Characteristics of Nutrition and Metabolism in Dogs and Cats

Abstract

Domestic dogs and cats have evolved differentially in some aspects of nutrition, metabolism, chemical sensing, and feeding behavior. The dogs have adapted to omnivorous diets containing taurine-abundant meat and starch-rich plant ingredients. By contrast, domestic cats must consume animal-sourced foods for survival, growth, and development. Both dogs and cats synthesize vitamin C and many amino acids (AAs, such as alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and serine), but have a limited ability to form de novo arginine and vitamin D₃. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for AAs (particularly arginine, taurine, and tyrosine), B-complex vitamins (niacin, thiamin, folate, and biotin), and choline; exhibit greater rates of gluconeogenesis; are less sensitive to AA imbalances and antagonism; are more capable of concentrating urine through renal reabsorption of water; and cannot tolerate high levels of dietary starch due to limited pancreatic α-amylase activity. In addition, dogs can form sufficient taurine from cysteine (for most breeds); arachidonic acid from linoleic acid; eicosapentaenoic acid and docosahexaenoic acid from α-linolenic acid; all-trans-retinol from β-carotene; and niacin from tryptophan. These synthetic pathways, however, are either absent or limited in all cats due to (a) no or low activities of key enzymes (including pyrroline-5-carboxylate synthase, cysteine dioxygenase, ∆⁶-desaturase, β-carotene dioxygenase, and quinolinate phosphoribosyltransferase) and (b) diversion of intermediates to other metabolic pathways. Dogs can thrive on one large meal daily, select high-fat over low-fat diets, and consume sweet substances. By contrast, cats eat more frequently during light and dark periods, select high-protein over low-protein diets, refuse dry food, enjoy a consistent diet, and cannot taste sweetness. This knowledge guides the feeding and care of dogs and cats, as well as the manufacturing of their foods. As abundant sources of essential nutrients, animal-derived foodstuffs play important roles in optimizing the growth, development, and health of the companion animals.

4.1 Introduction

The metabolism of most nutrients in domestic dogs and cats is similar to that in other mammals (Baker and Czarnecki-Maulden 1991). Thus, the qualitative dietary requirements of dogs and cats for most nutrients [e.g., amino acids (AAs) that are not formed de novo in animal cells] are similar to those for omnivores (e.g., humans and pigs). However, dogs and cats have a relatively short digestive tract (He et al. 2024) and have evolved to have some unique feeding behaviors and metabolic characteristics that are distinct from most of other nonruminant mammals such as pigs, rats, and humans (Legrand-Defretin 1994; Morris 2002). In addition, dogs differ from cats in some of these physiological and nutritional aspects (Che et al. 2021; Oberbauer and Larsen 2021). For example, the dog has adapted to omnivorous diets containing taurine-rich meat and starch-rich plant ingredients, but the cat must consume at least a portion of animal-sourced foods or the same essential nutrients from synthetic supplements for survival, growth, and development because of their limited ability to synthesize taurine (Baker and Czarnecki-Maulden 1991). In addition, there are peculiar differences in the syntheses and requirements of certain nutrients between dogs and cats, including taurine (essential for tissue integrity; Pion et al. 1987), arginine (essential for maintaining the urea cycle in an active state; Wu and Morris 1998), ω3 and ω6 polyunsaturated fatty acids (PUFAs; essential for cell structure and function; Bauer 2007), vitamin A (essential for retinal health; Case et al. 2011), and niacin (essential for nutrient metabolism; NRC 2006). Therefore, the concentrations of many AAs, fatty acids, and their metabolites in plasma differ between dogs and cats fed the same diet (Hall 2018b). Regardless of their sources of foods, adequate knowledge of nutrient metabolism and requirements by dogs and cats is crucial to ensure their optimal growth, development, and health. The major objective of this article is to integrate the metabolic characteristics of dogs and cats with their nutrition and feeding management.

4.2 Energy Metabolism

Energy is defined as the capacity to do work (Wu 2018). Energy metabolism is the sum of the metabolism of carbohydrates, lipids, and AAs in animals. Resting metabolic rate (measured after an overnight fast) of a dog and a cat, like other animals, is an estimate of its minimum energy requirement. In dogs living in a thermoneutral environment with a 12-h light period/a 12-h dark period, their heat production rate varies with a circadian rhythm, with the lowest rate in the morning (e.g., 124 kcal/kg BW^0.75/day in beagles), gradually increasing to a maximum in the evening (e.g., 195 kcal/kg BW^0.75/day in beagles), and then declining through the night (Besch and Woods 1977). Such changes in heat production are largely independent of food intake. The rate of heat production by dogs during the dark period is similar to that during the light period (Woods and Besch 1971). In contrast to dogs, no circadian rhythm in heat production was detected in cats (Riond et al. 2003). Furthermore, thermoneutral zones differ between dogs and cats, and even among different breeds of the same species or different sizes of the same breed (Hill and Scott 2004; Middleton et al. 2017). At an ambient temperature of 25 °C, dogs, but not cats, are within their thermoneutral zone.

In dogs and cats, as in other mammals, the maintenance energy requirement supports energy equilibrium over a long period of time and is calculated as 70.5 × BW^0.73 kcal (BW is body weight in kg; Brody et al. 1934), which can be simplified as 70 × BW^0.75 kcal (BW is body weight in kg; Kleiber 1961). Metabolizable energy (ME) is commonly used to express the requirements of dogs and cats for dietary energy. The energy requirements of adult dogs and cats may be related to their body surface areas [0.105 and 0.110 m²/kg of BW^0.67, respectively (Hill and Scott 2004)]. NRC (2006) recommended that intact active adult dogs, normal adult lean cats, and obese cats be fed ∼130 kcal ME/kg BW^0.75/day, 100 kcal ME/kg BW^0.67/day, and 130 kcal ME/kg BW^0.4/day, respectively. Through an extensive meta-analysis, Bermingham et al. (2010) indicated that the maintenance energy requirement of adult cats based on their BW alone may not be accurate and many factors (e.g., age, sex, neuter status, and composition) other than BW should be taken into consideration. Their proposed allometric equations for predicting the maintenance energy requirements of adult light, normal, and heavy cats were 53.7 kcal/kg BW^–1.061, 46.8 kcal/kg BW^–1.115, and 131.8 kcal/kg BW^–0.366, respectively, and were 58.4 kcal/kg lean mass^−1.140.

Chronic imbalances between energy intake and expenditure result in obesity in dogs and cats, as in other animals (Larsen and Oberbauer 2023; Zoran 2023). As reported for many other mammals (e.g., humans, rats, and sheep), brown adipose tissue (BAT) is present in neonatal dogs and nearly absent from adult dogs (Holloway et al. 1985). Likewise, there is more BAT in neonatal cats than in adult cats (Clark et al. 2013; Loncar and Afzelius 1989). Maintaining BAT in an active state plays an important role in preventing obesity in cats. A recent study has indicated the browning and beiging of adipose tissue in dogs (Lyer et al. 2019). Both shivering and non-shivering mechanisms are responsible for increasing heat production in dogs exposed to a cold environment. Epinephrine and norepinephrine stimulate lipolysis in both BAT and white adipose tissues to provide fatty acids and glycerol as metabolic fuels (Wu 2018).

4.3 Lipid Metabolism in Dogs and Cats

4.3.1 Overview of Lipid Metabolism

Lipids are hydrocarbon compounds (e.g., fats, long-chain fatty acids, and cholesterol) that are soluble in organic solvents and, except for short- and medium-chain fatty acids, are insoluble in water. Fats contribute to the palatability and texture of pet foods, and also have important metabolic functions in animals (Wu 2018). The carbon of a fatty acid can be counted from either the carboxyl group (the ∆ nomenclature system) or the methyl group (the ω nomenclature system). In dogs and cats, as in other animals, no double bond can be introduced beyond the ∆⁹ carbon, and therefore, a ω3, ω6, ω7, or ω9 fatty acid (with the first double bond appearing in the 3rd, 6th, 7th, and 9th carbon counted from the methyl group, respectively) remains in the same class of the fatty acid despite its desaturation and elongation (Baker and Czarnecki-Maulden 1991). Animal-sourced, but not plant-sourced foodstuffs contain cholesterol. Cholesterol is an essential component of cell membranes and plays an important role in the metabolism, growth, development, and reproduction of animals, but an excessive amount of this lipid in the blood increases risk for cardiovascular disorders (Wu 2018).

