lactation compared with non-pregnant and fat-soluble vitamins, and maintain health (particularly non-lactating adult dogs. Such recommendations do not retinal, cardiac, skeletal, reproductive, and metabolic appear to have physiological bases and should be revised health) [14, 43]. These animals do not have preference in the future. for plant products that generally contain high amounts of carbohydrates including sweet sugars [14]. In recent Important roles of animal‑sourced foodstuffs years, much work has shown that animal-derived ingre- in providing AAs in diets for cats and dogs dients are abundant sources of both arginine and taurine Abundant sources of both arginine and taurine for the diets of animals [54]. For example, the content of in animal-sourced foodstuffs for cats and dogs taurine in common animal-derived foods (mg/kg food, Compared with adult humans, adult dogs have a 90% on an as-fed basis) is: blood meal, 1520; chicken by-prod- greater rate of whole-body arginine catabolism and, uct meal, 2096; chicken visceral digest, 1317; spray-dried consequently, a much higher requirement for arginine peptone from enzyme-treated porcine mucosal tissues, [63]. This necessitates a higher intake of good-quality 1638; poultry by-product meal (pet-food grade), 3884; protein by dogs than humans to both directly supply and spray-dried poultry plasma, 2455. These foodstuffs exogenous arginine and endogenously generate argi- also contain creatine that is essential for energy metab- nine from its precursor AAs (glutamine/glutamate olism and anti-oxidative reactions in excitable tissues Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 15 of 21 Table 7 Recommended allowances of dietary AAs for post-weaning growing dogs and cats, as well as adults Nutrient Dogs, Amt/kg DM Cats, Amt/kg DM Pigs, Amt/kg DM Growing Adult (NP) LG PL Growing Adult LG PL 6-kg 110-kg P1G P1LS (NP) BW BW NRC (2006) NRC (2006) NRC (2012) ME, kcal 4000 4000 4000 4000 4000 4000 4000 4000 3778 3667 3667 3667 Crude protein, g 225 100 200 200 225 200 213 213 252 116 142 228 EAAs, g Arg 7.9 3.5 10.0 10.0 9.6 7.7 15 15 8.3 3.6 4.4 5.3 His 3.9 1.9 4.4 4.4 3.3 2.6 4.3 4.3 6.4 2.8 1.9 3.9 Ile 6.5 3.8 7.1 7.1 5.4 4.3 7.7 7.7 9.8 4.3 4.6 5.4 Leu 12.9 6.8 20.0 20.0 12.8 10.2 18 18 19.4 7.9 7.9 11.0 Lys 8.8 3.5 9.0 9.0 8.5 3.4 11 11 18.9 7.9 8.4 9.6 Met 3.5 3.3 3.1 3.1 4.4 1.7 5.0 5.0 5.4 2.3 2.4 2.6 Met + Cys 7.0 6.5 6.2 6.2 8.8 3.4 9.0 9.0 10.7 4.8 5.7 5.2 Phe 6.5 4.5 8.3 8.3 5.0 4.0 – – 11.2 4.8 4.7 5.2 Phy + Tyr 13.0 7.4 12.3 12.3 19.1 15.3 19.1 19.1 17.8 7.8 8.3 10.9 Thr 8.1 4.3 10.4 10.4 6.5 5.2 8.9 8.9 11.7 5.4 6.1 6.4 Trp 2.3 1.4 1.2 1.2 1.6 1.3 1.9 1.9 3.1 1.4 1.6 1.8 Val 6.8 4.9 13.0 13.0 6.4 5.1 10 10 12.2 5.4 6.1 8.3 Taurine (Cats) – – – – 0.40 0.40 0.53 0.53 – – c d f NEAAs, gLi and Wu Che et al. [140] Wu and Li [139] Ala 15.2 5.63 10.6 12.1 23.3 20.1 29.1 34.9 14.4 7.99 8.91 10.5 Asn 10.6 3.95 7.06 8.46 17.1 14.8 21.5 25.8 10.5 5.75 6.46 8.53 Asp 15.2 5.63 10.6 12.1 21.0 18.3 26.5 31.8 14.7 8.17 7.88 12.1 Glu 26.6 9.88 17.6 21.2 38.4 33.3 48.3 58.0 25.6 14.1 11.5 23.4 Gln 24.0 8.92 15.9 19.1 25.5 22.1 32.0 38.4 23.8 12.6 20.7 18.3 Gly 16.9 5.82 10.4 12.5 17.3 14.9 21.6 25.9 16.2 9.07 6.20 9.58 Pro + Hyp 19.0 6.27 11.2 13.4 17.8 15.4 22.3 26.8 18.6 10.2 11.5 15.8 Ser 9.30 3.46 6.18 7.42 18.1 15.6 22.6 27.1 8.74 4.87 5.81 9.24 Amt Amount, BW Body weight, DM Dry matter of diet, EAAs traditionally nutritionally essential amino acids, Hyp 4-hydroxyproline, LG Late gestation, ME Metabolizable energy, NEAAs traditionally nutritionally nonessential amino acids, NP Non-pregnant and non-lactating, NRC National Research Council, PL Peak lactation, P1G the last 24 d of pregnancy of first-parity swine, P1LS first-parity lactating sows “–” data are not available Referring to 4- to-14-week-old dogs for EAAs. The recommended allowance of dietary crude-protein for ≥14-week-old growing dogs is 175 g/kg DM The ratio of proline to 4-hydroxyproline is 18.6:1.0, g/g Present work. It is assumed that the recommended allowance of dietary crude-protein for adult dogs is 10%, dry matter basis It is assumed that the recommended allowances of dietary crude-protein for growing and young adult cats are 30% and 26%, respectively, on the dry matter basis [140]. The recommended allowances of dietary EAAs for growing cats (g/kg dry matter of diet) are: Arg, 26.8; Cys, 6.3; His, 16.3; Ile, 21.0; Leu, 34.1; Lys, 36.8; Met, 12.9; Phe, 17.1; Thr, 18.9; Trp, 5.1; Tyr, 15.4; and Val, 24.3. The recommended allowances of dietary EAAs for young adult cats (g/kg dry matter of diet) are: Arg, 23.3; Cys, 6.3; His, 14.0; Ile, 18.3; Leu, 29.5; Lys, 31.9; Met, 11.3; Phe, 14.9; Thr, 16.4; Trp, 4.4; Tyr, 13.4; and Val, 21.0. The recommended allowances of dietary crude-protein for young and adult cats are 1.25 times their minimum dietary requirements for crude protein Different breeds of swine have different body weights at young and adult ages. For the offspring of Yorkshire × Landrace sows and Duroc × Hampshire boars with a normal birth weight, 6- and 110-kg body weights correspond to approximately 3 weeks and 6 months of age. At about 18 months of age (adult), the lean-tissue weight curve of swine is flattened. The dietary requirement of adult swine for crude protein is approximately 100 g/kg dry matter Values refer to amino acid content, rather than digestible amino acid content, in the diet Abundant sources of glycine, proline, 4-hydroxyproline, (brain and skeletal muscle) [61]. In contrast, all plant- cysteine, and serine in animal-sourced foodstuffs for cats sourced foodstuffs lack taurine and creatine [54] and, and dogs therefore, should not be fed solely to either cats or some Another unique feature of animal-derived products is breeds of dogs. As for dogs, hydrolyzed feather meal can be included in diets as an abundant source of both argi that they contain high amounts of either collagens (e.g., meat and bone meal and poultry by-product meal) or nine and taurine for cats. Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 16 of 21 keratins (e.g., hydrolyzed feather meal) [61]. Collagen is and galactosamine from glycosaminoglycans, whereas comprised of two-thirds of AAs as glycine, proline, and chondroitinase and deacetylase hydrolyze chondroitin 4-hydroxyproline, whereas keratins (present in feather to galactosamine and glucuronic acid [61]. The amino - and hair) are also rich in these three AAs plus cysteine sugars are absorbed into enterocytes and then the portal and serine (the immediate precursor of glycine). After vein. Within cells, 4-epimerase converts galactosamine feather and hair are properly hydrolyzed, their AAs into glucosamine. Thus, poultry meal is a source of glu - are nutritionally available for both cats and dogs to use cosamine for animals, including cats and dogs. Of par- [54]. For example, the content of the most abundant ticular note, glucosamine has anti-inflammatory and AAs in chicken feather keratin (% of total AAs, mol/ anti-oxidative effects in immunologically challenged mol) is: glycine, 13.7; proline, 9.8; cysteine, 7.8; and ser- mammalian cells by inhibiting the expression of induc- ine, 14.1 [142]. Thus, hydrolyzed feather meal contains ible NO synthase and excessive NO production [143]. high amounts of glycine, proline, 4-hydroxyproline, This may explain why glucosamine plus chondroitin has cysteine, and serine (8.97%, 11.64%, 4.97%, 4.17%, and been used to effectively treat dogs (particularly elderly 8.92%, respectively, as-fed basis) [54]. Dietary provision dogs and working dogs) [144] and cats [145] with joint of hydrolyzed feather meal can spare energy and materi- pain or osteoarthritis. Research is warranted to define the als that would be needed for de novo syntheses of these efficacy of dietary supplementation with poultry meal in AAs in animals, thereby reducing energy expenditure as improving the health of cat and dog joints. well as the associated production of oxidants (e.g., for- maldehyde) and ammonia [61]. Of note, cysteine, glycine, Improvement of immune responses in cats and dogs and proline are crucial for the synthesis of hair proteins AAs are essential for immune responses in all ani- (e.g., cysteine-rich α-keratin and β-keratin) as are both mals (including cats and dogs) through a plethora glycine and proline for the synthesis of collagen and of mechanisms, such as the syntheses of proteins elastin [61]. These unique proteins maintain the normal (including antibodies and cytokines) and glutathione structures and integrity of hair and connective tissue (a potent anti-oxidative tripeptide consisting of gly- while preventing its abnormalities particularly in associa- cine, cysteine, and glutamate), as well as the killing of tion with aging. Indeed, hair quality is considered by pet pathogens via production of NO from arginine and of owners as a very important indicator of the nutritional chlorotaurine and bromotaurine from taurine [61]. Sup- adequacy of commercially manufactured pet foods or plementing arginine to a low-protein (23% CP) [146] or home-made meals [137]. Thus, hydrolyzed feather meal high-protein (60% CP) [147] diet has beneficial immu - may be a desirable pet-food ingredient to provide nutri- nomodulating effects in cats. In addition, intravenous tionally and physiologically significant AAs (including administration of alanyl-glutamine to dogs undergoing arginine, cysteine, glycine, proline, 4-hydroxyproline, and a treatment with methylprednisolone sodium succinate serine) [54]. This new knowledge can help to dispel the enhanced phagocytic capacity and respiratory burst unfounded myth that poultry-sourced hydrolyzed feather activity of leukocytes [148]. Currently, there is a global meal is of little nutritive value in feeding companion pandemic of COVID-19 caused by the severe acute res- animals. piratory syndrome coronavirus 2 that can also infect dogs [149] and cats [150]. Adequate AA nutrition [e.g., Provision of glucosamine in animal-sourced foodstuffs sufficient provision of AAs (such as lysine, cysteine, for cats and dogs methionine, tryptophan, glycine, and proline) that are Glucosamine is a normal metabolite of glutamine and abundant in animal proteins but are relatively low in fructose-6-phosphate in animals [61]. Poultry meal is plant proteins] may be crucial for improving both innate manufactured from raw materials containing chicken and acquired immune systems to mitigate risk for infec- cartilage, which consists of glycosaminoglycans (includ- tion in the animals. In addition, some animal-sourced ing chondroitin) and proteoglycans (formed from gly- foods, such as spray-dried animal plasma [151] and cosaminoglycans and protein backbones) in addition to spray-dried egg products [152] contain a large amount collagens and elastins. Glycosaminoglycans are com- of immunoglobulins and directly contribute to neutral- posed of N-acetylglucosamine and N-acetylgalactosa- izing the pathogens that invade the body. Furthermore, mine, as well as their sulfate derivatives [61]. In the small spray-dried animal plasma provides other functional intestine, proteoglycans are hydrolyzed to proteases molecules, such as albumin, essential fatty acids, (e.g., trypsin) to generate glycosaminoglycans, pep- B-complex vitamins, and minerals such as calcium, tides, and AAs; the combined actions of exoglycosidase, phosphorus, sodium, chloride, potassium, magnesium, endoglycosidase, sulfohydrolase, and hyaluronidase-like iron, zinc, copper, manganese, and selenium [153, 154]. enzymes along with deacetylase release glucosamine This animal-sourced foodstuff is also a useful binder Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 17 of 21 in canned pet food products due to its high content of AAs as well as large amounts of functional nutrients globulins and fibrinogen as well as its desired physico - (e.g., taurine, 4-hydroxyproline, creatine, and carnos- chemical properties [155]. Finally, animal plasma prod- ine), lipids, and minerals [61]. In addition, feather meal ucts are highly palatable to both cats and dogs [156]. can be used as an ingredient or supplement in dry or wet Thus, animal-derived ingredients used in dry or wet pet foods for cats and dogs to meet their high requirements foods can help to improve the immune responses and for both arginine and taurine [54]. Animal-derived ingre- health of all companion animals by providing not only dients alone or in combination are abundant sources of AAs but also other essential nutrients (e.g., macro- and both proteinogenic AAs and taurine for adequate nutri- micro-minerals). tion and metabolism in cats and dogs to optimize their growth, development, health, and well-being. Conclusions and perspectives Both cats and dogs are carnivores from the taxonomi- Abbreviations cal order Carnivora. During evolution, domestic dogs AA Amino acid have adapted to omnivorous diets that contain both tau- AAFCO Association of American Feed Control Officials BCAA Branched-chain amino acid rine-rich meat and starch-rich plant ingredients, while BW Body weight domestic cats remain obligate carnivores. Thus, dogs CP Crude protein differ from cats in many aspects of AA nutrition and DM Dry matter EAA Nutritionally essential amino acid metabolism, and dogs can thrive on taurine-free vegetar- ME Metabolizable energy ian diets supplemented with non-taurine nutrients that NEAA Nutritionally nonessential amino acid are inadequately synthesized de novo or absent from NO Nitric oxide NRC National Research Council plants. Much evidence shows that there are marked dif- P5C Pyrroline-5-carboxylate ferences in both qualitative (i.e., presence or absence) and quantitative (i.e., amounts) requirements for protein and Acknowledgments We thank Prof. Nancy Ing (DVM and PhD, Department of Animal Science at certain AAs (arginine, taurine, methionine, and cysteine, Texas A&M University) for critically reviewing and providing helpful com- as well as NEAAs) between cats and dogs. In comparison ments on this manuscript. We are also grateful to Prof. Jürgen Zentek (DVM with swine [53, 139], recommended minimum require- and PhD, Freie Universität, Berlin, Germany) for kindly sharing his published articles with us. ments and allowances of dietary EAAs for growing and adult cats and dogs are summarized in Table 7. We sug- Authors’ contributions gest that companion animals have dietary requirements PL and GW conceived the idea for this article, performed the literature research and data analysis, and wrote the manuscript. The author(s) read and for NEAAs as do other mammals due to insufficient syn - approved the final manuscript. thesis de novo. Cats have greater endogenous nitrogen losses, as well as higher requirements for dietary protein Funding This work was supported by Texas A&M AgriLife Research (H-8200). (including arginine) and taurine than do dogs and, there- fore, should not be fed dog foods. Because the composi- Availability of data and materials tion of the milk of both cats and dogs differ from that of This manuscript reviews published work and does not contain new experi- mental data. farm mammals, young pets should not be fed replacer diets formulated based on goat or cow milk. As compan- Declarations ion animals lose tremendous amounts of lean body mass with aging, their diets should contain adequate levels of Ethics approval and consent to participate high-quality protein, which may be much greater than This review article does not require either human consent or the approval of animal use. the current AAFCO [157] and NRC [16] recommenda- tions to support muscle protein synthesis and mitigate Consent for publication muscle loss. Not applicable. Effects of an excessive intake of a single AA in cats Competing interests [140] and dogs [108] may be different, depending on die - All authors declare no conflict of interest. tary intakes of other AAs. We are not aware that compar- isons of the metabolism or dietary requirements of any Received: 8 October 2022 Accepted: 21 December 2022 nutrients between modern breeds of cats and dogs were made in the same experiment. 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Publisher: Springer Journals
Copyright: Copyright © The Author(s) 2023
eISSN: 2049-1891
DOI: 10.1186/s40104-022-00827-8
Publisher site: See Article on Publisher Site

Abstract

Domestic cats and dogs are carnivores that have evolved differentially in the nutrition and metabolism of amino acids. This article highlights both proteinogenic and nonproteinogenic amino acids. Dogs inadequately synthesize citrul- line (the precursor of arginine) from glutamine, glutamate, and proline in the small intestine. Although most breeds of dogs have potential for adequately converting cysteine into taurine in the liver, a small proportion (1.3%–2.5%) of the Newfoundland dogs fed commercially available balanced diets exhibit a deficiency of taurine possibly due to gene mutations. Certain breeds of dogs (e.g., golden retrievers) are more prone to taurine deficiency possibly due to lower hepatic activities of cysteine dioxygenase and cysteine sulfinate decarboxylase. De novo synthesis of arginine and taurine is very limited in cats. Thus, concentrations of both taurine and arginine in feline milk are the greatest among domestic mammals. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for many amino acids (e.g., arginine, taurine, cysteine, and tyrosine), and are less sensitive to amino acid imbalances and antagonisms. Throughout adulthood, cats and dogs may lose 34% and 21% of their lean body mass, respectively. Adequate intakes of high-quality protein (i.e., 32% and 40% animal protein in diets of aging dogs and cats, respectively; dry matter basis) are recommended to alleviate aging-associated reductions in the mass and func- tion of skeletal muscles and bones. Pet-food grade animal-sourced foodstuffs are excellent sources of both proteino - genic amino acids and taurine for cats and dogs, and can help to optimize their growth, development, and health. Keywords Animal-sourced foodstuffs, Cats, Dogs, Health, Metabolism, Nutrition gradually between 2013 and 2022 by 677% and 147%, Introduction respectively [3]. In the United States, 25.4% and 38.4% of The domestic dog (Canis familiaris ) and the domes- households owned cats and dogs, respectively, in 2018, tic cat (Felis catus) have been human companions for as companions or family members [4]. Most petfoods at least 12,000 and 9000 years, respectively [1, 2]. These are commercially manufactured, although some peo- animals contribute to the mental health and well-being ple choose to prepare meals for their own pets by using of children, adolescents, and adults, and have become animal- and plant-sourced ingredients. Thus, the global increasingly popular in many countries and worldwide petfood industry has grown substantially in recent years. over the past decades (Table 1). For example, the num- The compound annual growth rate of the global petfood bers of domestic cats and dogs in China have increased market is expected to be 4.6% between 2020 and 2027 (monetary value, US $124.9 billion by 2027) [5]. The dog is a domesticated descendant of the grey wolf *Correspondence: Guoyao Wu (an obligate carnivore), and was from the taxonomical g-wu@tamu.edu order Carnivora over 15,000 years ago [6]. The cat, which North American Renderers Association, Alexandria, Virginia 22314, USA was also from the order Carnivora, is the only domesti- Department of Animal Science, Texas A&M University, College Station, TX 77843, USA cated species in the family Felidae [7]. The feline domes - tication occurred approximately 10,000 years ago [7]. To © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 2 of 21 Table 1 Numbers of domestic dogs and cats as pets worldwide that are distinct from omnivorous mammals such as pigs, 6 a (× 10 ) rats, and humans [10, 14, 15]. The National Research Council (NRC) [16] recognizes that the dog is a carni- World or country Year Dogs Cats vore anatomically but has many metabolic characteris- World 2018 471 373 tics of omnivores, including the conversion of β-carotene 2020 510 400 to vitamin A, tryptophan to niacin, cysteine to taurine, Brazil 2010 34 18 and linoleic acid to arachidonic acid. Dogs also differ 2018 52 22 from cats in many of these aspects. For example, unlike 2020 56 23 cats that require much more dietary protein (expressed Canada 2012 6.5 8.0 as % of diet) than provided in grains and vegetables, most 2018 7.6 8.1 breeds of dogs can thrive on taurine-free vegetarian diets 2020 7.7 8.1 that are properly balanced and sufficient in non-tau - China 2013 55 22 rine nutrients through supplementation with those either 2017 85 69 absent from plants or inadequately synthesized de novo 2018 94 87 [16]. Regardless of their sources of food, adequate knowl- 2022 136 171 edge of nutrient metabolism and requirements by cats Europe 2010 74 85 and dogs is crucial to ensure their optimal growth, devel- 2018 85 104 opment, and health. The major objective of this article is 2020 90 110 to highlight the nutrition and metabolism of amino acids Japan 2013 10.9 9.7 (AAs) in cats and dogs. Unless indicated, AAs except for 2018 8.9 9.6 glycine and taurine refer to the L-isomers herein. 2020 8.5 9.6 India 2014 13 1.2 Digestibilities of amino acids in the diets of cats 2018 19 1.7 and dogs 2020 22 1.9 Determination of AA digestibilities in cats and dogs USA 2011 70 74 usually requires a 5-d period of adaptation and a 5-d 2018 77 58 period of sample collection [16]. To date, the fecal col- 2020 85 65 lection method has been largely used to measure the a apparent total tract digestibilities of AAs in cats, and Data for the USA are taken from the American Veterinary Medical Association at https:// www. avma. org/ resou rces- tools/ repor ts- stati stics/ us- pet- owner ship- some studies have involved sample collection at the stati stics. Data for other regions of the world are taken from Statista at https:// end of the ileum in dogs [17–20]. Data on total tract www. stati sta. com/ stati stics digestibilities of amino acids are not useful for formu- lating animal diets, but comparison between apparent ileal and total tract AA digestibilities may help to assess date, about 450 dog breeds [8] and 60 cat breeds [9] the extent of metabolism of individual AAs in the large with differences in shape, size, and color are recognized intestine [15]. Although there are substantial amounts globally. Puberty or sexual maturity occurs in domes- of endogenous AAs in the small intestine of both cats tic cats and dogs at 6–12 and about 6 months, respec- and dogs (Table 2) due to gastrointestinal secretions tively, whereas the life span of cats and dogs is 12–18 and such as sloughed mucosa and digestive enzymes [21– 10–16 years, respectively, depending on size, breed, and 24], there are no data on the true ileal digestibilities of nutritional status. Both cats and dogs have: (a) a rela- AAs and other nutrients in these animals due to both tively shorter digestive tract, longer canine teeth, and technical and ethical challenges. Nevertheless, stand- a tighter digitation of molars than omnivorous mam- ardized ileal digestibilities of individual AAs based mals such as humans and pigs, (b) a very low activity of on corrections for endogenous AA flow in the ileum salivary α-amylase, (c) a limited ability to synthesize de have been reported for dogs [15]. According to pub- novo arginine and vitamin D or to convert α-linolenic lished studies [25, 26], standardized ileal digestibili- acid to 5,8,11,14,17,20-docosahexaenoic acid, and (d) ties of AAs in dogs are similar to, or lower than, those instinct preferences for meat to plant products [10, 11]. for pigs (Table 3). Compared with dogs, cats generally Based on their anatomical, metabolic, and natural feed- have lower apparent digestibilities of AAs for lower- ing characteristics, dogs (facultative carnivores) and cats quality proteins due to a shorter small intestine relative (obligate carnivores) are classified as carnivores in clas - to body weight (BW) but have similar values for high- sic animal nutrition [12] and veterinary medicine [13] quality proteins (e.g., those with 90% or higher appar- textbooks, but these animals have evolved to have some ent digestibilities) [27]. unique feeding behaviors and metabolic characteristics Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 3 of 21 Table 2 Endogenous excretion of amino acids and nitrogen at the terminal ileum of adult cats, dogs, rats, and pigs a b a c AminoAdult dogsAdult catsAdult rats Pigs acid Protein-free diet Enzyme- Protein Enzyme- Protein-free Enzyme- 15-kg 75-kg hydrolyzed free diet hydrolyzed diet hydrolyzed BW BW casein casein casein μg/g of dry matter intake Ala 515 951 666 1380 269 474 437 454 Arg 370 728 540 948 159 328 480 435 Asp + Asn 960 2428 1283 2725 660 1095 755 640 Cys – – 452 853 – – – 136 Glu + Gln 1089 5993 1427 4240 696 1513 786 735 Gly 600 1001 665 1298 513 435 1660 1053 His 268 700 397 897 136 218 231 166 Ile 362 1191 398 1205 249 575 231 265 Leu 560 1180 884 1823 378 706 399 459 Lys 441 866 570 1101 193 390 312 427 Met 119 323 164 411 75 157 – 65 Phe 465 624 632 1015 171 316 237 265 Pro 643 1814 820 1913 442 691 3557 3104 Ser 925 3386 1013 2734 407 887 549 345 Taurine – – 2091 299 – – – – Thr 1168 2015 1235 2127 450 659 571 423 Tyr 444 648 599 1046 180 361 181 213 Val 536 1448 696 1687 273 599 321 387 mg/g of dry matter intake AA N 1.27 3.33 1.9 3.6 0.71 1.23 1.63 1.40 Total N 2.27 4.12 2.4 3.6 1.91 1.63 2.16 – “–” data are not available Hendriks et al. [21]. Adult dogs and rats were fed either a protein-free diet or a diet containing 23% enzyme-hydrolyzed casein Hendriks et al. [22]. Adult cats were fed either a protein-free diet or a diet containing 14% enzyme-hydrolyzed casein The values were determined in 15-kg growing pigs [23] and 75-kg growing pigs [24] fed a protein-free diet The cecectomized rooster model has been used destroys trypsin inhibitors, but prolonged heating to assess the digestibilities of AAs in foodstuffs for decreases the digestibilities of crude protein (CP) and cats and dogs [28]. Based on this assay, a very low AAs (e.g., Lys and Arg) due to the Maillard reaction [16]. true digestibility of glycine (e.g., only 22%) has been Cooking can result in improved digestibilities of AAs in reported for lamb and brown rice, compared with the dogs [30] and of starch in cats [31]. Increasing the dietary values of > 80% for most AAs [28], possibly due to an level of soluble fiber reduces CP digestibility in both dogs inaccurate measurement of endogenous glycine flow in [32, 33] and cats [31]. In contrast, dietary supplementa- the ileum. In addition, results from a study with a swine tion with yucca schidigera (60–120 mg/kg) enhances the model indicated that the apparent ileal digestibility of digestibility and absorption of AAs in these animals, cysteine and proline (i.e., 30%) in meat and bone meal thereby reducing the odor of pet stools [34]. Extrusion of as compared with the values of > 69% for most AAs [29] meat and bone meal via high temperature and pressure was likely due to analytical problems, as it is a techni- (135 °C, 3 bar, 20 min) increases the digestibility of dietary cal challenge to determine these two AAs by using ion- CP in adult cats but had no effect in adult dogs [27], indi - exchange chromatography. Caution over data accuracy cating species differences in nutrient digestion. and test animal models should be taken when interpret- The apparent ileal and total tract digestibilities of CP in ing and using literature data on AA digestibilities for diets containing both plant ingredients (primarily wheat feeding companion animals. and corn grains) and meat (lamb meal, poultry meal, or Many of the factors that influence apparent AA digest - fish meal) were 74% and 84%, respectively, in adult dogs ibilities in dogs have similar effects in cats. In manufac - [35]. The apparent ileal digestibilities of individual AAs turing pet foods, the heating of plant-sourced foodstuffs in this canine diet (%) were: Ala, 80.5; Arg, 85.1; Asp + Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 4 of 21 Table 3 Apparent ileal, total tract, and standardized digestibilities of amino acids in adult dogs, cats, and pigs, % a c NutrientsAdult dogs Apparent digestibility Pigs (total tract) by adult cats Apparent digestibility Standardized Hair Hair Apparent Standardized ileal removed included ileal ileal digestibility Ileal Total tract digestibility digestibility (fecal) Dry matter 75.1 81.2 – 80.1 72.0 – – Organic matter 79.4 85.3 – – – – – Crude protein 76.2 81.9 81.4 82.4 73.4 80.7 87.8 Lipids 96.5 92.4 – 95.3 93.4 – – Carbohydrates 88.9 95.5 – – – – – Ala 77.7 81.6 80.8 86.8 78.6 78.9 85.9 Arg 87.3 91.7 89.6 91.4 85.3 91.0 94.9 Asp + Asn 67.0 82.0 71.1 82.1 70.7 82.8 88.0 Cys 56.5 75.3 – 75.4 42.2 79.9 87.6 Glu + Gln 81.6 87.1 84.3 85.5 76.9 87.1 90.8 Gly 73.1 83.7 76.7 90.0 82.9 74.3 89.1 His 61.4 78.7 66.1 – – 87.5 91.3 Ile 77.7 80.6 81.6 – – 81.9 87.5 Leu 79.4 84.9 82.3 85.1 76.1 85.0 89.1 Lys 76.8 81.6 80.0 – – 83.8 88.9 Met 82.6 83.8 85.1 – – 86.4 90.1 Phe 80.3 84.6 84.7 – – 85.2 89.4 Pro 78.8 88.2 82.6 – – 80.5 104.2 Ser 69.0 82.6 78.1 84.9 72.9 84.7 90.6 Thr 62.0 79.0 75.1 83.5 72.5 78.5 86.2 Tyr 77.2 82.8 82.5 – – 85.1 90.5 Val 72.3 79.6 76.9 – – 79.9 86.0 AA nitrogen 76.7 84.6 80.2 85.7 76.5 83.1 90.0 AA Amino acid, “–” data are not available Hendriks et al. [15]. Adult female dogs with a mean body weight of 25.3 kg, a mean age of 3 years, and a mean food intake of 389.4 g/d (15.4 g food/kg body weight/d) were fed commercial dry canine foods containing 24.3%–32.7% crude protein (dry matter basis). Apparent ileal digestibility = (AA intake – AA in ileal digesta)/AA intake × 100; Apparent total tract (fecal) digestibility = (AA intake – AA in feces)/AA intake × 100; Standardized ileal digestibility = [AA intake – (AA in ileal digesta – basal endogenous AA flow in ileum)]/AA intake × 100 Kim et al. [25]. Adult domestic cats (7 males and 7 females) with a mean body weight of 4.5 kg and a mean age of 3.3 years were fed a dry extruded diet (chicken- based, grain-free commercial feed with 2% refined cellulose and 3% sugar beet pulp) containing 33% CP. Hair of cats was removed from or included in feces for determining the apparent digestibilities of nutrients Pigs (92-kg body weight) were fed a corn- and soybean meal-based diet [26] dogs fed plant (extruded wheat, corn meal, soybean meal, Asn, 52.0; Cys, 54.5; Glu + Gln, 82.2; Gly, 76.2; His, 76.0; and beet pulp)- and animal (fishmeal, poultry meal, meat 4-hydroxyproline, 79.5; Ile, 78.7; Leu, 80.2; Lys, 78.9; meal, and meat & bone meal)-based diets containing 30% Met, 82.0; Phe, 81.9; Pro, 81.5; Ser, 70.5; Thr, 69.7; Tyr, CP, 19% lipids, 29%–34% starch, and 6%–13% fiber [37], 77.4; and Val, 77.8 in the mixed breed Alaskan Husky and for adult dogs fed commercial dry foods (Table 3). [35]. 