Because no double bond can be formed beyond the ∆⁹ position in all animals, both α-linolenic acid (C_18:3, ω3; ∆^9,12,15) and linoleic acid (C_18:2, ω6; ∆^9,12) are not synthesized by dogs and cats (Table 4.1). These two fatty acids are integral components of the plasma membrane of cells (including erythrocytes and respiratory epithelial cells) for cell integrity, transport, and function (Bryan et al. 2001; Lemaitre et al. 2009), thereby exerting enormous physiological functions in tissues. In addition, α-linolenic acid and linoleic acid are the precursors for the synthesis of eicosapentaenoic acid (EPA; C_20:5, ω3; ∆^5,8,11,14,17) plus docosahexaenoic acid (DHA; C_22:6, ω3; ∆^{4,7,10,13,16,19}) and arachidonic acid (C_20:4, ω6; ∆^5,8,11,14), respectively (Wu 2018). EPA, DHA, and arachidonic acid are also essential components of cell membranes to affect cell integrity, signaling, and homeostasis. Thus, both linolenic acid and linoleic acid must be supplied in the diets for dogs and cats to ensure their survival, growth, development, and health (Table 4.2).

Table 4.1 De novo formation of nutrients in dogs, cats, and pigs

Table 4.2 Dietary requirements of nutrients by dogs, cats, and pigs

Beef tallow and butter contain low content of linoleic acid and α-linolenic acid. Therefore, when fed to dogs and cats, these two foodstuffs should be used along with other ingredients such as corn, soybean, and safflower oils that are rich in linoleic acid and α-linolenic acid (NRC 2006). As reported for humans (Eyjolfson et al. 2004), conjugated linoleic acid (CLA) improves metabolic profiles in obese or overweight dogs (Schoenherr and Jewell 1999) and cats (Bartges and Cook 1999). For example, dietary supplementation with 0.3% CLA to overweight dogs for 120 days decreased fat deposition in subcutaneous tissue and the mass of whole-body white fat mass but increased blood levels of high-density lipoproteins (Rivera et al. 2011). CLA-rich foods are ruminant meat, fats, and dairy products, with the predominant CLA isomer (>90%) being c9,t11 (Larsen et al. 2003).

4.3.2 Lipid Metabolism in Dogs

Like most other mammals (e.g., humans, pigs, and rats), dogs can introduce one or more double bonds to a long-chain fatty acid between its ∆¹ and ∆⁹ carbons through the action of ∆-desaturases (enzymes in the smooth endoplasmic reticulum). Dogs have ∆⁶ desaturase activity and, therefore, can readily convert linoleic acid into arachidonic acid (Dunbar and Bauer 2002). Likewise, growing and young adult dogs can actively form EPA and DHA from α-linolenic acid. However, the synthesis of EPA and DHA is insufficient to meet the needs of these dogs (NRC 2006). Furthermore, as reported for other animals (Bordoni et al. 1988), ∆⁶ desaturase activity in the tissues of dogs may decline markedly with increasing age, thereby reducing EPA and DHA syntheses in older dogs. By contrast, dogs express a high activity of stearoyl-CoA desaturase (a ∆⁹ desaturase in the smooth endoplasmic reticulum) to readily convert palmitic acid (C_16:0; a saturated long-chain fatty acid) into ω7 and ω9 unsaturated fatty acids (Bauer 2007). The desaturation and elongation of long-chain fatty acids to form mono- and polyunsaturated fatty acids in the smooth endoplasmic reticulum are shown in Fig. 4.1.

Fig. 4.1

Elongation of the unsaturated fatty acid chain beyond C₁₈, as well as the formation of monosaturated fatty acids in the smooth endoplasmic reticulum. This metabolic pathway also requires the addition of acetyl-CoA as malonyl-CoA to the hydrocarbon chain, as well as NADPH. The shortening of a polyunsaturated fatty acid chain from C₂₄ to C₂₂ through β-oxidation in the peroxisome is also shown. All desaturases are localized in the smooth endoplasmic reticulum. ∆⁵-D/E = ∆⁵-desaturase and elongase; EL = elongase; SCD = stearoyl-CoA desaturase (∆⁹ desaturase)

Dogs have a wide tolerance for dietary lipids (e.g., at least 40% fat in their diets) and can be maintained on diets containing 5–8% lipids on the DM basis (NRC 2006). A minimum content of 5% lipids in a canine diet is adequate if it provides sufficient essential fatty acids. To meet their requirement for energy, sled dogs are often fed a diet containing at least 50% fat during the racing season (Hill 1998; Loftus et al. 2014). During fasting, dogs mobilize fat in white adipose tissue and rapidly utilize ketone bodies as metabolic fuels (De Bruijne and van den Brom 1986; McCue 2010). This may explain, in part, why the concentrations of acetoacetate and β-hydroxybutyrate in the plasma of fasting dogs are low (< 0.15 mM) during brief and long-term fasting (De Bruijne and van den Brom 1986).

4.3.3 Lipid Metabolism in Cats

In contrast to dogs, cats have a very limited ∆⁶ desaturase activity (Pawlosky et al. 1994; Rivers et al. 1975). Thus, when cats are fed a diet rich in linoleic acid, there is a modest increase in: (1) tissue concentrations of eicosadienoic acid (C_20:2, ω6; Δ^11,14) due to chain elongation, and (2) the formation of sciadonic acid (C_20:3, ω6; ∆^5,11,14) due to the action of ∆⁵ desaturase activity (Sinclair et al. 1981). These metabolic pathways are shown in Fig. 4.1. Sciadonic acid is a biomarker for limited ∆⁶ desaturase activity but active ∆⁵ desaturase in animal tissues (Bauer 1997). Accordingly, cats generate only a limited amount of arachidonic acid from linoleic acid (Sinclair et al. 1981), but dogs and omnivores (e.g., humans and pigs) readily convert linoleic acid to arachidonic acid as noted previously. Cats have a greater requirement for dietary arachidonic acid than dogs. In addition, a low activity of ∆⁶ desaturase in cats limits the conversion of α-linolenic acid into EPA and DHA. Thus, diets for cats must contain EPA and DHA. Daily supplementation of DHA and EPA is recommended for pregnant and lactating cats for the proper brain and nervous system development of their offspring (Vuorinen et al. 2020).

4.4 Carbohydrate Metabolism in Dogs and Cats

4.4.1 Overview of Carbohydrate Metabolism

Glucose is the exclusive energy source for the brain, red blood cells, kidney medulla, and retinal cells of dogs and cats in fed and post-absorptive states (Wu 2018). Because the brain is relatively large in newborn dogs and cats (accounting for 2.74% and 3.61% of BW, respectively; Latimer 1967), a large amount of glucose is used by the brain of puppies and kittens. For comparison, the brain of adult dogs and cats represents 0.85% and 0.80% of BW, respectively (Latimer 1967), which is greater than the brain of cattle and sheep (0.2% of BW, Wu 2018). In addition, glucose is a major metabolic fuel for immune cells (e.g., lymphocytes and macrophages), as well as the primary source of NADPH for anti-oxidative reactions and nitric-oxide synthesis. As noted previously, dogs, unlike cats, have adapted during evolution to omnivorous diets that contain both animal- and plant-sourced foods (Baker and Czarnecki-Maulden 1991). Gluconeogenesis (formation of glucose from non-glucose substrates such as AAs, glycerol, lactate, and odd-carbon number fatty acids) plays an important role in maintaining glucose homeostasis and carbon balances in all mammals, including both dogs and cats (Wu 2018). There are no established requirements of dogs and cats for dietary carbohydrates.

4.4.2 Carbohydrate Metabolism in Dogs

When diets do not provide sufficient starch, glycogen, or glucose, dogs must synthesize glucose from glucogenic AAs, lactate, and glycerol in their liver and kidneys (Belo et al. 1976). In the canine liver and kidneys, phosphoenolpyruvate carboxykinase (PEPCK) is localized to both the cytosol and mitochondria (e.g., equal distribution in the liver but 50% and 65%, respectively, in the kidney under fed conditions) (Croniger et al. 2002; Wolf and Mehlman 1972). Such an intracellular compartmentation of PEPCK allows for the use of all glucogenic substrates (including glucogenic AAs) for glucose synthesis (Feng et al. 1996; Wolf and Mehlman 1972). Note that if PEPCK is exclusively localized to the mitochondria of the liver and kidneys, there is no formation of glucose from AAs due to the absence of NADH provision, as reported for the liver of fed or fasted chickens (Watford 1985). Interestingly, the activities of mitochondrial PEPCK (converting oxaloacetate into phosphoenolpyruvate) and pyruvate carboxylase (converting pyruvate into oxaloacetate) in the liver and kidneys of dogs are enhanced by the feeding of a carbohydrate-free diet (Feng et al. 1996). Intakes of dietary carbohydrates (including starch and fiber) affect postprandial concentrations of glucose and insulin in the plasma of dogs (Carciofi et al. 2008).

In dogs, water-soluble fibers (e.g., plant pectin, fructans, gums, psyllium, and β-glucan) and water-insoluble fibers (e.g., plant cellulose and hemicellulose) play an important role in their intestinal health (Nogueira et al. 2019). Soluble fibers in oats, peas, beans, apples, citrus fruits, carrots, barley, and psyllium form a viscous solution upon contact with water and are highly fermentable in the large intestine (NRC 2006; Silvio et al. 2000). Insoluble fibers in plant cell walls, wheat bran, vegetables, and whole grains retain water, do not form a viscous solution, and are much less fermentable than soluble fibers in the gut. Insoluble fibers stimulate intestinal peristalsis, increase fecal mass, and decrease the transit time of food through the GIT (Schneeman 1994). Some fermentable fibers (e.g., fructooligosaccharides) are beneficial prebiotics for the intestinal health of dogs (Montserrat-Malagarriga et al. 2024; Nogueira et al. 2019).

4.4.3 Carbohydrate Metabolism in Cats

As noted previously, cats consume meat (containing low glycogen content, < 5% on a DM basis) and no starch. Cats, just like dogs, have little or no salivary α-amylase activity (McGeachin and Akin 1979). In addition, pancreatic α-amylase activity is low in cats as compared with dogs. For example, pancreatic α-amylase activity in adult cats is only 2.3% and 2% of that in adult dogs and pigs, respectively (Kienzle 1993a), and the abundance of glucose transporters in the small intestine of cats is much lesser than that in dogs (Batchelor et al. 2011). Therefore, cats have a much lower ability to digest and use dietary starch in the small intestine than dogs, and their consumption of carbohydrate-rich diets can result in intestinal inflammation (Morris et al. 1977; Verbrugghe and Hesta 2017).

Because of a limited intake of glycogen, starch, and sugars in their natural foods, cats primarily depend on constant gluconeogenesis (mainly from AAs) to provide glucose (Legrand-Defretin 1994). This metabolic pathway is particularly significant for newborn cats with relatively a large brain (accounting for 3.62% of BW; Latimer 1938), where a large amount of glucose is used via glycolysis and the Krebs cycle. For comparison, the brain of adult cats represents 0.80% of BW (Latimer 1967), which is similar to that for adult dogs. Unlike dogs, glucokinase activity is absent from the feline liver (Washizu et al. 1999) just like the ruminant liver (Wu 2018). By contrast, the activities of other glycolytic enzymes (e.g., hexokinase I, phosphofructokinase, and pyruvate kinase) in the feline liver are substantially greater than those in the canine liver (Washizu et al. 1999). Furthermore, the activities of rate-controlling enzymes for gluconeogenesis (e.g., pyruvate carboxylase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase) in the feline liver are also much greater than those in the canine liver (Washizu et al. 1999). This is consistent with a greater rate of whole-body glucose production in cats than in dogs (Legrand-Defretin 1994). Interestingly, hepatic PEPCK activity is not altered in cats that are fasted after consuming high-protein diets (Kettelhut et al. 1980) or after the protein content of their diet is increased from 17.5% (low) to 70% (high) (Rogers et al. 1977). These results support the view that cats express high basal activities of hepatic gluconeogenic enzymes to ensure the rapid conversion of excess dietary AAs into glucose. In cats, gluconeogenesis plays an essential role in the provision of glucose to their brain, red blood cells, and immunocytes and therefore their survival.

Just like dogs, carbohydrates (including water-soluble and insoluble fibers) are also extensively fermented by microbes in the large intestine of cats (Brosey et al. 2000; Legrand-Defretin 1994; Morris et al. 1977). For example, in adult cats, the prececal apparent digestibility of starch (e.g., cooked corn starch, 72%; raw corn starch, 46%; raw potato starch, 0%) is lower than its total apparent (fecal) digestibility (cooked corn starch was digested to nearly 100%, raw corn starch to 78%, raw potato starch to 36%), but the pattern of apparent digestibility of raw starch is similar to that for the cooked starch (Kienzle 1993b). This may help to explain the observation that the concentration of glucose in adult cats fed a 31.7%-starch diet was continuously elevated from 4.6 mM at 3 h after the meal, compared with the pre-meal baseline (4.8 mM) to 6.2 mM at 19 h after feeding, whereas the concentration of glucose in adult cats fed a 23%-starch diet did not differ from the pre-meal level or during the period of 3 and 19 h after the meal (Hewson-Hughes et al. 2011). Due to its microbial fermentation, starch led to an acidification of the large bowel chyme and feces, and the effect of dietary undigested starch can be alleviated by oral antibiotics in cats (Kienzle 1993b). This may explain, in part, why cats should not be fed dog food and require carefully formulated diets. Commercial diets for cats generally contain a small amount of water-soluble and insoluble fibers (a total of 3–5% fibers, DM basis) to maintain intestinal health (Zoran 2023).