4-Hydroxyproline is converted into glycine via the Comparisons between the apparent ileal and total tract 4-hydroxyproline oxidase pathway in animal tissues digestibilities of methionine and lysine in dogs indicate [36]. For comparison, the apparent total tract digestibili- that, in contrast to pigs [38], (a) a substantial amount of ties of individual AAs in this canine diet (%) were: Ala, protein metabolites is absorbed by the canine large intes- 87.3; Arg, 90.2; Asp + Asn, 78.3; Cys, 71.0; Glu + Gln, tine and (b) the microbes of the canine large intestine do 88.9; Gly, 88.3; His, 86.9; 4-hydroxyproline, 94.4; Ile, 85.1; not have a net synthesis of methionine and lysine. These Leu, 86.8; Lys, 83.8; Met, 86.1; Phe, 86.4; Pro, 89.9; Ser, findings suggest an important difference in gut microbial 82.3; Thr, 82.6; Tyr, 84.5; and Val, 84.5 in these dogs [35]. metabolism among animal species. Similar results were reported for adult German shepherd Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 5 of 21 Digestibilities of AAs are influenced by the breed Metabolism of AAs by cats and dogs and age of dogs, as well as diet and the method of its The metabolism of most AAs by cats and dogs is similar preparation. For example, at 11, 21, 35, and 60 weeks to that of other mammals [43]. In support of this view, of age, apparent CP digestibilities were greater in the plasma concentrations of most AAs from differ - large breeds than small breeds of dogs [39] and were ent research groups [47–50] are similar between adult increased with age, possibly due to a greater activ- cats and dogs, except for Asn, Asp, Citrulline, Glu, Gly, ity of intestinal microbes. The apparent digestibil- His and Pro (that are lower in dogs than in cats) and for ity of CP was reduced by 5% in 11-week-old puppies lysine (that is higher in dogs than in cats) (Table 4). Inter- compared with 2- to 4-year-old adult dogs, but no estingly, the plasma concentrations of Arg, citrulline, and differences were detected between 0.5- and 2-year- ornithine in both cats and dogs were lower than those old dogs [40]. Likewise, no difference in CP digest- for pigs [51, 52] (Table 4), suggesting differences in the ibility was noted between 2- and 17-year-old beagles whole-body metabolism of AAs among the three animal [41]. Dietary supplementation with β-mannanase species. (the enzyme hydrolyzing polysaccharides made from The qualitative dietary requirements of dogs for most D-mannose) enhanced the apparent digestibilities of AAs are similar to those for omnivores (e.g., humans and CP in dogs fed a diet containing a large amount of pigs) [16, 53]. However, in contrast to most breeds of plant-sourced protein ingredients, but had no effect dogs, cats have a very limited ability to synthesize taurine in dogs fed a diet containing a large amount of ani- and arginine [14]. Because taurine is present in animal mal-sourced protein ingredients [42]. Compared with products but absent from plants [54], cats must be pro- the extruded diet, slight cooking enhanced CP digest- vided with at least a portion of animal-sourced foods or ibility in dogs [30]. In adult dogs, increasing the con- the same essential nutrients from synthetic supplements tent of dietary CP from 18% to 42% did not affect the [43]. This is consistent with the much greater concen - standardized ileal digestibilities of AAs [17], indicat- trations of both taurine and arginine in the milk of cats ing a high ability of these animals in digesting dietary [55–58] as compared with ruminants and pigs (Table 5) protein. Likewise, the inclusion of 7.5% fiber in diets [59–61]. Thus, there are peculiar differences in the did not adversely affect the apparent ileal digestibili- requirements of certain AAs, such as taurine (essential ties of CP and AAs in adult dogs [30], further sup- for tissue integrity) [62] and arginine (essential for main- porting the notion that these animals can adapt well taining the urea cycle in an active state) [63] between cats to an appropriate proportion of plant-sourced ingre- and dogs. Likewise, the concentrations of many AAs in dients in their diets [43]. plasma differ between cats and dogs offered diets high Hair, which is lost from the skin, should be removed in carbohydrate, high in fat, or high in protein [64]. Cats from feces for accurately measuring nutrient digestibil- and dogs chose a different mix of food, which is consist - ity in cats [25]. Harper and Turner [44] reported that ent with cats needing a higher protein concentration in 19-week-old cats had higher apparent CP digestibilities food than dogs [64]. In these two animal species, the syn- than younger kittens. In adult cats, the apparent digest- thesis of glucose from AAs in the liver and kidneys plays ibilities of CP were 91%–94%, 87%–88%, or 89%–90%, an important role in maintaining glucose homeostasis respectively, in a meat (a mixture of beef and mutton)- [43]. When diets do not provide sufficient starch, glyco - based diet containing 22% cornstarch, 14.3% corn, or gen or glucose, dogs must synthesize glucose from glu- 14.3% wheat grains, and were not affected by fine grind - cogenic AAs in their liver and kidneys [65]. In contrast to ing or cooking [31]. Likewise, the apparent digestibili- modern breeds of dogs that consume both animal- and ties of CP by healthy adult cats did not differ between plant-sourced foods, a natural food (i.e., meat) for cats two diets containing 36%–55% high-quality proteins contains only a small amount of glycogen (primarily from (a high proportion of meat) and 37%–56% low-quality muscle and liver; < 5%, DM basis) and no starch [14]. proteins (a low proportion of meat) (i.e., 89%–90% ver- u Th s, in cats, gluconeogenesis from AAs plays an essen - sus 88%–91%, respectively) [45], but most likely did tial role in the provision of glucose to the brain, red blood not reflect the true digestibilities of the proteins due cells, and immunocytes, and therefore their survival [12]. to microbial AA metabolism in the large intestine. However, the apparent digestibility of dietary CP was Endogenous nitrogen excretion decreased in cats fed raw corn starch and raw potato In adult dogs (BW ranging from 2.8 to 51 kg) fed a pro- starch compared with cooked foods [46]. These results tein-free, semi-purified diet, the outputs of endogenous indicate effects of age and dietary composition on the urinary nitrogen, metabolic fecal nitrogen [nitrogen digestion of dietary protein and microbial AA metabo- originating from the sloughed gastrointestinal epithelium lism in the gut. and bacteria, as well as other endogenous sources (e.g., Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 6 of 21 Table 4 Concentrations of amino acids (AAs) in the plasma of dogs, cats, and pigs AAs Dogs Cats Pigs < 2 wk 2 wk to 2 12 to 60 > 60 mo Young Adults 4 d 2.5 mo 18 mo c e f g of age mo of age mo of age of age kittens (n = 120)of ageof ageof age (n = 7) (n = 7) (n = 7) (n = 8) (n = 8) (n = 10) (n = 8) (n = 8) Ala 315 ± 46 296 ± 180 400 ± 129 332 ± 162 603 ± 62 462 ± 160 1049 ± 478 596 ± 91 352 ± 58 Arg 278 ± 80 245 ± 101 103 ± 29 104 ± 36 – 95 ± 38 163 ± 63 159 ± 45 165 ± 19 Asn 121 ± 43 121 ± 48 52 ± 16 47 ± 10 – 91 ± 25 118 ± 28 62 ± 8 66 ± 10 Asp 19 ± 9 11 ± 3 trace trace – 28 ± 12 25 ± 9 13 ± 6 12 ± 5 Cit 88 ± 39 107 ± 45 34 ± 10 38 ± 12 – 18 ± 6 89 ± 25 64 ± 6 60 ± 8 h g Cys 42 ± 38 56 ± 36 32 ± 22 42 ± 26 – 26 ± 9 165 ± 33 173 ± 12 170 ± 23 Gln 564 ± 174 461 ± 139 593 ± 175 658 ± 213 – 664 ± 134 469 ± 120 513 ± 79 491 ± 59 Glu 112 ± 82 73 ± 32 35 ± 8 38 ± 12 – 73 ± 38 156 ± 63 172 ± 40 92 ± 15 Gly 283 ± 85 364 ± 107 181 ± 61 171 ± 42 – 398 ± 279 824 ± 237 664 ± 62 688 ± 81 His 140 ± 45 85 ± 28 66 ± 13 65 ± 15 – 116 ± 24 114 ± 47 103 ± 20 97 ± 16 Ile 83 ± 24 70 ± 23 67 ± 21 63 ± 18 51 ± 8 63 ± 29 159 ± 57 112 ± 28 104 ± 15 Leu 212 ± 39 147 ± 69 142 ± 34 135 ± 48 91 ± 14 146 ± 49 186 ± 51 246 ± 42 196 ± 18 Lys 275 ± 68 195 ± 121 149 ± 46 191 ± 55 – 108 ± 61 223 ± 82 88 ± 48 103 ± 35 Met 52 ± 33 73 ± 20 46 ± 15 52 ± 16 116 ± 51 64 ± 28 100 ± 51 35 ± 6 47 ± 6 Orn 85 ± 31 43 ± 16 19 ± 9 17 ± 7 – 21 ± 12 107 ± 38 85 ± 17 82 ± 20 Phe 60 ± 13 64 ± 25 61 ± 14 63 ± 18 72 ± 11 70 ± 15 117 ± 35 87 ± 11 89 ± 14 Pro 389 ± 106 290 ± 105 145 ± 61 114 ± 15 – 258 ± 76 628 ± 373 395 ± 59 382 ± 50 Ser 241 ± 40 262 ± 91 123 ± 28 138 ± 31 – 179 ± 85 267 ± 123 153 ± 45 157 ± 38 Taurine – – – 77 ± 24 – 118 ± 55 169 ± 57 56 ± 17 60 ± 21 Thr 523 ± 245 250 ± 151 196 ± 63 157 ± 50 168 ± 51 173 ± 54 369 ± 145 89 ± 45 93 ± 32 Trp 66 ± 27 55 ± 15 69 ± 27 54 ± 17 – 60 ± 17 38 ± 19 47 ± 8 45 ± 9 Tyr 88 ± 31 54 ± 24 48 ± 7 43 ± 8 36 ± 8 57 ± 15 242 ± 73 105 ± 17 112 ± 24 Val 248 ± 42 200 ± 83 199 ± 43 199 ± 63 131 ± 34 164 ± 62 350 ± 114 280 ± 37 216 ± 31 Cit Citrulline, mo months, Orn Ornithine, Tau Taurine, wk weeks, “–” data were not available Values are means ± SD Blazer-Yost and Jezyk [47] for all amino acids except taurine Delaney et al. [48]. Adult cats (n = 131) had a median age of 5.3 years (ranging from 2 to 14 years) Hargrove et al. [49]. Kittens (1 to 2 kg body weight) were fed a casein-, soy protein-, and AA mix-based diet Heinze et al. [50] Flynn and Wu [51] Wu et al. [52] Wu G (unpublished work). Adult gilts were fed, twice daily at 1% of body weight per meal, a corn- and soybean meal-based diet containing 12.2% CP [53]. Blood samples were obtained from the jugular vein at 2 h after feeding to prepare plasma for AA analysis [52] cysteine + ½ cystine saliva, mucus, bile, and pancreatic and intestinal secre- The endogenous excretions of total, urea, ammonia, tions)], and total endogenous nitrogen were 210, 63, and and creatinine (a metabolite of creatine) nitrogen for cats 0.75 273 mg/kg BW /d, respectively [66]. This is equivalent fed the protein-free diet were 360, 243, 27.6, and 14.4 mg/ 0.75 0.75 to the catabolism of 1.71 g protein/kg B W /d. There kg BW /d, respectively [67]. Similarly, Earle [68] was no significant effect of either sex or BW on the meas - reported that adult cats could maintain nitrogen balance ured variables expressed per metabolic BW, but endog- or had minimal endogenous nitrogen loss at 1.4–1.7 g enous urinary nitrogen output was positively correlated protein/kg BW/d. These values are greater than those for with BW loss during the 14-d feeding period [66]. The dogs and pigs (Table 2). For comparison, the rates of uri- 0.75 maintenance requirement of adult dogs for CP is 82 g/kg nary nitrogen excretion (mg nitrogen/kg BW /d) when of the diet containing 4.0 kcal ME/g diet, and the NRC- fed a nitrogen-free diet were: human, 62; marmoset, 110; recommended allowance for CP is 100 g/kg of the diet rat, 128; pig, 163; dog, 210; and cat, 360 [16]. Accord- containing 4.0 kcal ME/g (DM basis) [16]. ingly, adult cats require 2 to 3 times more dietary protein Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 7 of 21 Table 5 Concentrations of total amino acids (free plus peptide-bound) in the mature milk of cats, dogs, cows, goats, and pigs a b b c f Nutrient Dogs Cats Cows Goats Pigs (n = 16) (n = 4) (n = 4) (n = 30) (n = 10) Water, g/kg milk 773 790 877 870 799 Dry matter, g/kg milk 227 210 123 130 201 Crude protein, g/kg milk 75 75 33 35 48 Total amino acids, g/L milk Ala 2.50 2.80 ± 0.5 1.08 ± 0.03 1.18 1.97 ± 0.06 Arg 2.93 4.85 ± 0.5 1.14 ± 0.03 1.36 1.43 ± 0.08 Asp + Asn 9.53 6.51 ± 2.0 2.35 ± 0.17 2.51 5.12 ± 0.12 Cys 2.48 0.91 ± 0.5 0.30 ± 0.03 0.31 0.72 ± 0.05 Glu + Gln 8.90 15.8 ± 0.5 6.99 ± 0.07 6.95 9.44 ± 0.37 Gly 1.56 0.76 ± 0.5 0.61 ± 0.03 0.56 1.12 ± 0.07 His 1.35 2.04 ± 0.5 0.81 ± 0.03 1.23 0.92 ± 0.05 4-Hydroxyproline – – 0.48 ± 0.04 – 0.82 ± 0.04 Ile 1.97 3.26 ± 0.5 1.58 ± 0.03 1.61 2.28 ± 0.10 Leu 5.48 8.93 ± 0.5 3.33 ± 0.03 3.41 4.46 ± 0.17 Lys 3.17 4.32 ± 0.5 2.89 ± 0.07 3.43 4.08 ± 0.15 Met 1.41 2.42 ± 0.5 0.87 ± 0.03 0.78 1.04 ± 0.04 Phe 3.53 2.27 ± 0.5 1.68 ± 0.03 1.76 2.03 ± 0.11 Pro 3.62 7.12 ± 1.0 3.36 ± 0.13 3.11 5.59 ± 0.26 Ser 3.26 3.33 ± 0.5 1.88 ± 0.03 1.53 2.35 ± 0.11 Thr 3.44 3.48 ± 0.5 1.41 ± 0.03 1.39 2.29 ± 0.14 Trp 0.26 – 0.43 ± 0.02 – 0.66 ± 0.02 Tyr 3.57 3.41 ± 0.5 1.58 ± 0.03 1.63 1.94 ± 0.05 Val 3.80 3.56 ± 0.5 1.75 ± 0.03 2.10 2.54 ± 0.09 d d f e f Taurine 0.33 ± 0.14 0.36 ± 0.04 0.007 ± 0.001 0.098 0.19 ± 0.013 Adapted from Rezaei et al. [55] for the content of water, dry matter, and crude protein in milk. Values for total amino acids were calculated on the basis of their intact molecular weights, and are expressed as either means or means ± SEM when data are available Ferrando et al. [56] for proteinogenic amino acids Davis et al. [57] for proteinogenic amino acids Ceballos et al. [58] for proteinogenic amino acids Rassin et al. [59] Prosser [60] Wu [61] than adult dogs and herbivores (e.g., cows, sheep, and of AAs in a tissue-specific manner [61]. This must be horses) [14]. The maintenance requirement and the rec - taken into consideration when determining AA require- ommended allowance of dietary CP by the adult cat are ments of cats. 160 and 200 g/kg diet containing 4.0 kcal ME/g diet (DM basis), respectively [16]. This is equivalent to the mini - Metabolism of arginine mum maintenance requirement of adult cats for dietary There have been many studies of arginine nutrition in protein energy (16% of dietary ME). For comparison, dogs since the pioneering work of Rose and Rice in 1939 a dietary intake of protein energy accounting for 3.5%– [72]. Dogs can synthesize arginine from dietary glu- 4.5% of dietary ME is sufficient to maintain BW, nitrogen tamine/glutamate and possibly dietary proline, as well balance, and carcass nitrogen content in adult rats [69, as arterial glutamine via the intestinal-renal axis [63]. In 70]. The obligatory loss of nitrogen in cats appears to be adult dogs [73, 74], as in many other adult mammals (e.g., similar when they are fed a nitrogen-free diet or are food humans, pigs, rats, and sheep) [61], the small intestine deprived [16]. Interestingly, nitrogen balance in adult cats synthesizes and releases citrulline, which is taken up by fed a low-protein diet may be maintained when lean body extraintestinal tissues (primarily the kidneys) for arginine mass is reduced [71], possible due to reduced oxidation synthesis. The small intestine and other organs of dogs Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 8 of 21 express arginase for the hydrolysis of arginine to urea and not ornithine, can restore growth in young cats fed an ornithine [75]. arginine-free diet [80]. Such results can be explained by In adult dogs, the activity of arginase (expressed on the the findings that dietary or arterial blood ornithine is not basis of tissue protein) is similar between the duodenum used for the intestinal synthesis of citrulline in cats [63], and jejunum, with values for the ileum being 24%–37% as reported for other mammals including dogs [73] and of those for the upper parts of the small intestine [75]. pigs [53]. This is due to both the preferential metabolism Unlike pigs [61], the small intestine of postabsorptive of dietary ornithine into proline by enterocytes and the dogs does not release arginine [73], likely due to either lack of uptake of arterial blood ornithine by the gut [63]. low activities of argininosuccinate synthase and lyase for We suggest that higher protein requirements by cats than arginine synthesis or the further hydrolysis of arginine by dogs may result, in part, from a much lower ability to arginase in enterocytes. Thus, the homeostasis of argi - synthesize arginine in cats. The sensitivity of mammals to nine in the body depends on the rates of its endogenous dietary arginine deficiency is cats > dogs > rats [14]. synthesis and catabolism. Growing and adult dogs cannot synthesize sufficient arginine to meet functional needs Metabolism of aspartate, glutamate, and glutamine (e.g., ammonia detoxification via the urea cycle) beyond In canine and feline nutrition, aspartate, glutamate, and maintaining nitrogen balance [43]. Thus, a dietary level glutamine are among the traditionally classified nutri - of 0.4% and 0.28% arginine is needed for the maximum tionally nonessential AAs (NEAAs), but this has been growth of young dogs and the hepatic ureagenesis in disputed [81]. Mammals, including dogs, use dietary adult dogs, respectively, if other AAs are sufficient [43]. aspartate, glutamate, and glutamine as well as arterial This indicates that both mature and immature dogs have glutamine as the major metabolic fuels in their small an inadequate or limited ability to synthesize arginine de intestine [82], but cannot adequately synthesize aspar- novo. Syndromes of arginine deficiency in dogs include tate, glutamate, and glutamine [75]. Both aspartate and decreased food intake, hyperammonemia, severe emesis, glutamate are essential for intestinal metabolism, but are frothing at the mouth, and muscle tremors, and can be not taken up by the small intestine from the arterial prevented by dietary supplementation with arginine or blood [61]. Thus, these two AAs are required in cat and citrulline [16]. Dietary or arterial blood ornithine is not dog diets. used for arginine synthesis and cannot correct arginine There is a large database on the metabolism of the deficiency symptoms in dogs [43]. Interestingly, dog’s glutamine family of AAs in the small intestine [75] and milk contains much more arginine than the milk of herbi- kidneys [83] of dogs. Dietary aspartate, glutamate, and vores (e.g., cows) and omnivores (e.g., humans and pigs) glutamine are extensively degraded in the mucosa of the [76] to ensure that canine neonates receive adequate argi- canine small intestine as metabolic fuels [82]. Glutamine, nine for survival and growth. When fed a milk-replacer but not aspartate and glutamate, in the arterial blood, is diet containing inadequate arginine, dog puppies develop taken up by the canine small intestine for metabolism cataract [77]. Dietary arginine deficiency also occurs in [73]. The small intestine accounts for ~ 30% of the arte- human infants (causing hyperammonemia and death) rial blood glutamine utilized by healthy adult dogs, and and adults [reducing nitric oxide (NO) synthesis, sperm release ammonia, alanine, proline, and citrulline but little production, and fetal growth], and in rats (impairing or no ornithine and arginine [84–86]. This involves the growth and spermatogenesis) [61]. conversion of glutamine into alanine, proline and citrul- Cats have a very limited ability to synthesize citrulline line via a series of enzymes including glutaminase, glu- and arginine de novo because of the low activities of pyr- tamate transaminases, P5C synthase, and P5C reductase roline-5-carboxylate (P5C) synthase and ornithine ami- [61]. In addition, glutamine is the major source of gluta- notransferase [78]. The latter also limits the formation mate for gluconeogenesis and ammoniagenesis for the of citrulline from proline via the proline oxidase path- regulation of acid-base balance in the canine kidneys way. There is evidence for the synthesis of arginine from [87]. The utilization of arterial glutamine by the small citrulline and the catabolism of arginine via arginase in intestine of dogs is increased by ~ 80% during treadmill feline renal tubules [79]. In cats, when dietary intake of exercise due to elevated concentrations of glucagon, lead- arginine is insufficient, food ingestion is reduced, fol - ing to a 17% decrease in plasma glutamine concentration lowed by hyperammonemia (occurring within 1–3 h after [88]. Likewise, an intraluminal infusion of glucose, which feeding) due to impaired ureagenesis in the liver, vom- stimulate the release of glucagon from the pancreas, can iting, neurological signs, severe emesis, ataxia, tetanic enhance the uptake of arterial blood glutamine and the spasms, and death [80]. Dietary supplementation with release of ammonia, alanine, glutamate, and citrulline citrulline or ornithine to cats can prevent hyperammone- by the small intestine of dogs [89]. In response to meta- mia due to arginine deficiency. However, citrulline, but bolic acidosis, the uptake of glutamine by the canine Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 9 of 21 kidneys is markedly increased to meet the demand for At present, little is known about BCAA metabolism renal ammoniagenesis. Interestingly, in contrast to rats in the small intestine and other tissues of cats. However, [82], the extraction of glutamine by the small intestine of increasing the dietary protein content from 15% to 65% dogs is increased during progressive fasting (up to 4 d) of dietary ME increases the whole-body oxidation of leu- via unknown biochemical mechanisms [90], indicating cine and urea production in adult cats by about 3 times, 13 15 another species difference between dogs and omnivo - as measured with [1- C]leucine and [ N ]urea [95]. This rous mammals in the regulation of intestinal glutamine finding indicates that cats can adapt well to dietary AA metabolism in response to food deprivation. intake through modulating AA oxidation. In contrast Like dogs, the small intestine of cats takes up arte- to dogs, cats, like other Felidae species, use leucine and rial blood glutamine and releases ammonia [82]. In the valine to synthesize isovalthine and isobuteine, respec- fasted state, the feline small intestine extracts ~ 20% of tively, with hitherto unknown physiological function glutamine from the arterial blood [86]. Little is known [96, 97]. We are not aware of studies to compare tissue- about the metabolism of other AAs in the small intestine specific or whole-body BCAA metabolism among animal of cats. Besides published data on arginine synthesis from species (including cats). citrulline and the catabolism of arginine via arginase in renal tubules [79], there are no reports on the metabo- Metabolism of methionine, cysteine, and taurine lism of other AAs in the feline kidneys. We are not aware In the liver of dogs, methionine is catabolized to cysteine of studies to compare the tissue-specific- or whole-body and then to taurine, but neither taurine nor cysteine is metabolism of aspartate, glutamate, and glutamine converted to methionine [14]. The rate of oxidation of among animal species (including cats). cysteine to taurine depends on the dietary intakes of sul- fur-AAs. Methionine is often the first or the second (after Metabolism of branched-chain AAs (BCAAs) lysine) most limiting AA in plant-based diets for dogs. Dietary BCAAs (~ 30%) are extracted by the small Dietary cysteine can replace up to 50% dietary methio- intestine of fed dogs in first-pass metabolism, with 55% nine in these animals [43]. and 45% of the utilized leucine entering the transami- Most breeds of dogs can synthesize sufficient tau - nation and protein synthesis pathways, respectively rine when fed a methionine- and cysteine-adequate [91]. As reported for rats, the liver of dogs does not diet [43]. However, an inadequate intake of methionine degrade BCAAs [92] due to the near absence of BCAA and cysteine in diets [e.g., plant (e.g., peas, lentils, and transaminase in hepatocytes. Rates of BCAA uptake by rice)-based foods with no or insufficient taurine] may extrahepatic tissues determine the availability of these contribute to the development of dilated cardiomyo- nutrients for metabolic utilization. In fasted dogs (20 kg pathy characterized by thin heart muscle and enlarged BW; a food deprivation period of ~ 36 h), skeletal mus- chambers in some breeds of dogs [98, 99]. For example, cle takes up leucine from the arterial blood at the rate of a small proportion (1.3%–2.5%) of Newfoundland dogs 0.89 μmol/kg BW/min [93]. Based on the uptake of total fed commercially available diets that were considered to BCAAs (0.143 μmol/kg BW/min) by the skeletal muscle be complete and balanced in nutrition have a deficiency of 12 h-fasted pigs (20–25 kg BW) [94] and the ratio of of taurine [100] due to reduced taurine synthesis pos- leucine:isoleucine:valine (0.32:0.23:0.45) in the plasma sibly as a result of gene mutations [101]. When fed pro- of 12-h fasted pigs [75], it can be estimated that the skel- tein-restricted diets, certain breeds of dogs (e.g., golden etal muscle of pigs fasted for ~ 12 h takes up leucine from retrievers) are more prone to taurine deficiency and the the arterial blood at the rate of 0.046 μmol/kg BW/min. development of dilated cardiomyopathy even when fed It remains to be determined whether the reported large meat-based diets due to a combination of factors, includ- discrepancy in leucine uptake by skeletal muscle between ing complex interactions among dietary, metabolic, and dogs and pigs results from differences in animal species, genetic factors [102]. This disorder may result from low age, nutritional state, and research methodology. In dogs activities of cysteine dioxygenase and cysteine sulfinate and other mammals, most of the diet-derived BCAAs decarboxylase, as well as a limited availability of cysteine. bypass the liver and are used by extra-hepatic tissues An ability of some breeds of dogs to form taurine does (mainly skeletal muscle) for the synthesis of alanine and not necessarily mean that they do not require dietary glutamine in the presence of α-ketoglutarate and ammo- taurine for optimum health. Only in the breeds of dogs nia [92, 93]. The ammonia is derived from the blood as that possess sufficient enzymes for taurine synthesis well as the intramuscular catabolism of purines and AAs can the adequate provision of methionine plus cysteine [61]. In the post-absorptive state, alanine and glutamine in their diets prevent metabolic diseases such as dilated account for about 50% of the AAs released from the skel- cardiomyopathy. Although concentrations of taurine in etal muscle of dogs, pigs, rats, and humans [61]. plasma and skeletal muscle reflect its availability in these Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 10 of 21 animals, those in the whole blood may not be a sensitive no production of ammonia by the gut [107]. Less than indictor of taurine depletion caused by a low intake of 3% of glycine absorbed by the canine jejunum intralu- bioavailable sulfur AAs in dogs, especially in large dogs minally infused with 10 mmol/L glycine is released as [99]. serine [107], suggesting either a low activity of serine Methionine is generally the most limiting AA for cats hydroxymethyltransferase or an insufficient availability 5 10 fed a meat-based conventional diet [16]. The feline liver of N ,N -methylenetetrahydrofolate as a methyl group can convert methionine into cysteine. However, young donor in the intestinal tissues. In postabsorptive dogs, and adult cats have a limited ability to synthesize taurine there is no release of glycine or serine by the small intes- from cysteine due to low activities of cysteine dioxyge- tine [73], indicating the lack of their synthesis under this nase and cysteinesulfinic acid decarboxylase; therefore, nutritional condition. they have a requirement for dietary taurine [43]. In these Aromatic AAs are utilized via multiple metabolic path- animals, taurine deficiency results in dilated cardiomyo - ways in animals, including dogs [108–110]. Interestingly, pathy, heart failure, central retinal degeneration, blind- dogs require at least twice as much phenylalanine plus ness, deafness, and poor reproduction [103]. Thus, all tyrosine for maximal black hair color (adequate eumela- foods for cats must include sufficient taurine. nin in hair) as for growth [108, 110]. Furthermore, sup- Cats synthesize felinine, isovalthine, and isobuteine plementation with tryptophan (0.145% of diet, the from cysteine plus acetyl-CoA, cysteine plus isovaleryl- precursor of serotonin) can reduce territorial aggression CoA (a metabolite of leucine), and cysteine plus in dogs fed a low (19%)-CP diet (with the basal trypto- isobutyryl-CoA (a metabolite of valine) as unique sulfur- phan content of 0.18%) but has no effect in dogs fed a containing AAs [96, 97]. In intact adult male cats, the high (31%)-CP diet (with the basal tryptophan content of rates of urinary excretion of felinine and isovalthine are 0.24%) [111]. In addition, Pereira [112] found that dietary 122 and 1.8 μmol/kg BW/d, respectively [96]. At present, supplementation with tryptophan (12.5 mg/kg BW/d) there are no quantitative data on the urinary excretion of reduced bark and stare behavior in multi-housed dogs. isobuteine by cats. The direct source of cysteine for these At present, we are not aware of studies regarding proline synthetic pathways is glutathione. Felinine, isovalthine, catabolism in the intestine and other tissues of dogs. and isobuteine may serve as pheromones in cats for the Little is known about the catabolism of glycine, pro- purpose of territorial marking, intra-species commu- line, serine, threonine, phenylalanine, tyrosine, or tryp- nications, and chemical signals to attract females [104]. tophan in the feline intestine. However, these AAs are Indeed, males produce 239% more felinine than females degraded in the liver of cats [16]. As for dogs, the mainte- (122 vs. 36 μmol/kg BW/d) [105]. In addition, it is pos- nance of adequate eumelanin in the hair of cats requires sible that the production of unique cysteine metabolites twice as much phenylalanine plus tyrosine as for whole- (non-toxic, non-reactive, and relatively stable) helps body growth [109]. As a precursor of serotonin (a neu- to prevent excessive formation of toxic and acidic sub- rotransmitter and antioxidant), tryptophan along with stances (e.g., H S, SO , and H SO ) in cats [61]. α-casozepine (a bioactive peptide originating from the 2 2 2 4 Cysteine is the most abundant AA (accounting for S1 casein protein in cow’s milk) relieved anxiety while ~ 16% of total protein) in the hair of cats and dogs [43]. alleviating stress and aggression in cats [113]. Dietary During the growth period, these animals have greater supplementation with tryptophan (12.5 mg/kg BW/d) dietary requirements for cysteine than other mammals reduced vocalization, agonistic behavior, exploring, with fewer hair in the skin (e.g., pigs and humans) [61]. scratching, and agonistic interactions in multi-housed Although sulfur AA restriction has been reported to cats [114]. Interestingly, unlike omnivores, hepatic tryp- improve health (e.g., delayed aging and longer lifespans) tophan 2,3-dioxygenase [the enzyme oxidizing trypto- in adult rodents by altering the intestinal microbiome phan to N-formylkynurenine (the immediate precursor profile [106], we are not aware of such studies with cats of kynurenine)] is not induced by glucocorticoids in cats or dogs. [14]. This indicates another species difference in the regulation of AA metabolism between cats and other Metabolism of other AAs mammals. The mucosa of the canine intestine does not degrade threonine (the major AA in mucins), phenylalanine, AA imbalances and antagonisms tyrosine, or tryptophan, and its ability to catabolize or AA imbalances (improper ratios of AAs) occur in dogs interconvert glycine and serine is very limited [85, 107], fed commercial plant-based diets containing either a but these AAs are degraded in the canine liver [108]. small amount of or no animal-sourced ingredients (e.g., u Th s, when the canine small intestine is intraluminally meat) [98, 102]. This is largely because most plant pro - infused with each of these AAs (10 mmol/L), there is teins (particularly those in cereals) are deficient in some Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 11 of 21 EAAs (particularly lysine, tryptophan, threonine, methio- of lysine from 1.1% to 3.6%, 6.1%, and 8.6% in a diet con- nine, and cysteine), as well as glycine and proline (the first taining 1.3% arginine and 4.09 kcal ME/g diet does not and second most abundant AAs in the animal body) [54]. result in lysine-arginine antagonism in adult cats [119]. For AAs with similar chemical structures (e.g., BCAAs) Further increasing the dietary content of lysine to 11.1% or net electric charges (e.g., arginine and lysine), their and 13.1% gradually reduced the food intake of adult cats improper ratios often result in antagonisms because they by 30%–35% without affecting health [119]. Such a severe share the same transporters in the cell membrane for imbalance between lysine and arginine would be detri- absorption and uptake, the same enzymes for catabolism, mental for omnivores and herbivores [61]. and/or the same inhibitors of their respective pathways [61]. Effects of dietary AA imbalances in dogs are more Metabolic adaptation to low or high protein intakes similar to those in rats than in cats [14]. For example, in Most omnivores (e.g., rats and pigs) and herbivores (e.g., response to diets deficient in one or two EAAs, the food sheep and cattle) can adapt to: (1) low-protein diets by intake of dogs is not decreased as rapidly and severely increasing food intake and reducing AA catabolism, and as that for cats. Inclusion of adequate animal-sourced (2) high-protein diets by initially reducing food intake foodstuffs or dietary supplementation with deficient AAs for 1–5 d (depending on dietary protein levels), followed may aid in preventing and correcting AA imbalances and by up-regulating the expression and/or activities of AA- antagonisms in canine nutrition. catabolic enzymes [61]. For example, rats can down- or Based on changes in hepatic activities of AA-cata- up-regulate AA-catabolic and urea-cycle enzymes in bolic and urea-cycle enzymes as well as blood and urine response to a low or high intake of AAs [14]. In contrast, nitrogenous metabolites, cats are less sensitive to imbal- low protein intake did not affect whole-body protein deg - ances of most AAs (except for arginine, methionine, and radation in the postabsorptive state, and increasing die- lysine) in diets than dogs, rats, chicks, and pigs [14]. tary protein intake from 32 g CP/Mcal ME (low intake) This is because, compared with many omnivores, cats to 63 g CP/Mcal ME (medium intake) and to 148 g CP/ have a lower ability to sense protein-free or protein- Mcal ME (high intake) did not affect the rate of leucine deficient (e.g., 6% protein) diets and the diets containing oxidation in adult dogs under these nutritional condi- excess protein (e.g., 