4.5 Protein and AA Metabolism in Dogs and Cats

4.5.1 Overview of Protein and AA Metabolism

Both dogs and cats have a high ability to digest dietary protein in the gastrointestinal tract and degrade dietary AAs in a tissue-specific manner (Galim et al. 1980; Beliveau and Freedland RA 1982; Hammer et al. 1996; Hu et al. 2021; Li and Wu 2023a; Wu 1998). These mammals, just like all other animals, cannot form de novo the carbon skeletons of Cys, His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Tyr, and Val, but can convert Phe into Tyr, Met into Cys, and Trp to serotonin (Wu 2022). Some metabolites of tyrosine (melanins) are crucial for the color of hair and skin health (Anderson et al. 2002; Biourge and Sergheraert 2002; Yu et al. 2001), whereas serotonin regulates animal behavior (Landsberg et al. 2017) and intestinal function (Chiocchetti et al. 2021). In addition, these two animal species, young or adult, are not capable of synthesizing sufficient arginine from glutamine/glutamate and proline (Burns et al. 1981; Czarnecki and Baker 1984; Weber et al. 1977; Yu et al. 1996). This is likely due to a deficiency of pyrroline-5-carboxylate synthase, proline oxidase, carbamoylphosphate synthetase I, and/or N-acetylglutamate synthetase as well as ornithine aminotransferase in enterocytes of the canine and feline small intestines (Dillion and Wu 2021; Morris 2002; Rogers and Phang 1985; and Wu and Morris 1998). In addition, most breeds of dogs,with possible exceptions of a few, can synthesize sufficient taurine when fed methionine- and cysteine-adequate diets (Jacobsen and Smith 1968). Interestingly, golden retrievers, American Cocker Spaniels, and a small population (1.3–2.5%) of the Newfoundlands may be genetically predisposed to taurine deficiency possibly due to gene mutations (Backus et al. 2003, 2006; Kaplan et al. 2018; Kittleson et al. 1997). By contrast, all breeds of cats have a very limited ability to convert cysteine into taurine due to low activities of cysteine dioxygenase and cysteinesulphinic acid decarboxylase in the liver (Knopf et al. 1978). Thus, Arg, His, Ile, Leu, Lys, Met, Phe, Thr, Trp, and Val must be included in canine and feline diets; taurine must be provided to all cats and some breeds of dogs. As reported for pigs, rats, fish and crustaceans (He and Wu 2022; He et al. 2023; Hou et al. 2015, 2016; Hou and Wu 2015; Li et al. 2020, 2021b,c, 2023; Wu 2009; Wu and Li 2022), dogs and cats may have dietary requirements for AAs (e.g., glutamate, glutamine, glycine, and proline) that are synthesized de novo and also use dietary AAs as major metabolic fuels. Their quantitative amounts have recently been suggested for both animal species during the growth period and in adulthood (Li and Wu 2023a). Growing evidence shows that AAs are signaling molecules to activate the mechanistic target of rapamycin pathway for initiating protein synthesis in animal cells (Rezaei and Wu 2022), as well as regulating energy sensing and metabolism (Jobgen et al. 2022a, b, 2023). Both dogs and cats have dietary requirements for AAs but not protein or nitrogen. In the following sections, we focus on arginine and cysteine in canine and feline nutrition, as well as AA imbalances and antagonisms, metabolic adaptation to AA intakes, and AA requirements.

4.5.2 Arginine Metabolism in Dogs and Cats

In most mammals, including dogs and cats, the small intestine synthesizes de novo and releases citrulline, which is taken up by extraintestinal tissues (primarily the kidneys and, to a lesser extent, all other tissues) for arginine synthesis (Levillain et al. 1996; Wu and Morris 1998). However, endogenous synthesis of arginine is limited in both animal species, as noted previously. All mammalian tissues contain arginase for the hydrolysis of arginine into urea and ornithine, with ornithine being either converted into proline, glutamate, polyamines, agmatine, and NO or oxidized to CO₂ in a cell-specific manner (Wu et al. 2016b). Thus, the dietary intake, endogenous synthesis, and catabolism of arginine regulate its homeostasis in the body.

Consistent with the lower rate of arginine synthesis in cats than in dogs, the milk of cats contains more arginine than the milk of dogs (Davis et al. 1994a, b) to support feline survival, growth, and development. In addition, the content of arginine in the milk of dogs is greater than that in the milk of herbivores (e.g., cows) and omnivores (e.g., humans and pigs) to compensate for a lower rate of arginine synthesis in the dogs (Heinze et al. 2014). Current commercial vegan foods may not provide sufficient arginine for cats (Zafalon et al. 2020b).

A deficiency of arginine results in decreased food intake, impaired ureagenesis, hyperammonemia, abnormal hemodynamics, hypertension, severe emesis, frothing at the mouth, muscle tremors, and cataract in dogs (Burns et al. 1981; Czarnecki and Baker 1984; Ranz et al. 2002). Cats have a lower rate of endogenous arginine synthesis, ingest more AAs, and are more sensitive to a deficiency of dietary arginine than dogs. Thus, a lack of dietary arginine rapidly causes hyperammonemia (occurring within 1–3 h after feeding), vomiting, neurological signs, severe emesis, ataxia, tetanic spasms, and death in cats (Morris and Rogers 1978). Dietary supplementation with arginine, citrulline, or ornithine to dogs and cats can prevent hypoargininemia-induced hyperammonemia and mortality. Dietary or intravenous administration of arginine or citrulline, but not ornithine, can restore growth in young cats fed arginine-free or deficient diets (Czarnecki and Baker 1984). This is due to the complex compartmentalization of ornithine metabolism in enterocytes, as extracellular ornithine is preferentially channeled into the formation of proline but not citrulline in these cells (Wu and Morris 1998). Greater protein requirements by cats than dogs may be explained, in part, by a lower ability of cats to synthesize arginine than dogs and a need for the greater provision of arginine from feline than canine diets (Li and Wu 2023a).

4.5.3 Cysteine Metabolism in Dogs and Cats

All cats and some breeds of dogs have a limited ability to synthesize taurine from cysteine (a metabolite of methionine; Fig. 4.2), as noted previously. This metabolic pathway occurs in the liver of dogs and cats, with dietary cysteine being capable to replace up to 50% dietary methionine (Che et al. 2021; Oberbauer and Larsen 2021). In all animals, the synthesis of cysteine requires methionine and serine, as the sulfur atom of cysteine is derived from methionine, whereas serine provides both the amino group and the whole carbon skeleton of cysteine (Wu 2022). Of particular note, taurine is abundant in animals-sourced foodstuffs but is absent from plants (Hou et al. 2017, 2019; Li et al. 2021a). In dogs and cats, the primary bile acids are conjugated exclusively and almost exclusively with taurine, respectively, to form bile salts, which are required for the digestion and absorption of dietary lipids (including long-chain fatty acids and lipid-soluble vitamins). In healthy dogs, the primary bile acids are cholic acid, deoxycholic acid, and chenodeoxycholic acid; their tauro-conjugated forms account for approximately 73%, 20%, and 6%, respectively, of the total bile acid pool in the gallbladder (Washizu et al. 1994); and there are no glycine-conjugated bile acids in the liver or bile (Wildgrube et al. 1986). In healthy cats, the major primary bile acids are cholic acid and deoxycholic acid, and their distribution percentages (%) are 98.4% taurine-conjugated form (79.1% cholic acid and 19.3% deoxycholic acid), 1.1% glycine-conjugated cholic acid, and 0.5%of unconjugated cholic acid (Rabin et al. 1976).

Because the content of methionine and cysteine in plant-sourced ingredients is generally lower than that in animal-sourced ingredients, an inadequate intake of methionine and cysteine in plant-based foods can result in a deficiency of taurine in both dogs and cats (Cavanaugh et al. 2021; Knopf et al. 1978; McCauley et al. 2020). A deficiency of taurine in these companion animals can result in dilated cardiomyopathy that is characterized by thin heart muscle and enlarged chambers (Cavanaugh et al. 2021; McCauley et al. 2020; Morris 2002). Additional symptoms in cats include heart failure, central retinal degeneration, blindness, deafness, poor reproduction, and impaired immune responses (Morris 2002; Pion et al. 1992). All foods for cats and some breeds of dogs must provide sufficient taurine to maintain the integrity and health of their tissues. Morris et al. (1990) reported that the content of 0.12% taurine (on a DM basis) in commercial expanded (dry) cat foods was sufficient for cats, but the content of taurine in canned diets should be increased to 0.20–0.25% (DM basis) because heating during the canning process forms products that promotes the loss of taurine via the enterohepatic circulation.

Fig. 4.2

Synthesis of taurine from L-cysteine in the liver of dogs and cats. In the presence of serine, L-methionine is degraded on the liver via the transsulfuration pathway to generate cysteine and α-ketobutyrate. Subsequently, L-cysteine is oxidized to hypotaurine via reactions catalyzed by cysteine dioxygenase and cysteinesulfinate decarboxylase. Hypotaurine is spontaneously oxidized to taurine. Most breeds of dogs can synthesize sufficient taurine when fed L-methionine- or L-cysteine-adequate diets. However, this synthetic pathway is limited in cats due to low activities of cysteine dioxygenase and cysteinesulfinate decarboxylase

Another metabolite of cysteine via cooperation of the liver and kidneys in both dogs and cats is isovalthine, the formation of which also requires leucine (Wu 2022). Interestingly, male cats produce more felinine than females (Hendriks et al. 2004). In cats but not dogs, cysteine is also used to generate felinine and isobuteine from isopentenyl pyrophosphate (formed from acetyl-CoA) and valine, respectively, via cooperation of the liver and kidneys (Che et al. 2021; Hendriks et al. 2008; Rutherfurd-Markwick et al. 2005). Cysteine participates in all these reactions in the form of glutathione. These synthetic pathways may contribute to greater requirements of cats for cysteine, as compared with dogs (Li and Wu 2023a). Physiological functions of felinine, isovalthine, and isobuteine remain unknown but may serve as pheromones in cats for the purpose of territorial marking and intra-species communications (e.g., chemical signals to attract females) (Miyazaki et al. 2008).

4.5.4 AA Imbalances and Antagonisms, as Well as Metabolic Adaptation to AA Intakes

Compared with non-collagen animal proteins, plant proteins (particularly those in cereals) generally contain low amounts of nearly all AAs, particularly the AAs (e.g., lysine, tryptophan, threonine, methionine, and cysteine) that are not synthesized de novo and the AAs (glycine and proline + hydroxyproline) that are most abundant in the body (Li et al. 2021d). Dietary imbalances of AAs (defined as improper ratios of AAs in diets) can occur in dogs and cats fed plant-based diets containing a small amount of or no animal-sourced ingredients (Li and Wu 2023a). Antagonisms among AAs with similar chemical structures (e.g., BCAAs) or net electric charges (e.g., arginine and lysine) that share the same transmembrane transporters can occur in response to their improper ratios in diets or tissue (Wu 2022). Effects of dietary AA imbalances or antagonisms in dogs are more similar to those in rats than in cats based on alterations in food intake, plasma metabolic patterns, and hepatic enzyme activities (Morris 2002). Animal-sourced foods are abundant providers of all proteinogenic AAs as well as taurine and 4-hydroxyproline and play an important role in balancing AAs in diets for dogs and cats (Che et al. 2021; Li and Wu 2018; 2020; Li et al. 2021a; Wu et al. 2016a).

Dogs and cats have a high ability to use and oxidize dietary AAs, with the rate of AA catabolism in cats being greater than that in dogs under both fed and post-absorptive conditions (Morris 2002; Russell et al. 2002; Wester et al. 2015). For example, dogs and cats can tolerate at least 32% and 60% dietary protein, respectively (DeNapoli et al. 2000; Dodman et al. 1996; Green et al. 2008; Laflamme and Hannah 2013). These animals, just like most of other mammals (e.g., pigs and rats) can adapt to: (1) low-protein diets by increasing food intake and reducing whole-body AA catabolism, and (2) high-protein diets by reducing food intake and up-regulating the expression of AA-catabolic enzymes (Harper et al. 1984; Rogers et al. 1998). However, dogs are not as efficient as rats in metabolic adaptation to low- or high-protein intakes. In support of this view, studies with adult dogs have shown that the rate of whole-body protein degradation is not affected by low dietary protein intake and that the rate of whole-body leucine oxidation is not influenced by high dietary protein intake (e.g., increasing from 32 to 148 g CP/Mcal ME) (Humbert et al. 2002). In contrast, cats rapidly respond to protein-free diets by decreasing the rate of whole-body AA oxidation (Hendricks et al. 1996) and to elevated protein intake by increasing the rate of whole-body AA oxidation (Hendricks et al. 1997; Wester et al. 2015).

4.5.5 AA Requirements of Dogs and Cats

Compared with dogs, cats have greater a loss of endogenous nitrogen (urinary plus fecal nitrogen) when fed nitrogen-free diets (Hendriks et al. 1996 and 2002; Kendall et al. 1982). The whole-body degradation of protein provides AAs (e.g., arginine) for metabolic utilization. Accordingly, cats have greater requirements for dietary AAs than dogs (Mansilla et al. 2018; 2020a,b; NRC 2006). Quantitative requirements of dogs and cats for dietary AAs may be estimated from experiments involving nitrogen balance, the factorial analysis of use of AAs for growth and product (e.g., milk and hair) formation, and the oxidation of direct and indicator AAs (Lambie et al. 2024; Singh et al. 2024; Wu 2022). Interestingly, differences in dietary requirements for EAAs between these two animal species do not appear to be substantial (Rogers and Morris 1979). It is possible that cats degrade NEAAs at greater rates in a tissue-specific manner, as compared with dogs, with some of NEAAs being used for hepatic and renal gluconeogenesis. This is physiologically and nutritionally important because feline diets contain only a small amount of glycogen or starch (Washizu et al. 1999).

Skeletal muscle is the largest tissue in healthy animals. Like other mammals (e.g., humans), elderly dogs and cats experience sarcopenia (aging-related progressive losses of skeletal muscle mass and strength) because the rate of protein synthesis is lower than the rate of proteolysis (Kealy 1999; Laflamme 2008a,b). Sarcopenia results in not only physical weakness and disability but also tremendous increases in risks for falls, fractures, morbidity, and mortality, as well as health care costs. Exercise and AAs (e.g., arginine, branched-chain AAs, glutamine, glycine, and tryptophan) are co-stimuli to activate the mechanistic target of rapamycin cell signaling pathway and protein synthesis in skeletal muscle, thereby mitigating the loss of muscle protein with aging in humans (Glynn et al 2010; Holowaty et al. 2023; Mitchell et al. 2015). This is true for dogs and cats (Laflamme and Hannah 2013; Laflamme and Danièlle 2014; Williams et al. 2001), although the underlying signal transduction mechanisms have not yet been elucidated. There are suggestions that dietary protein requirements be increased by approximately 50% and 2.5-fold in older dogs and cats, respectively, compared with young adults (Laflamme 2008b; Laflamme and Hannah 2013; Wannemacher and McCoy 1966). This is equivalent to 2.55 and 5 g protein/kg BW/day for healthy adult dogs and cats, respectively (Churchill and Eirmann 2021). Considering long-term kidney health, we (Li and Wu 2023a) recommended adequate intakes of high-quality protein (i.e., 32% and 40% animal protein in diets for aging dogs and cats, respectively; DM basis) to alleviate sarcopenia. This goal can be achieved through the inclusion of pet-food grade animal-sourced foodstuffs in canine and feline diets (Li et al. 2021a, d; Li and Wu 2022, 2023b).

4.6 Vitamin Metabolism in Dogs and Cats

4.6.1 Vitamin Metabolism in Dogs

Like most animals, dogs can convert: (1) dietary β-carotene into vitamin A (all-trans-retinol) in enterocytes of the small intestine (Schweigert 1998), (2) tryptophan into niacin in the liver (Krehl 1981), and (3) glucose into vitamin C in the liver (McDowell 1989). Thus, β-carotene and tryptophan can replace some vitamin A and niacin in canine diets, respectively, and dogs do not have a dietary requirement for vitamin C. However, in contrast to other mammals (e.g., sheep, cattle, horses, pigs, rats, and humans), dogs have a limited ability to synthesize vitamin D₃ (cholecalciferol) from 7-dehydrocholesterol in the skin exposed to sunlight and, therefore, depend on the dietary intake of vitamin D₃ for bone health (How et al. 1994; Morris 1999). This is because the skin of dogs has a low concentration of 7-dehydrocholesterol and a high activity of 7-dehydrocholesterol ∆⁷ reductase to rapidly convert 7-dehydrocholesterol into cholesterol (Zafalon et al. 2020a). For example, concentrations of 7-dehydrocholesterol in the skin of dogs are only about 10% of those in the skin of rats (How et al. 1994). Cholecalciferol is generally added to the commercial diets for dogs, although dogs can effectively use the plant-sourced vitamin D₂ (ergocalciferol). Synthetic vitamin K₃ (menadione sodium bisulfite; water-soluble) is used in the commercial diets of dogs. Dogs do not synthesize other vitamins required for cell metabolism (NRC 2006). Lipid- and water-soluble vitamins are excreted from the body mainly via bile (feces) and urine, respectively (Wu 2018).