63% protein) or most AAs, even if tions [120]. Thus, dogs may not be as efficient as rats they exhibit protein malnutrition, lose weight, and grow in metabolic adaptation to low or high protein intakes. poorly [115, 116]. However, cats select for or against Experimental evidence shows that dogs can tolerate at some proteins (e.g., casein and soy protein) [115] and least 30%–32% dietary protein [111, 121]. the solution of some AAs [116], possibly based on their Adult cats have a 100% greater requirement for dietary chemical properties including taste. For example, in con- protein than do adult dogs, but dietary requirements for trast to omnivorous mammals (e.g., rats and pigs), cats some EAAs do not appear to differ appreciably between do not select a diet containing adequate (0.6%) threonine these two animal species [16]. This may be explained by versus a threonine-free diet [116]. In addition, only a higher requirements of cats for NEAAs to fulfill meta - mild AA antagonism is exhibited by cats when fed a diet bolic needs than do dogs [61]. First, dietary NEAAs (par- with isoleucine or valine as the limiting AA in the basal ticularly aspartate, glutamate, and glutamine) may be the diet, and these animals do not avoid a diet containing a major energy sources for the feline small intestine [61]. highly excessive amount of leucine (e.g., 10% of leucine in u Th s, based on the dietary AA intake, cats oxidize more diet) [117]. Thus, cats are less sensitive than omnivores to NEAAs than EAAs [122]. Second, there are metabolic leucine-isoleucine and valine antagonisms. Furthermore, demands for the increased use of BCAAs to synthesize a dietary deficiency of arginine greatly reduces food aspartate, glutamate, and glutamine in extra-intestinal intake by cats [80], likely because of neurological disor- tissues (including skeletal muscle, white adipose tissue, ders induced by hyperammonemia and reduced nitric- and heart) in cats as in other carnivorous mammals [61]. oxide availability [61]. Interestingly, Rogers et al. [116] Aspartate, glutamate, and glutamine in plasma may be reported that when adult cats were offered diets contain - the primary nutrients for ATP production in the feline ing either 0 or 0.2% methionine, they initially ate only the liver, skeletal muscle, and kidneys, as reported for car- methionine-containing diet, but beginning on d 3, they nivorous fish [123]. Third, most NEAAs are used as glu - consumed an increasing amount of the methionine-free cogenic substrates in the feline liver and kidneys because diet, and on d 6 selected the same amount of each diet. the natural diet (i.e., meat) of cats provides only a small It is unknown why cats select for methionine, but this amount of glucose [124]. When cats are fed diets (e.g., may be because of its bitter taste, just like leucine. Finally, some commercial foods) containing sufficient digest - although cats select for 0.5 to 50 mmol/L lysine (prepared ible carbohydrates, there is less gluconeogenesis from in saline) over saline [118], increasing the dietary content dietary AAs when compared with diets with insufficient Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 12 of 21 digestible carbohydrates. Finally, when the content of one to reduced muscle protein synthesis, increased prote- of 10 EAAs (Arg, Lys, His, Ile, Leu, Met, Phe, Thr, Trp, olysis, and augmented lipogenesis [61] while develop- and Val; provided at 1.8 to 3.3 times NRC requirements ing sarcopenia (defined as a loss of ≥3% lean body mass [16]) is decreased to one-half that present in the basal over 3 years) [128]. This can be illustrated by studies with diet, there is no decrease in the weight gain of young cats, Labrador retrievers (males plus females) that were fed a but the presence of one or more NEAAs is required for growth diet (containing 27.5% CP and 15.0 kJ ME/g diet) their maximal growth [14]. between 8 weeks and 3.25 years of age) and an adult diet Cats respond to nitrogen-free intake by decreasing (containing 21.2% CP and 14.8 kJ ME/g diet) between whole-body AA oxidation, leading to reduced excretion 3.25 and 14 years of age; those dogs lost 21% lean body of urinary total nitrogen, urea, and ammonia [22]. Thus, mass between 8 and 13 years of age and had a median life increasing the dietary content of protein from 0 to 4%, span of 11.2 years [129]. Because a 15% reduction in lean 7%, 10%, or 13% augmented the excretion of the nitrog- body mass of animals (including dogs) impairs organic enous metabolites in a dose-dependent manner [67]. and physiological functions, and a > 30% reduction may These animals can down-regulate AA catabolism and be fatal [130], it is imperative to mitigate sarcopenia in reduce urinary nitrogen excretion when protein intake is aging dogs. To alleviate insulin resistance with aging, pro- below their requirement (20% CP). Interestingly, regard- tein requirements should be increased by about 50% in ing AA metabolism, cats differ from omnivores (e.g., rats, older dogs compared with young adults [130, 131]. Pro- pigs, and chickens) and herbivores (e.g., sheep) that often tein restriction for healthy older dogs can be detrimen- exhibit 3- to 4-fold greater activities of hepatic AA-cat- tal to their health (particularly regarding the mass and abolic enzymes in response to a high-protein diet [125]. function of skeletal muscles and bones) and, therefore, For example, the activities of aminotransferases and urea- should be avoided in feeding practice. To meet the die- cycle enzymes in the liver do not differ in adult cats fed a tary requirements of dogs for high-quality protein, ani- high (70%)-CP diet and a low (17.5%)-CP diet [125]. It is mal-sourced foodstuffs [which provide proper ratios and likely that (1) tissues of cats express high basal levels of adequate amounts of proteinogenic AAs as well as func- enzymes for AA catabolism and urea-cycle enzymes; (2) tional nutrients (e.g., taurine, 4-hydroxyproline, creatine, when fed a low-protein diet, cats are unable to downreg- and carnosine)] can be useful ingredients for canine diets ulate these enzymes, but actual metabolic fluxes through [54]. In support of this view, increasing dietary CP intake the enzymes in vivo are decreased under conditions of from 16% to 32% enhanced whole-body protein synthesis such a nutritional state (e.g., reduced concentrations of in both young adult (2 years of age) and aging (8 years of AAs and cofactors in cells) to meet a need for conserv- age) dogs [132]. ing AAs; and (3) the activities of these enzymes are suf- Like dogs, cats lose lean body mass with age. For exam- ficient to degrade excessive AAs and remove excessive ple, aging cats lose 34% lean body mass over an 8-year ammonia as urea in healthy cats fed high-protein diets. period between 7 and 15 years of age [128]. As for older In support of this view, Russell et al. [126] reported that dogs, older cats need adequate high-quality protein (i.e., compared with adult cats fed a moderate-CP (35% of 40% animal protein in diet; DM basis) to alleviate aging- dietary ME) diet, feeding a high-CP (52% of ME) diet for associated reductions in the mass and function of skel- 50 d increased protein oxidation by 47% but decreased etal muscles and bones [71]. Such diets should also help fat oxidation by 37%. Likewise, increasing the dietary CP to improve the anti-oxidative and immune functions of level from 9.1% to 59.6% increased protein oxidation by senior cats. Consistent with this notion, adult cats (neu- 507% in adult cats (Table 6). Thus, cats are metabolically tered males) needed 1.5 g protein/kg BW/d (i.e., 2.1 g/kg 0.75 more capable of adapting to high protein intake than pre- BW /d) to maintain nitrogen balance but required 5.2 g 0.75 viously realized based on enzyme activity data [125]. Cats protein/kg BW/d (i.e., 7.8 g/kg BW /d) to maintain lean can tolerate at least 60% dietary protein [122]. In animals body mass [71]. This value is equivalent to 40% protein (including cats), AA oxidation increases to remove excess in the diet and greatly exceeds current NRC [16] recom- AAs [61]. mendations for dietary protein requirement of adult cats. Protein-restricted diets for healthy adult cats must be Loss of muscle protein with aging avoided in feeding practice. To meet the dietary require- According to the current NRC [16], the requirement of ments of adult cats and dogs for high-quality protein, non-pregnant and non-lactating adult dogs for dietary animal-sourced foodstuffs can be used to manufacture CP is similar to that of adult pigs but is only 50% of that diets for these animals [54]. of adult cats. Without nutritional intervention, aging Dietary AA intake by cats and dogs, like other ani- dogs usually lose a substantial amount of lean body mass mals, depends on dietary AA content and food con- and gain white adipose tissue [127, 128], possibly due sumption [61]. Multiple factors should be considered in Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 13 of 21 Table 6 Increasing dietary protein intake increases the oxidation of protein in adult cats Variable Diets containing different levels of protein Low Adequate Moderate High protein protein protein protein Body weight, kg 3.84 3.99 4.2 4.17 Dietary energy and nutrient content and food intake Protein content, % (as-fed basis) 9.1 17.2 32.7 59.6 Protein content, % of metabolizable energy 7.5 14.2 27.1 49.6 Fat content, % (as-fed basis) 20.9 21.0 20.9 20.9 Carbohydrate, % (as-fed basis) 65.1 57.0 41.0 13.5 Metabolizable energy, MJ/kg dry matter 20.3 20.3 20.2 20.1 Food intake, g dry matter/d 41.5 58.5 54.0 57.8 Nitrogen intake, excretion, and balance Nitrogen intake, g/d 0.60 1.61 2.82 5.52 Total urinary nitrogen, g/d 0.76 1.16 2.29 4.58 Urinary ammonia nitrogen, mg/d 117 152 157 198 Urinary creatinine nitrogen, mg/d 48.7 47.1 56.5 54.5 Fecal nitrogen, g/d 0.09 0.12 0.15 0.17 Nitrogen balance, g/d −0.25 0.33 0.39 0.76 Dietary intake and the oxidation of nutrients Protein intake, g/d 3.8 10.0 17.6 34.5 Protein oxidation, g/d 5.7 8.6 17.1 34.6 Fat intake, g/d 8.7 12.3 11.3 12.1 Fat oxidation, g/d 12.1 10.7 13.9 9.5 Carbohydrate intake, g/d 27.1 33.3 22.2 7.8 Carbohydrate oxidation, g/d 21.6 27.1 20.3 9.2 Concentrations of amino acids in plasma Total amino acids, μmol/L 1827 2579 4461 5363 Leucine + Isoleucine + Valine, μmol/L 154 221 564 1286 Urinary excretion of amino acids EAAs (excluding taurine) and NEAAs, μmol/d 418 563 458 527 EAAs/NEAAs, mol/mol 5.4 5.7 2.6 1.7 Felinine , μmol/d 388 458 1009 919 EAAs Nutritionally essential amino acids, NEAAs Nutritionally nonessential amino acids Adapted from Green et al. [122] Two males and two females formulating diets, including endogenous AA synthesis, growth) is minimal in the first 6 weeks of gestation and the digestibility and bioavailability of dietary nutrients, increases by 25% during the last 3 weeks of gestation the presence of antinutritive factors in foodstuffs, the fer - [134]. In cats, maternal weight gain (including both the maternal body fat gain and the conceptus growth) occurs mentability and quantity of dietary fiber, and interactions linearly during gestation and is 43% of their pre-preg among food constituents [33, 61, 133]. Furthermore, - requirements of cats and dogs for dietary AAs (including nancy BW [134]. In cats and dogs, 82% and 90% of fetal sulfur AAs) may critically depend on the catabolism of growth occurs in the last 3 weeks of gestation, respec- these nutrients by the intestinal microbiota [75, 99]. tively [134]. A deficiency of maternal dietary taurine (≤ 0.05%) causes early embryonic resorptions and fetal Requirements of cats and dogs for dietary amino defects in cats and their diets must contain > 0.05% tau- acids during pregnancy and lactation rine for optimum pregnancy outcomes [136]. At present, Average pregnancy length in cats and dogs appears to little is known about the dietary requirements of preg- similar (65 and 63 d, respectively), but there are differ - nant dogs for taurine. ences in the patterns of both maternal and fetal weight Lactation lasts approximately 7–8 weeks in cats and gains between these two species [134, 135]. In dogs, dogs, with peak milk production around weeks 3 to 4 maternal weight gain (almost exclusively the conceptus [134]. There are differences in maternal weight change Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 14 of 21 after parturition between cats and dogs. Specifically, and proline) via the intestinal-renal axis [63]. Of par- after giving births, the bitch generally returns to her ticular note, most dietary glutamine (~ 70%) and glu- pre-breeding BW immediately after delivery [134]. In tamate (~ 95%) as well as ~ 40% of dietary proline are contrast, the queen losses BW (mainly fats) gradually to extracted by the mammalian small intestine during reach her pre-breeding weight at 24 d post-partum and their first pass into the portal vein [75] and, thus, most her BW at 5 to 7 weeks post-partum is about 95% of her of these three AAs in the body are derived from endog- pre-breeding weight [137]. As in other mammals, dietary enous syntheses. Interestingly, glutamine is the only deficiencies of AAs impair milk production by lactating AA in the arterial blood that is taken up by the small cats and dogs [16]. intestine for citrulline and arginine production; there- To date, dietary requirements of pregnant and lactating fore, it is of nutritional and physiological importance cats and dogs for proteinogenic AAs have not been well to convert BCAAs (the source of the amino group and defined. It has been assumed that dietary CP require - amide nitrogen) into glutamine in extra-hepatic tissues ments for the growth of young cats and dogs would (primarily skeletal muscle) [61]. Because BCAAs are meet their requirements for gestation and lactation [16]. not formed de novo in all animals, these EAAs must However, embryos of mammals, including cats and dogs, be provided from high-quality and high-quantity pro- are highly sensitive to ammonia toxicity and, therefore, tein. Even the same ingredient may not supply the same maternal intakes of dietary CP and AAs should not be amount of nutrients depending on the method of food excessive [138, 139]. In gestating and lactating dogs, the processing, and some of diet-derived small peptides requirement for dietary CP is 260 g/kg diet containing can exert signaling and regulatory functions in the 4.0 kcal ME/g of the diet without dietary carbohydrate intestine and extraintestinal tissues [141]. Despite an or 200 g/kg diet containing 4.0 kcal ME/g of the diet with endogenous synthesis of arginine, both young and adult dietary digestible carbohydrate [16]. The NRC [16] rec - dogs must ingest adequate arginine in diets to main- ommends that the dietary requirements of dogs or cats tain its vital physiological functions beyond nitrogen for CP and AAs be the same during late pregnancy and balance [43], as noted previously. Interestingly, hydro- peak lactation, but it is likely that such estimates do not lyzed feather is a rich source of arginine (5.83%, as-fed reflect the true requirements of the animals because of basis) [54]. Inclusion of hydrolyzed feather meal in dry marked differences in physiological states (pregnancy or wet foods for dogs can meet their high requirements versus lactation) and products (conceptus versus milk). for arginine. In addition, even when fed a diet contain- In addition, the NRC [16] recommends substantial ing sufficient methionine and cysteine, some breeds of increases in the dietary contents of CP and most pro- dogs have a limited ability to synthesize taurine due to teinogenic AAs except methionine, cysteine and tryp- genetic mutations as noted previously and, therefore, tophan for adult dogs during late gestation and peak must be provided with adequate dietary taurine (e.g., lactation compared with non-pregnant and non-lactating 0.4% of dietary DM) [100]. counterparts (Table 7). Disappointingly, the NRC [16] did As noted previously, cats have a very limited ability to not explain its recommended 3%, 6%, and 14% decreases, synthesize both arginine and taurine and, therefore, must respectively, in the dietary content of cysteine, methio- consume diets containing these two AAs to ensure nor- nine and tryptophan for dogs