4.6.2 Vitamin Metabolism in Cats

Vitamin B₆, Vitamin B₁₂, folate, and choline, along with methionine, serine, and histidine, play an important role in one-carbon metabolism in cats, as in other animals (Verbrugghe and Bakovic 2013). Like dogs, cats synthesize vitamin C and do not require a dietary source of this vitamin under normal feeding and environmental conditions (McDowell 1989), and also have a very limited ability to synthesize vitamin D₃ from 7-dehydrocholesterol in response to sunlight (How et al. 1994). This is because of a very low concentration of 7-dehydrocholesterol and a high activity of 7-dehydrocholesterol ∆⁷ reductase to rapidly convert 7-dehydrocholesterol into cholesterol (How et al. 1994; Morris 1999), as reported for dogs. Cholecalciferol is generally added to the commercial diets for cats. Unlike dogs, cats cannot convert dietary β-carotene into vitamin A due to the absence of β-carotene 15,15-dioxygenase (Gershoff et al. 1957), although β-carotene is absorbed by the enterocytes of the small intestine (Schweigert et al. 2002). In addition, cats cannot use dietary vitamin D₂ (the vitamin D in sun-dried plants). Furthermore, cats cannot synthesize niacin from tryptophan due to (a) a high activity of picolinate carboxylase to divert 2-amino-3-carboxymuconate semialdehyde into α-ketoadipate, and (b) a low activity of quinolinate phosphoribosyltransferase for converting 2-amino-3-carboxymuconate semialdehyde into nicotinate mononucleotide (Badawy 2017). Therefore, feline diets must provide vitamin A, vitamin D₃, and niacin. Other vitamins are not synthesized by cats and are required in their diets for cell metabolism (NRC 2006).

4.6.3 Anti-vitamin Factors in Foods

Some foods contain anti-vitamin factors. For example, thiaminase is present in the viscera of some freshwater fish [e.g., common bream (Abramis brama), central stoneroller (Campostoma anomalum), goldfish (Carassius auratus), and common carp (Cyprinus carpio)], some marine fish [e.g., broad-striped anchovy (Anchoa hepsetus), Atlantic herring (Clupea harrengus), Atlantic cod (Gadus morhua), and Yellowfin tuna (Neothunnus macropterus)], and some bacteria (Greig and Gnaedinger 1971). This enzyme is heat-labile and is inactivated by cooking. Among the 32 species of freshwater fish tested, 18 of them were found to contain thiaminase. Among the 61 marine fish tested, 32 were found to contain thiaminase (Lichtenberger 2021). In other words, 56% of the freshwater fish examined contained thiaminase as compared with 51% of the marine fish species. Interestingly, there is little or a small difference in thiaminase activity between freshwater and marine fish. Furthermore, raw egg white contains avidin (a glycoprotein) that binds biotin very tightly. Fortunately, avidin is heat-labile and can be inactivated by cooking. Caution should be taken when feeding dogs and cats with raw fish or raw eggs.

4.7 Mineral Metabolism in Dogs and Cats

4.7.1 Mineral Metabolism in Dogs

In dogs, nutritionally essential macrominerals are Ca, P, Na, Cl, K, and Mg, whereas nutritionally essential microminerals are Fe, Zn, Cu, Mn, I, and Se (NRC 2006). Possibly essential minerals for these animals are Mo, boron, and Cr. Methionine and cysteine provide the sulfur atom required for the structures of protein and other molecules in dogs. Dietary inorganic sulfur is not required by these animals. Minerals are essential for nutrient transport, bone growth and development, the activities of proteins (including enzymes), and acid–base balance (Wu 2018). However, excessive Ca intake (e.g., > 0.31% of dietary DM) results in skeletal abnormalities in growing dogs (Hazewinkel et al. 1991; Hedhammar et al. 1974). Likewise, excessive Ca and P intake (0.31% Ca and 0.28% P of dietary DM) during early maturation (3 to 17 weeks of age) in dogs alters Ca and P balances by as early as 9 weeks of age, but dietary normalization (0.1% Ca and 0.08% P of dietary DM) during 17 to 27 weeks of age can effectively mitigate any long-term adverse effects on Ca and P balance (Schoenmakers et al. 1999).

Dogs ingesting 4 g NaCl/kg BW/day showed no sign of appreciable salt retention for 6 days if they have free access to drinking water (Ladd and Raisz 1949). However, seizures, hypernatremia [serum sodium concentration (211 mM; reference range, 145–158 mM) and serum chloride concentration (180 mM; reference range, 105–122 mM), and death occur after consuming an excessive amount of sodium from a salt–flour mixture (finely ground salt; Khanna et al. 1997). Dogs consuming excessive salts should have free access to sufficient drinking water.

4.7.2 Mineral Metabolism in Cats

Cats have the same requirements for minerals as dogs but can tolerate a higher intake of dietary salt than dogs (NRC 2006). Cats are commonly beset with the problem of urolithiasis, namely the formation of sediment (consisting of one or more poorly soluble urine crystalloids) within the urinary tract (Houston et al. 2003). Magnesium ammonium phosphate (also known as struvite) and calcium oxalate (CaOx) are often present in the majority of such deposits in alkaline urine (Robertson et al. 2002). In North America, struvite and CaOx stones are the most and second most common mineral types found in the feline uroliths. Thus, caution must be exercised to avoid an excess of dietary magnesium and calcium. In practice, sufficient water intake can reduce risk for urolithiasis in cats. In addition, a high dietary intake of phosphorus and a low ratio of calcium-to-phosphorus in diets increases risks for kidney damage in cats (Summers et al. 2020). Furthermore, the content of minerals in the diet can affect its palatability for cats and must be monitored carefully. Of note, there is evidence that current commercial vegan foods for dogs and cats may not provide: (a) adequate calcium, potassium, and sodium for dogs; or (b) sufficient potassium and proper Ca/P ratio for cats (Zafalon et al. 2020b). This nutritional problem can be prevented by the inclusion of animal-sourced foodstuffs, which contain relatively a large amount of minerals (Li and Wu 2022, 2023b).

4.8 Water Metabolism in Dogs and Cats

4.8.1 Overview of Water Metabolism

Water is the quantitatively most important and most abundant nutrient for all animals, and its loss occurs via evaporation, respiration, urine, and feces (Wu 2018). Drinking and dietary (moist food) water are by far the major sources of water in animals, as the production of metabolic water from the oxidation of nutrients is limited in most mammals including dogs and cats (e.g., 1.09, 0.60, and 0.41 g water for the oxidation of 1 g fat, glucose, and protein, respectively (Wu 2018). Thirst results from both intracellular and extracellular dehydration (e.g., a loss of 0.5–1% of body water), as well as an increase in plasma osmolality, leading to the stimulation of water consumption (NRC 2006). It usually takes only days for animals (including dogs and cats) to be dehydrated particularly in an environment with elevated temperatures, but weeks or longer before clinical signs of a deficiency of a non-water nutrient are evident. In both dogs and cats, clinical signs of water deficiency include the loss of elasticity of the skin, reduced saliva secretion, reduced appetite, reduced blood volume, impaired blood circulation, the formation of bladder and kidney stones, and the dysfunction of tissues. These abnormalities appear when water stores in dogs are decreased by 5%, and death may occur when the body loses about 15% water (Kirk and Bistner 1981).

4.8.2 Water Metabolism in Dogs

Thirsty dogs can drink sufficient water within minutes to restore the losses of water, whereas starved dogs reduce water consumption by 67–80%. By contrast, increasing protein or salt intake increases water consumption in dogs. Voluntary water consumption by dogs is affected by the moisture of ingested foods. The ratio of total water intake to DM intake is 2.33 ml water for 1 g DM food. At 22–25 °C, a dog needs 50–60 ml water/kg BW/day (Schaer 1989). Water intake is increased when dogs exercise or are exposed to heat stress. Exposure to cold does not appear to affect water consumption.