during both late gestation mal blood flow, the proper digestion of dietary lipids and and peak lactation compared with non-pregnant and fat-soluble vitamins, and maintain health (particularly non-lactating adult dogs. Such recommendations do not retinal, cardiac, skeletal, reproductive, and metabolic appear to have physiological bases and should be revised health) [14, 43]. These animals do not have preference in the future. for plant products that generally contain high amounts of carbohydrates including sweet sugars [14]. In recent Important roles of animal‑sourced foodstuffs years, much work has shown that animal-derived ingre- in providing AAs in diets for cats and dogs dients are abundant sources of both arginine and taurine Abundant sources of both arginine and taurine for the diets of animals [54]. For example, the content of in animal-sourced foodstuffs for cats and dogs taurine in common animal-derived foods (mg/kg food, Compared with adult humans, adult dogs have a 90% on an as-fed basis) is: blood meal, 1520; chicken by-prod- greater rate of whole-body arginine catabolism and, uct meal, 2096; chicken visceral digest, 1317; spray-dried consequently, a much higher requirement for arginine peptone from enzyme-treated porcine mucosal tissues, [63]. This necessitates a higher intake of good-quality 1638; poultry by-product meal (pet-food grade), 3884; protein by dogs than humans to both directly supply and spray-dried poultry plasma, 2455. These foodstuffs exogenous arginine and endogenously generate argi- also contain creatine that is essential for energy metab- nine from its precursor AAs (glutamine/glutamate olism and anti-oxidative reactions in excitable tissues Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 15 of 21 Table 7 Recommended allowances of dietary AAs for post-weaning growing dogs and cats, as well as adults Nutrient Dogs, Amt/kg DM Cats, Amt/kg DM Pigs, Amt/kg DM Growing Adult (NP) LG PL Growing Adult LG PL 6-kg 110-kg P1G P1LS (NP) BW BW NRC (2006) NRC (2006) NRC (2012) ME, kcal 4000 4000 4000 4000 4000 4000 4000 4000 3778 3667 3667 3667 Crude protein, g 225 100 200 200 225 200 213 213 252 116 142 228 EAAs, g Arg 7.9 3.5 10.0 10.0 9.6 7.7 15 15 8.3 3.6 4.4 5.3 His 3.9 1.9 4.4 4.4 3.3 2.6 4.3 4.3 6.4 2.8 1.9 3.9 Ile 6.5 3.8 7.1 7.1 5.4 4.3 7.7 7.7 9.8 4.3 4.6 5.4 Leu 12.9 6.8 20.0 20.0 12.8 10.2 18 18 19.4 7.9 7.9 11.0 Lys 8.8 3.5 9.0 9.0 8.5 3.4 11 11 18.9 7.9 8.4 9.6 Met 3.5 3.3 3.1 3.1 4.4 1.7 5.0 5.0 5.4 2.3 2.4 2.6 Met + Cys 7.0 6.5 6.2 6.2 8.8 3.4 9.0 9.0 10.7 4.8 5.7 5.2 Phe 6.5 4.5 8.3 8.3 5.0 4.0 – – 11.2 4.8 4.7 5.2 Phy + Tyr 13.0 7.4 12.3 12.3 19.1 15.3 19.1 19.1 17.8 7.8 8.3 10.9 Thr 8.1 4.3 10.4 10.4 6.5 5.2 8.9 8.9 11.7 5.4 6.1 6.4 Trp 2.3 1.4 1.2 1.2 1.6 1.3 1.9 1.9 3.1 1.4 1.6 1.8 Val 6.8 4.9 13.0 13.0 6.4 5.1 10 10 12.2 5.4 6.1 8.3 Taurine (Cats) – – – – 0.40 0.40 0.53 0.53 – – c d f NEAAs, gLi and Wu Che et al. [140] Wu and Li [139] Ala 15.2 5.63 10.6 12.1 23.3 20.1 29.1 34.9 14.4 7.99 8.91 10.5 Asn 10.6 3.95 7.06 8.46 17.1 14.8 21.5 25.8 10.5 5.75 6.46 8.53 Asp 15.2 5.63 10.6 12.1 21.0 18.3 26.5 31.8 14.7 8.17 7.88 12.1 Glu 26.6 9.88 17.6 21.2 38.4 33.3 48.3 58.0 25.6 14.1 11.5 23.4 Gln 24.0 8.92 15.9 19.1 25.5 22.1 32.0 38.4 23.8 12.6 20.7 18.3 Gly 16.9 5.82 10.4 12.5 17.3 14.9 21.6 25.9 16.2 9.07 6.20 9.58 Pro + Hyp 19.0 6.27 11.2 13.4 17.8 15.4 22.3 26.8 18.6 10.2 11.5 15.8 Ser 9.30 3.46 6.18 7.42 18.1 15.6 22.6 27.1 8.74 4.87 5.81 9.24 Amt Amount, BW Body weight, DM Dry matter of diet, EAAs traditionally nutritionally essential amino acids, Hyp 4-hydroxyproline, LG Late gestation, ME Metabolizable energy, NEAAs traditionally nutritionally nonessential amino acids, NP Non-pregnant and non-lactating, NRC National Research Council, PL Peak lactation, P1G the last 24 d of pregnancy of first-parity swine, P1LS first-parity lactating sows “–” data are not available Referring to 4- to-14-week-old dogs for EAAs. The recommended allowance of dietary crude-protein for ≥14-week-old growing dogs is 175 g/kg DM The ratio of proline to 4-hydroxyproline is 18.6:1.0, g/g Present work. It is assumed that the recommended allowance of dietary crude-protein for adult dogs is 10%, dry matter basis It is assumed that the recommended allowances of dietary crude-protein for growing and young adult cats are 30% and 26%, respectively, on the dry matter basis [140]. The recommended allowances of dietary EAAs for growing cats (g/kg dry matter of diet) are: Arg, 26.8; Cys, 6.3; His, 16.3; Ile, 21.0; Leu, 34.1; Lys, 36.8; Met, 12.9; Phe, 17.1; Thr, 18.9; Trp, 5.1; Tyr, 15.4; and Val, 24.3. The recommended allowances of dietary EAAs for young adult cats (g/kg dry matter of diet) are: Arg, 23.3; Cys, 6.3; His, 14.0; Ile, 18.3; Leu, 29.5; Lys, 31.9; Met, 11.3; Phe, 14.9; Thr, 16.4; Trp, 4.4; Tyr, 13.4; and Val, 21.0. The recommended allowances of dietary crude-protein for young and adult cats are 1.25 times their minimum dietary requirements for crude protein Different breeds of swine have different body weights at young and adult ages. For the offspring of Yorkshire × Landrace sows and Duroc × Hampshire boars with a normal birth weight, 6- and 110-kg body weights correspond to approximately 3 weeks and 6 months of age. At about 18 months of age (adult), the lean-tissue weight curve of swine is flattened. The dietary requirement of adult swine for crude protein is approximately 100 g/kg dry matter Values refer to amino acid content, rather than digestible amino acid content, in the diet Abundant sources of glycine, proline, 4-hydroxyproline, (brain and skeletal muscle) [61]. In contrast, all plant- cysteine, and serine in animal-sourced foodstuffs for cats sourced foodstuffs lack taurine and creatine [54] and, and dogs therefore, should not be fed solely to either cats or some Another unique feature of animal-derived products is breeds of dogs. As for dogs, hydrolyzed feather meal can be included in diets as an abundant source of both argi that they contain high amounts of either collagens (e.g., meat and bone meal and poultry by-product meal) or nine and taurine for cats. Li and Wu Journal of Animal Science and Biotechnology (2023) 14:19 Page 16 of 21 keratins (e.g., hydrolyzed feather meal) [61]. Collagen is and galactosamine from glycosaminoglycans, whereas comprised of two-thirds of AAs as glycine, proline, and chondroitinase and deacetylase hydrolyze chondroitin 4-hydroxyproline, whereas keratins (present in feather to galactosamine and glucuronic acid [61]. The amino - and hair) are also rich in these three AAs plus cysteine sugars are absorbed into enterocytes and then the portal and serine (the immediate precursor of glycine). After vein. Within cells, 4-epimerase converts galactosamine feather and hair are properly hydrolyzed, their AAs into glucosamine. Thus, poultry meal is a source of glu - are nutritionally available for both cats and dogs to use cosamine for animals, including cats and dogs. Of par- [54]. For example, the content of the most abundant ticular note, glucosamine has anti-inflammatory and AAs in chicken feather keratin (% of total AAs, mol/ anti-oxidative effects in immunologically challenged mol) is: glycine, 13.7; proline, 9.8; cysteine, 7.8; and ser- mammalian cells by inhibiting the expression of induc- ine, 14.1 [142]. Thus, hydrolyzed feather meal contains ible NO synthase and excessive NO production [143]. high amounts of glycine, proline, 4-hydroxyproline, This may explain why glucosamine plus chondroitin has cysteine, and serine (8.97%, 11.64%, 4.97%, 4.17%, and been used to effectively treat dogs (particularly elderly 8.92%, respectively, as-fed basis) [54]. Dietary provision dogs and working dogs) [144] and cats [145] with joint of hydrolyzed feather meal can spare energy and materi- pain or osteoarthritis. Research is warranted to define the als that would be needed for de novo syntheses of these efficacy of dietary supplementation with poultry meal in AAs in animals, thereby reducing energy expenditure as improving the health of cat and dog joints. well as the associated production of oxidants (e.g., for- maldehyde) and ammonia [61]. Of note, cysteine, glycine, Improvement of immune responses in cats and dogs and proline are crucial for the synthesis of hair proteins AAs are essential for immune responses in all ani- (e.g., cysteine-rich α-keratin and β-keratin) as are both mals (including cats and dogs) through a plethora glycine and proline for the synthesis of collagen and of mechanisms, such as the syntheses of proteins elastin [61]. These unique proteins maintain the normal (including antibodies and cytokines) and glutathione structures and integrity of hair and connective tissue (a potent anti-oxidative tripeptide consisting of gly- while preventing its abnormalities particularly in associa- cine, cysteine, and glutamate), as well as the killing of tion with aging. Indeed, hair quality is considered by pet pathogens via production of NO from arginine and of owners as a very important indicator of the nutritional chlorotaurine and bromotaurine from taurine [61]. Sup- adequacy of commercially manufactured pet foods or plementing arginine to a low-protein (23% CP) [146] or home-made meals [137]. Thus, hydrolyzed feather meal high-protein (60% CP) [147] diet has beneficial immu - may be a desirable pet-food ingredient to provide nutri- nomodulating effects in cats. In addition, intravenous tionally and physiologically significant AAs (including administration of alanyl-glutamine to dogs undergoing arginine, cysteine, glycine, proline, 4-hydroxyproline, and a treatment with methylprednisolone sodium succinate serine) [54]. This new knowledge can help to dispel the enhanced phagocytic capacity and respiratory burst unfounded myth that poultry-sourced hydrolyzed feather activity of leukocytes [148]. Currently, there is a global meal is of little nutritive value in feeding companion pandemic of COVID-19 caused by the severe acute res- animals. piratory syndrome coronavirus 2 that can also infect dogs [149] and cats [150]. Adequate AA nutrition [e.g., Provision of glucosamine in animal-sourced foodstuffs sufficient provision of AAs (such as lysine, cysteine, for cats and dogs methionine, tryptophan, glycine, and proline) that are Glucosamine is a normal metabolite of glutamine and abundant in animal proteins but are relatively low in fructose-6-phosphate in animals [61]. Poultry meal is plant proteins] may be crucial for improving both innate manufactured from raw materials containing chicken and acquired immune systems to mitigate risk for infec- cartilage, which consists of glycosaminoglycans (includ- tion in the animals. In addition, some animal-sourced ing chondroitin) and proteoglycans (formed from gly- foods, such as spray-dried animal plasma [151] and cosaminoglycans and protein backbones) in addition to spray-dried egg products [152] contain a large amount collagens and elastins. Glycosaminoglycans are com- of immunoglobulins and directly contribute to neutral- posed of N-acetylglucosamine and N-acetylgalactosa- izing the pathogens that invade the body. Furthermore, mine, as well as their sulfate derivatives [61]. In the small spray-dried animal plasma provides other functional intestine, proteoglycans are hydrolyzed to proteases molecules, such as albumin, essential fatty acids, (e.g., trypsin) to generate glycosaminoglycans, pep- B-complex vitamins, and minerals such as calcium, tides, and AAs; the combined actions of exoglycosidase, phosphorus, sodium, chloride, potassium, magnesium, endoglycosidase, sulfohydrolase, and hyaluronidase-like iron, zinc, copper, manganese, and selenium [153, 154]. enzymes along with deacetylase release glucosamine This animal-sourced foodstuff is also a useful binder Li and W u Journal of Animal Science and Biotechnology (2023) 14:19 Page 17 of 21 in canned pet food products due to its high content of AAs as well as large amounts of functional nutrients globulins and fibrinogen as well as its desired physico - (e.g., taurine, 4-hydroxyproline, creatine, and carnos- chemical properties [155]. Finally, animal plasma prod- ine), lipids, and minerals [61]. In addition, feather meal ucts are highly palatable to both cats and dogs [156]. can be used as an ingredient or supplement in dry or wet Thus, animal-derived ingredients used in dry or wet pet foods for cats and dogs to meet their high requirements foods can help to improve the immune responses and for both arginine and taurine [54]. Animal-derived ingre- health of all companion animals by providing not only dients alone or in combination are abundant sources of AAs but also other essential nutrients (e.g., macro- and both proteinogenic AAs and taurine for adequate nutri- micro-minerals). tion and metabolism in cats and dogs to optimize their growth, development, health, and well-being. Conclusions and perspectives Both cats and dogs are carnivores from the taxonomi- Abbreviations cal order Carnivora. During evolution, domestic dogs AA Amino acid have adapted to omnivorous diets that contain both tau- AAFCO Association of American Feed Control Officials BCAA Branched-chain amino acid rine-rich meat and starch-rich plant ingredients, while BW Body weight domestic cats remain obligate carnivores. Thus, dogs CP Crude protein differ from cats in many aspects of AA nutrition and DM Dry matter EAA Nutritionally essential amino acid metabolism, and dogs can thrive on taurine-free vegetar- ME Metabolizable energy ian diets supplemented with non-taurine nutrients that NEAA Nutritionally nonessential amino acid are inadequately synthesized de novo or absent from NO Nitric oxide NRC National Research Council plants. Much evidence shows that there are marked dif- P5C Pyrroline-5-carboxylate ferences in both qualitative (i.e., presence or absence) and quantitative (i.e., amounts) requirements for protein and Acknowledgments We thank Prof. Nancy Ing (DVM and PhD, Department of Animal Science at certain AAs (arginine, taurine, methionine, and cysteine, Texas A&M University) for critically reviewing and providing helpful com- as well as NEAAs) between cats and dogs. In comparison ments on this manuscript. We are also grateful to Prof. Jürgen Zentek (DVM with swine [53, 139], recommended minimum require- and PhD, Freie Universität, Berlin, Germany) for kindly sharing his published articles with us. ments and allowances of dietary EAAs for growing and adult cats and dogs are summarized in Table 7. We sug- Authors’ contributions gest that companion animals have dietary requirements PL and GW conceived the idea for this article, performed the literature research and data analysis, and wrote the manuscript. The author(s) read and for NEAAs as do other mammals due to insufficient syn - approved the final manuscript. thesis de novo. Cats have greater endogenous nitrogen losses, as well as higher requirements for dietary protein Funding This work was supported by Texas A&M AgriLife Research (H-8200). (including arginine) and taurine than do dogs and, there- fore, should not be fed dog foods. Because the composi- Availability of data and materials tion of the milk of both cats and dogs differ from that of This manuscript reviews published work and does not contain new experi- mental data. farm mammals, young pets should not be fed replacer diets formulated based on goat or cow milk. As compan- Declarations ion animals lose tremendous amounts of lean body mass with aging, their diets should contain adequate levels of Ethics approval and consent to participate high-quality protein, which may be much greater than This review article does not require either human consent or the approval of animal use. the current AAFCO [157] and NRC [16] recommenda- tions to support muscle protein synthesis and mitigate Consent for publication muscle loss. Not applicable. Effects of an excessive intake of a single AA in cats Competing interests [140] and dogs [108] may be different, depending on die - All authors declare no conflict of interest. tary intakes of other AAs. We are not aware that compar- isons of the metabolism or dietary requirements of any Received: 8 October 2022 Accepted: 21 December 2022 nutrients between modern breeds of cats and dogs were made in the same experiment. 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Journal

Journal of Animal Science and Biotechnology – Springer Journals

Published: Feb 21, 2023

Keywords: Animal-sourced foodstuffs; Cats; Dogs; Health; Metabolism; Nutrition

References

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Callon, MC; Cargo-Froom, C; DeVries, TJ; Shoveller, AK
Differences in food preferences between individuals and populations of domestic cats Felis silvestris catus

Bradshaw, JWS; Healey, LM; Thorne, CJ; Macdonald, DW; Arden-Clark, C
Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations

Morris, JG
Comparison of ileal and fecal nutrient digestibility of dry canine foods

Hendriks, WH; Bosch, G; Fahey, GC
Protein and amino acid bioavailability estimates for canine foods

Hendriks, WH; Bakker, EJ; Bosch, G
Selected animal and plant protein sources affect nutrient digestibility and fecal characteristics of ileally cannulated dogs

Bednar, GE; Murray, SM; Patil, AR; Flickinger, AE; Merchen, NR; Fahey, GC
Ileal and total tract nutrient digestibilities and fecal characteristics of dogs as affected by soybean protein inclusion in dry, extruded diets

Clapper, GM; Grieshop, CM; Merchen, NR; Russet, JC; Brent, JL; Fahey, GC
Effects of species raw material source, ash content, and processing temperature on amino acid digestibility of animal by-product meals by cecectomized roosters and ileally cannulated dogs

Johnson, ML; Parsons, CM; Fahey, GC; Merchen, NR; Aldrich, CG
Comparison of the endogenous ileal and faecal amino acid excretion in the dog (Canis familiaris) and the rat (Rattus rattus) determined under protein-free feeding and peptide alimentation

Hendriks, WH; Sritharan, K; Hodgkinson, SM
Gut endogenous nitrogen and amino acid excretions in adult domestic cats fed a protein-free or an enzymatically hydrolyzed casein-based diet

Hendriks, WH; Moughan, PJ; Tarttelin, MF
Endogenous amino acid flow in the stomach and small intestine of the young growing pig

Moughan, PJ; Schuttert, G; Leenaars, M
Apparent and standardized ileal digestibilities of amino acids for pigs fed corn- and soybean meal-based diets at varying crude protein levels

Zhai, H; Adeola, O
Effects of coat length and faecal hair removal on measured nutrient digestibility in cats

Kim, HS; Hong, JS; Park, CW; Cho, KH; Kim, YY
Additivity of values for apparent and standardized ileal digestibility of amino acids in mixed diets fed to growing pigs

Stein, HH; Pedersen, C; Wirt, AR; Bohlke, RA
Digestibility for dogs and cats of meat and bone meal processed at two different temperature and pressure levels

de Oliveira, LD; de Carvalho Picinato, MA; Kawauchi, IM; Sakomura, NK; Carciofi, AC
True nutrient and amino acid digestibility of dog foods made with human-grade ingredients using the precision-fed cecectomized rooster assay

Oba, PM; Utterback, PL; Parsons, CM; Swanson, KS
Amino acid digestibility in six sources of meat and bone meal, blood meal, and soybean meal fed to growing pigs

Navarro, DMDL; Mathai, JK; Jaworski, NW; Stein, HH
Apparent total-tract macronutrient digestibility, serum chemistry, urinalysis, and fecal characteristics, metabolites and microbiota of adult dogs fed extruded, mildly cooked, and raw diets

Algya, KM; Cross, TL; Leuck, KN; Kastner, ME; Baba, T; Lye, L
Carbohydrate digestion by the domestic cat (Felis catus)

Morris, JG; Trudell, J; Pencovic, T
Nutrient digestion by ileal cannulated dogs as affected by dietary fibers with various fermentation characteristics

Muir, HE; Murray, SM; Fahey, GC; Merchen, NR; Reinhart, GA
Influence of fiber fermentability on nutrient digestion in the dog

Silvio, J; Harmon, DL; Gross, KL; McLeod, KR
The effect of Yucca schidigera extract on canine and feline faecal volatiles occurring concurrently with faecal aroma amelioration

Lowe, JA; Kershaw, SJ; Taylor, AJ; Linforth, RS
Ileal, colonic and total tract nutrient digestibility in dogs (Canis familiaris) compared with total tract digestibility in mink (Neovison vison)

Tjernsbekk, MT; Tauson, AH; Ahlstrøm, O
Hydroxyproline in animal metabolism, nutrition, and cell signaling

Hu, SD; He, WL; Wu, G
Comparative apparent total tract digestibility of major nutrients and amino acids in dogs (Canis familiaris), blue foxes (Alopex lagopus) and mink (Mustela vison)

Vhile, SG; Skrede, A; Ahlstrøm, Ø; Hove, K
Effects of level of feed intake on nitrogen, amino acid and energy digestibilities measured at the end of the small intestine and over the total digestive tract of growing pigs

Haydon, KD; Knabe, DA; Tanksley, TD
Influence of age and body size on orocecal transit time as assessed by use of the sulfasalzine method in healthy dogs

Weber, MP; Martin, LJ; Biourge, VC; Nguyen, PG; Dumon, HJ
Digestibility and metabolizable energy measurement in dogs and cats

Shields, RG
Lack of effect of age on digestibility of protein, fat and dry matter in Beagle dogs

Buffington, CA; Branam, JE; Dunn, GC; Burger, IH; Rivers, JPW
Effect of β-mannanase on the digestibility of diets with different protein sources in dogs determined by different methodologies

Félix, AP; Gabeloni, LR; Brito, CBM; Oliveira, SG; Silva, AVF; Maiorka, A
Comparative nutrition of cats and dogs

Baker, DH; Czarnecki-Maulden, GL
Age-related changes in apparent digestibility in growing kittens

Harper, EJ; Turner, CL
Influence of protein concentration and quality in a canned diet on urine composition, apparent nutrient digestibility and energy supply in adult cats

Paßlack, N; Kohn, B; Doherr, MG; Zentek, J
Carbohydrate metabolism of the cat. 2. Digestion of starch

Kienzle, E
Free amino acids in the plasma and urine of dogs from birth to senescence

Blazer-Yost, B; Jezyk, PF
Plasma and whole blood taurine in normal dogs of varying size fed commercially prepared food

Delaney, SJ; Kass, PH; Rogers, QR; Fascetti, AJ
Effects of dietary excesses of the branched-chain amino acids on growth, food intake and plasma amino acid concentrations of kittens

Hargrove, DM; Rogers, QR; Calvert, CC; Morris, JG
Plasma amino acid and whole blood taurine concentrations in cats eating commercially prepared diets

Heinze, CR; Larsen, JA; Kass, PH; Fascetti, AJ
An important role for endogenous synthesis of arginine in maintaining arginine homeostasis in neonatal pigs