4.8.3 Water Metabolism in Cats

Like dogs, cats require water as the most abundant nutrient and can maintain body water balance when fed meat containing ~ 70% water. At 22–25 °C, a cat needs 45–55 ml water/kg BW/day (NRC 2006). Cats have a lower physiological thirst drive than dogs and concentrate their urine (through the reabsorption of water in the lumen of kidney tubules into the blood) to a greater degree than dogs, which helps to conserve water (Anderson 1982; Chew 1965). In addition, the dense hair of cats can minimize the evaporative loss of water through the skin. Thus, cats may have a lower requirement for exogenous water than dogs under similar feeding conditions and can tolerate mild dehydration better than dogs (NRC 2006).

A loss of 20% water from cats can be fatal (Anderson 1982). Compared with continuous feeding, periodic feeding reduces the daily consumption of food and water by cats (Finco et al. 1986). In both dogs and cats, drinking too much water may result in adverse effects. As important as it is to avoid dehydration, water intoxication in dogs due to their excessive water intake may occur during swimming, diving, or water-retrieving (Flaim 2019). A cat drinking an excessive amount of water (e.g., > 100 ml/kg BW/day) may appear thirsty, but may actually have potential health problems (e.g., diabetes mellitus, chronic kidney disease, and hyperthyroidism; Sparkes et al. 2015).

4.9 Feeding Behavior and Management of Dogs and Cats

4.9.1 General Considerations

The evolution of domestic dogs and cats has resulted in the idiosyncrasies of their digestive tract, metabolism, and nutrient requirements. Both dogs and cats can develop learned taste aversions (Mugford 1977), avoid arginine-deficient diets (Morris 2002), prefer flavor components in meat and peptides (e.g., protein hydrolysates) and free AAs (e.g., alanine, histidine, leucine, lysine, and proline, as well as monosodium glutamate plus 5’-nucleotides (Hargrove et al. 1994; Kumazawa and Kurihara 1990a,b). In both animal species, age and the percentage of lean or fat body mass can influence food intake (Hall et al. 2018a, b). Their appetite is regulated by signals produced in the hypothalamus and peripheral organs (e.g., the digestive tract and white adipose tissue), as well as by the concentrations of glucose, fatty acids, amino acids, and their metabolites in the blood (Wu 2018). In addition, overweight or obesity occurs in dogs and cats if they are allowed to eat as much as they want at all times during the day without adequate exercise (Zoran 2010). Furthermore, spaying (the removal of ovaries from females) and neutering (the removal of testes from males) are performed as a management procedure before reaching sexual maturity (e.g., at or before 5 and 6 months of age in cats and dogs, respectively) with adequate nutritional support to prevent any unwanted behaviors (Hart et al. 2020; Vendramini et al. 2020). In both dogs and cats, care must be taken to prevent gastrointestinal parasites, as well as infectious diseases [e.g., canine and feline parvovirus (highly contagious viral diseases of dogs and cats, respectively) that cause acute gastrointestinal illness] (Laflamme and Hannah 2010). Of particular note, cow or goat milk is not a suitable replacement milk for both puppies and kittens, because the ruminant milk contains much less protein, arginine, taurine, methionine, fat, ME, calcium, phosphorus, sodium, copper, iron, and oleic acid than dog and cat milk (Debraekeleer et al. 2010; Gross et al. 2010), as shown in Table 4.3. Orphaned puppies and kittens can be fed commercial milk replacers especially produced for them. However, despite the above shared similarities, dogs and cats differentially select diets containing different compositions of macronutrient when given choices to eat foods with similar palatability (Hall et al. 2018a). These peculiar aspects of canine and feline nutrition are highlighted in the following paragraphs. Such knowledge will aid in our understanding of the different feeding behaviors of dogs and cats. This, in turn, can help to develop optimal management methods to raise, nurture, and care for them.

Table 4.3 Concentrations of nutrients in the milks of cats, dogs, cows, goats, and pigs^a

4.9.2 Weaning of Dogs and Cats

Weaning refers to the full transition of the puppy's diet from its mother's liquid milk to the solid growth diet. This is a significant and stressful period for young dogs. In dogs, weaning can begin at 3–4 weeks of age (Case et al. 2011). A small amount of canned pet food (usually consisting of ingredients of animal origin, such as meat, poultry, fish, and animal by-products) mixed with water (1:1, g/g) can be introduced to puppies 1 week before weaning so that they can adapt more easily to solid foods. Care must be taken to protect the intestinal microflora and prevent intestinal disorders in post-weaning dogs, such as gastritis, abnormal stool, gut atrophy, diarrhea, enteritis, leaky gut syndrome, inflammatory bowel disease, and vomiting (Case et al. 2011). The weanling diet must contain sufficient proteinogenic AAs (particularly arginine, glutamate, glutamine, glycine, methionine, proline, and tryptophan) and taurine (Wu 2018).

Weaning of cats usually begins when kittens are ~ 4 weeks of age by separating them from their mother for a few hours at a time each day and is completed at ~ 8 weeks of age (Case et al. 2011). Moistened kitten food and clean water bowls can be provided to kittens before the nursing phase ends, so as to stimulate the development of their digestive tract and facilitate their transition from the mother’s liquid milk to solid weaning diets. As for dogs, weaning allows kittens to gradually progress from the sole maternal care to social independence. This is a critical period in neonatal life when environmental stresses should be minimized or prevented. During weaning, cats must be provided with diets containing high-quality protein and taurine to ensure optimal nutrition for the small intestine and the whole body.

4.9.3 Food Selection of Dogs and Cats

Dogs are similar to other carnivores in the GIT anatomy but to omnivores in the metabolism of most nutrients. This characteristic is useful to guide canine feeding. Dogs have evolved in their ability to digest a relatively large amount of cooked starch (Axelsson et al. 2013; Félix et al. 2012) and select a diet lower in protein (30% of ME from protein) than a high-protein (52% of ME from protein) diet of wild wolves (Buff et al. 2014). Thus, dogs have more latitude in the selection of food ingredients and more flexible adaptability to both animal- and plant-sourced diets (NRC 2006). In addition, dogs have receptors for sweet substances and, therefore, have preference for 10% sucrose solution, compared to water. When offered complete and balanced diets with varying levels of protein, fat, and carbohydrate, dogs will choose a high-fat (63% of ME from fat) diet over a low-fat (15% of ME from fat) diet (Bradshaw 2006; Buff et al. 2014; Hewson-Hughes et al., 2016). Regarding dietary protein choices, dogs are more like rats than cats. When offered various levels of protein in diets, dogs will select the diets containing 25–30% of energy as protein (Romos and Ferguson 1983; Tôrres et al. 2003). Dogs can eat powdered, dry, semi-moist, or canned diets. Food should be stored in under air-tight, dry, and cool conditions.

Cats are picky eaters (Pekel et al. 2020). Preferring meat with palatability factors (e.g., fats, AAs, peptides, and nucleotides) as food, cats eat small prey, such as rats, mice, birds, lizards, and insects. Odor, taste, texture, particle size, and temperature of diets affect the food preference of cats. Cats may avoid a particular diet because of the textural difference but not the difference in its nutrient composition. Clearly, cats prefer (1) moist foods to dry foods, (2) warm foods to cold or hot foods, (3) high-protein (e.g., 50% protein on a DM basis) foods to low-protein (e.g., 20% protein on a DM basis) or protein-free foods, (4) animal fat to plant fat, and (5) diets containing 25% total lipids to diets containing 10% or 50% total lipids (Cook et al. 1985, 1996; Eyre et al. 2022; Morris 2002; Rogers et al. 2004). Thus, cats would select beef tallow over butter and chicken fats, but have similar preference for beef tallow, lard, and partially hydrogenated vegetable oil (Kane et al. 1981). A diet consisting of high fat content and 5% hydrogenated beef tallow is particularly palatable to cats (Rogers et al. 2004). By contrast, medium-chain fatty acids, present in either the free form (e.g., 0.1% caprylic acid) or TAG (e.g., 5% medium-chain TAG), and 25% hydrogenated coconut oil in diets can reduce food intake by cats (MacDonald et al. 1985). Of particular note, cats cannot taste sweetness as indicated previously, because of the lack of sweet taste receptors due to the deletion of the Tas1r gene and, unlike dogs, do not select sweet substances such as sucrose (Li et al. 2005, 2006). For this reason, cats show no preference for sugar-rich foods such as fruits and juice.