Flynn, NE; Wu, G
Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs

Wu, G; Davis, PK; Flynn, NE; Knabe, DA; Davidson, JT
Amino acids in swine nutrition and production

Zhang, Q; Hou, YQ; Bazer, FW; He, WL; Posey, EA; Wu, G
Composition of amino acids in foodstuffs for humans and animals

Li, P; He, WL; Wu, G
Amino acids and mammary gland development: Nutritional implications for neonatal growth

Rezaei, R; Wu, ZL; Hou, YQ; Bazer, FW; Wu, G
Amounts of amino acids in the Beagle bitch milk

Ferrando, R; Klur, H; Coquet, B; Rouaud, JL
Amino acid composition of human milk is not unique

Davis, TA; Nguyen, HV; Garcia-Bravo, R; Fiorotto, ML; Jackson, EM; Lewis, DS
Composition of goat and cow milk produced under similar conditions and analyzed by identical methodology

Ceballos, LS; Morales, ER; de la Torre, AG; Castro, JD; Martínez, LP; Sanz Sampelayo, MR
Taurine and other free amino acids in milk of man and other mammals

Rassin, DK; Sturman, JA; Gaull, GE
Compositional and functional characteristics of goat milk and relevance as a base for infant formula

Prosser, CG
Myocardial failure in cats associated with low plasma taurine: A reversible cardiomyopathy

Pion, PD; Kittleson, MD; Rogers, QR; Morris, JG
Arginine metabolism: nitric oxide and beyond

Wu, G; Morris, SM
Comparison of circulating metabolite concentrations in dogs and cats when allowed to freely choose macronutrient intake

Hall, JA; Jackson, MI; Vondran, JC; Vanchina, MA; Jewell, DE
Influence of diet on glucose tolerance, on the rate of glucose utilization and on gluconeogenic enzyme activities in the dog

Belo, PS; Romsos, DR; Leveille, GA
Assessment of endogenous nitrogen output in adult dogs of contrasting size using a protein-free diet

Kendall, PT; Blaze, SE; Holme, DW
Urinary excretion of endogenous nitrogen metabolites in adult domestic cats using a protein-free diet and the regression technique

Hendricks, WH; Moughan, PJ; Tarttelin, MF
Protein and amino acid requirements of the adult cat

Earle, KE
Nitrogen balance studies in aging rats

Baldwin, JK; Griminger, P
Carcass nitrogen as a predictor of protein requirement for mature female rats

Sheehan, PM; Clevidence, BA; Reynolds, LK; Thye, FW; Ritchey, SJ
Discrepancy between use of lean body mass or nitrogen balance to determine protein requirements for adult cats

Laflamme, DP; Hannah, SS
The significance of the amino acids in canine nutrition

Rose, WC; Rice, EE
Amino acid metabolism of dog jejunum before and during absorption of keto analogues

Weber, FL; Maddrey, WC; Walser, M
Quantitative aspects of interorgan relationships among arginine and citrulline metabolism

Yu, YM; Burke, JF; Tompkins, RG; Martin, R; Young, VR
Amino acid metabolism in the small intestine

Wu, G
Comparison of the nutrient composition of commercial dog milk replacers with that of dog milk

Heinze, CR; Freeman, LM; Martin, CR; Power, ML; Fascetti, AJ
Nutritional lens opacities in two litters of Newfoundland dogs

Ranz, D; Gutbrod, F; Corinna Eule, C; Kienzle, E
Deficiency of pyrroline-5-carboxylate synthase in the intestinal mucosa of the cat

Rogers, QR; Phang, JM
Arginine metabolism in cat kidney

Levillain, O; Parvy, P; Hus-Citharel, A
Arginine: an essential amino acid for the cat

Morris, JG; Rogers, QR
Nutritionally nonessential amino acids: A misnomer in nutritional sciences

Hou, YQ; Wu, G
Glutamine utilization by the small intestine

Windmueller, HG
Regulation of glutaminase activity and glutamine metabolism

Curthoys, NP; Watford, M
Phosphate-dependent glutaminase of small intestine: Localization and role in intestinal glutamine metabolism

Pinkus, LM; Windmueller, HG
The importance of the small intestine in gut ammonium production in the fasting dog

Weber, FL; Veach, GL
Uptake and metabolism of plasma glutamine by the small intestine

Windmueller, HG; Spaeth, AE
Regulation of glutamine metabolism in dog kidney in vivo

Vinay, P; Lemieux, G; Gougoux, A; Halperin, M
Interaction of gut and liver in nitrogen metabolism during exercise

Wasserman, DH; Geer, RJ; Williams, PE; Becker, T; Lacy, DB; Abumrad, NN
Stimulation of ammonia production from glutamine by intraluminal glucose in small intestine of dogs

Weber, FL; Veach, G; Friedman, DW
Regulation of gut glutamine metabolism: Role of hormones and cytokines

Abumrad, NN; Kim, S; Molina, PE
Quantitative role of splanchnic region in leucine metabolism: L-[1-13C,15N]leucine and substrate balance studies

Yu, YM; Wagner, DA; Tredget, EE; Walaszewski, JA; Burke, JF; Young, VR
Amino acid movements between gut, liver, and periphery in unanesthetized dogs

Elwyn, DH; Parikh, HC; Shoemaker, WC
Branched-chain amino acid nitrogen transfer to alanine in vivo in dogs

Galim, EB; Hruska, K; Bier, DM; Matthews, DE; Haymond, MW
Simultaneous measurement of metabolic flux in portally-drained viscera, liver, spleen, kidney and hindquarter in the conscious pig

Ten Have, GAM; Bost, MCF; Suyk-Wierts, JCAW; van den Bogaard, AEJM; Deutz, NEP
Amino acid oxidation increases with dietary protein content in adult neutered male cats as measured using [1-13C]leucine and [15N2]urea

Wester, TJ; Weidgraaf, K; Hekman, M; Ugarte, CE; Forsyth, SF; Tavendale, MH
Mammalian isovalthine metabolism

Rutherfurd-Markwick, KJ; Rogers, QR; Hendriks, WH
Urinary felinine excretion in intact male cats is increased by dietary cystine

Hendriks, WH; Rutherfurd-Markwick, KJ; Weidgraaf, K; Morton, RH; Rogers, QR
Short-term amino acid, clinicopathologic, and echocardiographic findings in healthy dogs fed a commercial plant-based diet

Cavanaugh, SM; Cavanaugh, RP; Gilbert, GE; Leavitt, EL; Ketzis, JK; Vieira, AB
Plasma and whole blood taurine concentrations in dogs may not be sensitive indicators of taurine deficiency when dietary sulfur amino acid content is reduced

Tôrres, CL; Biourge, VC; Backus, RC
Taurine deficiency in Newfoundlands fed commercially available complete and balanced diets

Backus, RC; Cohen, G; Pion, PD; Good, KL; Rogers, QR; Fascetti, AJ
Low plasma taurine concentration in Newfoundland dogs is associated with low plasma methionine and cyst(e)ine concentrations and low taurine synthesis

Backus, RC; Ko, KS; Fascetti, AJ; Kittleson, MD; MacDonald, KA; Maggs, DJ
Taurine deficiency and dilated cardiomyopathy in golden retrievers fed commercial diets

Kaplan, JL; Stern, JA; Fascetti, AJ; Larsen, JA; Skolnik, H; Peddle, GD
Clinical findings in cats with dilated cardiomyopathy and relationship of findings to taurine deficiency

Pion, PD; Kittleson, MD; Thomas, WP; Skiles, ML; Rogers, QR
The biological function of cauxin, a major urinary protein of the domestic cat

Miyazaki, M; Yamashita, T; Taira, H; Suzuki, A; Hurst, JL; Beynon, RJ; Roberts, SC; Wyatt, TD
Urinary isovalthine excretion in adult cats is not gender dependent or increased by oral leucine supplementation

Hendriks, WH; Vather, R; Rutherfurd, SM; Weidgraaf, K; Rutherfurd-Markwick, KJ
Disease prevention and delayed aging by dietary sulfur amino acid restriction: translational implications

Dong, Z; Sinha, R; Richie, JP
Ammonia production from intraluminal amino acids in canine jejunum

Weber, FL; Friedman, DW; Fresard, KM
Amino acids in dog nutrition and health

Oberbauer, AM; Larsen, JA
Cats require more dietary phenylalanine or tyrosine for melanin deposition in hair than for maximal growth

Anderson, PJ; Rogers, QR; Morris, JG
Hair pigmentation can be affected by diet in dogs

Biourge, V; Sergheraert, R
Effect of dietary protein content and tryptophan supplementation on dominance aggression, territorial aggression, and hyperactivity in dogs

DeNapoli, JS; Dodman, NH; Shuster, L; Rand, WM; Gross, KL
Therapeutic effects of an alpha-casozepine and L-tryptophan supplemented diet on fear and anxiety in the cat

Landsberg, G; Milgram, B; Mougeot, I; Kelly, S; de Rivera, C
Self selection of dietary casein and soy-protein by the cat

Cook, NE; Kane, E; Rogers, QR; Morris, JG
Cats select for adequate methionine but not threonine

Rogers, QR; Wigle, AR; Laufer, A; Castellanos, VH; Morris, JG
Kittens choose a high leucine diet even when isoleucine and valine are the limiting amino acids

Hargrove, DM; Morris, JG; Rogers, QR
Taste preferences of the cat for neurophysiologically active compounds

White, TD; Boudreau, JC
Excess dietary lysine does not cause lysine-arginine antagonism in adult cats

Fascetti, AJ; Maggs, DJ; Kanchuk, ML; Clarke, HE; Rogers, QR
Dietary protein level affects protein metabolism during the postabsorptive state in dogs

Humbert, B; Martin, L; Dumon, H; Darmaun, D; Nguyen, P
Effect of dietary protein content on behavior in dogs

Dodman, NH; Reisner, I; Shuster, L; Rand, W; Luescher, UA; Robinson, I
Cats are able to adapt protein oxidation to protein intake provided their requirement for dietary protein is met

Green, AS; Ramsey, JJ; Villaverde, C; Asami, DK; Wei, A; Fascetti, AJ
Oxidation of energy substrates in tissues of largemouth bass (Micropterus salmoides)

Li, XL; Zheng, SX; Jia, SC; Song, F; Zhou, CP; Wu, G
Comparison of the activities of enzymes related to glycolysis and gluconeogenesis in the liver of dogs and cats

Washizu, T; Tanaka, A; Sako, T; Washizu, M; Arai, T
Lack of hepatic enzymatic adaptation to low and high levels of dietary protein in the adult cat

Rogers, QR; Morris, JG; Freedland, RA
Net protein oxidation is adapted to dietary protein intake in domestic cats (Felis silvestris catus)

Russell, K; Murgatroyd, PR; Batt, RM
Factors influencing lean body mass in aging dogs

Kealy, RD
Effect of nutritional interventions on longevity of senior cats

Cupp, CJ; Jean-Philippe, C; Kerr, WW; Patil, AR; Perez-Camargo, G
Diet restriction and ageing in the dog: Major observations over two decades

Lawler, DF; Larson, BT; Ballam, JM; Smith, GK; Biery, DN; Evans, RH
Pet food safety: Dietary protein

Laflamme, DP
Determination of optimal dietary protein requirements of young and old dogs

Wannemacher, RW; McCoy, JR
Effects of dietary protein on whole-body protein turnover and endocrine function in young-adult and aging dogs

Williams, CC; Cummins, KA; Hayek, MG; Davenport, GM
The association between pulse ingredients and canine dilated cardiomyopathy: Addressing the knowledge gaps before establishing causation

Mansilla, WD; Marinangeli, CPF; Ekenstedt, KJ; Larsen, JA; Aldrich, G; Columbus, DA
Food intake and nutrition during pregnancy, lactation and weaning in the dam and offspring

Fontaine, E
The growing conceptus of the domestic cat

Nelson, NS; Cooper, J
High Dietary Taurine effects on feline tissue taurine concentrations and reproductive performance

Sturman, JA; Messing, JF
Impacts of maternal dietary protein intake on fetal survival, growth and development

Herring, CM; Bazer, FW; Johnson, GA; Wu, G
The “ideal protein” concept is not ideal in animal nutrition

Wu, G; Li, P
Protein hydrolysates in animal nutrition: Industrial production, bioactive peptides, and functional significance

Hou, YQ; Wu, ZL; Dai, ZL; Wang, GH; Wu, G
Feather keratin: Composition, structure and biogenesis

Gregg, K; Rogers, GE; Bereiter-Hahn, J; Matoltsy, AG; Richards, KS
Glucosamine inhibits inducible nitric oxide synthesis

Meininger, CJ; Kelly, KA; Li, H; Haynes, TE; Wu, G
Glucosamine and chondroitin use in canines for osteoarthritis: A review

Bhathal, A; Spryszak, M; Louizos, C; Frankel, G
Osteoarthritis in the cat: 2. How should it be managed and treated?

Bennett, D; Zainal Ariffin, SM; bt, Johnston P.
The potential for enhancement of immunity in cats by dietary supplementation

Rutherfurd-Markwick, KJ; Hendriks, WH; Morel, PCH; Thomas, DG
Effects of arginine and ornithine supplementation to a high-protein diet on selected cellular immune variables in adult cats

Paßlack, N; Kohn, B; Zentek, J
Effect of parenteral l-alanyl-l-glutamine administration on phagocytic responses of polymorphonuclear neutrophilic leukocytes in dogs undergoing high-dose methylprednisolone sodium succinate treatment

Kang, JH; Kim, SS; Yang, MP
Infection of dogs with SARS-CoV-2

Sit, THC; Brackman, CJ; Ip, SM; Tam, KWS; Law, PYT
Detection of SARS-CoV-2 in a cat owned by a COVID-19-affected patient in Spain

Segales, J; Puig, M; Rodon, J; Avila-Nieto, C; Carrillo, J; Cantero, G
Early-life dietary spray-dried plasma influences immunological and intestinal injury responses to later-life Salmonella typhimurium challenge

Boyer, PE; D’Costa, S; Edwards, LL; Milloway, M; Susick, E; Borst, LB
Egg yolk antibodies (IgY) and their applications in human and veterinary health: A review

Pereira, EPV; van Tilburg, MF; Florean, EOPT; Guedes, MIF
Effects of spray-dried animal plasma on the growth performance of weaned piglets–A review

Balan, P; Staincliffe, M; Moughan, PJ
Effects of spray-dried animal plasma on intake and apparent digestibility in dogs

Quigley, JD; Campbell, JM; Polo, J; Russell, LE
Effects of spray-dried animal plasma on food intake and apparent nutrient digestibility by cats when added to a wet pet food recipe

Rodríguez, C; Saborido, N; Ródenas, J; Polo, J
Functional properties of spray-dried animal plasma in canned petfood

Polo, J; Rodríguez, C; Saborido, N; Ródenas, J

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Amino acid nutrition and metabolism in domestic cats and dogs

Amino acid nutrition and metabolism in domestic cats and dogs

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Amino acid nutrition and metabolism in domestic cats and dogs

Amino acid nutrition and metabolism in domestic cats and dogs

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