When cats are offered complete and balanced diets with varying levels of protein, fat, and carbohydrate (without controlling palatability), cats will choose high-protein diets (Bradshaw 2006; Hewson-Hughes et al. 2016). Thus, high-starch diets over a prolonged period of time could be a major contributing to poor feline health and must be avoided in feeding both young and adult cats particularly those with overweight or diabetes (Verbrugghe and Hesta 2017). Likewise, domestic cats select a macronutrient profile (52% and 48% of ME from protein and fat, respectively) similar to the diet (52%, 46%, and 2% of ME from protein, fat, and glycogen, respectively) of wild cats. With regards to AAs, cats have preference for proline, cysteine, ornithine, lysine, histidine, alanine, and leucine (Bradshaw et al. 1996), but against arginine, isoleucine, phenylalanine, and tryptophan (Oliveira et al. 2016; Zaghini and Biagi 2005). This is consistent with the notion that cats are more sensitive to bitter taste than dogs. Exposure of kittens to numerous different flavors and textures early in life can result in wider preference for such foods later in life (Kuo 1967). However, adult cats can learn to select new and better flavors in addition to the ones early in life (Mugford 1977).

Healthy cats do not select sodium even if they are deficient in sodium, but acidotic cats will select diets containing excess sodium (Yu et al. 1997). Furthermore, cats can develop a learned taste aversion to diets that causes metabolic acidosis (Cook et al. 1996). Healthy cats are normally neophilic, namely selecting a new food (or flavor) if offered a variety of foods (or flavors) throughout their lives (Beauchamp et al. 1977). Furthermore, cats do not consume a powdered diet but will ingest the same diet in a pelleted or gel form. If a diet is too dry or powdery, cats will eat very little even when no alternative food is offered (NRC 2006).

4.9.4 Meal Frequency of Dogs and Cats

Dogs eat small and large prey, as well as plant-sourced foods. These animals can consume a large meal within 10 min that is sufficient to meet daily nutrient requirements, but they usually eat 4 to 8 meals/day (depending on breed; Bradshaw 2006). Growing puppies should have free access to food or be fed 2–3 times daily. Young and adult dogs generally ingest food and drink water during the light period and may eat at night if they are hungry or dehydrated, but some breeds of dogs also eat and drink during the dark period (Mugford and Thorne 1980). On average, dogs drink more water than cats per kg BW per day. Although adult dogs may do well on a regimen of eating a meal per day with possible health benefits (Bray et al. 2021), these animals should be offered foods at least twice daily to ensure adequate nutrition and welfare (Brooks et al. 2014). Multiple meals per day may help dogs to alleviate boredom, while reducing the risk for the problem of gastric dilatation-volvulus in susceptible breeds of dogs (Bataller 1995). If overweight or obese, they should be fed either less or lower energy-dense diets. Caloric restriction may reduce oxidative stress and delay aging in dogs (Kealy et al 2002; Lawler et al. 2008). Of particular note, intermittent fasting on a ketogenic diet containing medium-chain triglycerides may confer both metabolic and immunological effects in healthy dogs (Leung et al. 2020). Dogs with special health issues or dietary needs may require individualized feeding programs.

In contrast to dogs, cats do not have clear-cut circadian rhythms (Hawking et al. 1971; Randall et al. 1985). Some cats are nocturnal (e.g., resting during the daytime and going out or walking at night), and their existing rhythms can be altered by minor disturbances such as human noises (Macdonald and Apps 1978). Cats generally eat frequent, small meals and drink water throughout the day and night and can voluntarily eat 12–20 meals/day, but fresh food should be offered daily (Kane et al. 1981). After they adapt to a new diet for 1 week, the size and number of meals are not affected by the type of diet. When cats eat a big portion of food at one time, they may immediately vomit it due to gastric irritation. Therefore, appropriate and frequent meals are critical for feline health. Pet owners can decide meal size and frequency based on breed, age, health condition, ambient environment, and practicality for their dogs and cats rather than simply food availability.

Camara et al. (2020) recently reported that compared with the feeding of the same canned food four times/day, adult cats fed only once daily had greater plasma concentrations of glucagon-like protein-1, gastric inhibitory protein, and some AAs within the initial postprandial hours (as expected because of more nutrient intakes at a single time), but a lower value of respiratory quotient (RQ) in the fasting state. These authors interpreted the data as that a reduced RQ value may result from a greater rate of whole-body fatty acid oxidation and that feeding once daily may be a beneficial feeding management strategy for indoor cats to promote satiation and lean body mass. However, a reduced RQ value in the fasting state may also result from a greater rate of the conversion of AAs (released from the breakdown of muscle protein; possibly an adverse effect) into glucose, because this process has an RQ value of 0.333 (Wu 2018). In the work (Study 1) of Camara et al. (2020), neither body weight nor feed (energy) intake differed between the two groups of cats. Clearly, more studies are warranted to assess whether feeding once daily may have health benefits for cats.

4.10 Conclusion and Perspectives

Both dogs and cats naturally prefer and effectively digest meat, but can also digest appropriate amounts of properly (e.g., suitable temperature and period) processed and formulated plant-sourced ingredients. These two animal species differ markedly in some aspects of nutrition and metabolism: including (1) the ability to digest the quantities (i.e., small vs large) of dietary cooked starch in the small intestine; (2) thermoneutral zones and rates of basal energy metabolism; (3) qualitative (i.e., presence or absence) and quantitative (i.e., amounts) requirements for many dietary nutrients, particularly protein, certain AAs (arginine, taurine, methionine, and cysteine, as well as NEAAs), ω3 and ω6 PUFAs, and vitamins (e.g., vitamin A and niacin); (4) sensitivity to dietary AA imbalances and antagonisms; (5) felinine synthesis from cysteine; (6) feeding behavior and meal frequencies; (7) food preference and refusal; (8) the selection for foods containing various amounts of protein and fats; (9) the form of diets; and (10) the occurrence of gastrointestinal and metabolic disorders (Table 4.1). Recommended minimum requirements and allowances of dietary nutrients other than AAs for growing and adult dogs and cats are summarized in Table 4.4. Recommended values for their dietary AA requirements have been recently summarized (Li and Wu 2023a, b). Unlike cats, dogs have adapted by expressing a high activity of pancreatic α-amylase activity during evolution to omnivorous diets containing taurine-rich meat and plant ingredients with relatively a large amount of starch. Thus, a high intake of starch (e.g., 40% on a DM basis) can result in metabolic disorders in cats but not in dogs, whereas dietary fiber is beneficial for the intestinal health of both cats and dogs. Given the increasing incidences of obesity or overweight in both dogs and cats, dietary supplementation with arginine may be a promising solution to prevent metabolic syndromes in these animals, as reported for rats (Jobgen et al. 2009, 2022a, b, 2023). In practice, cats should not be fed canine foods. Because the composition of the milk of both dogs and cats differ from that of farm mammals, the young pets should not be fed replacer diets formulated based on goat or cow milk. For both dogs and cats, there may be breed differences in dietary requirements for nutrients and thermoneutral zones even among different sizes of the same breed. The fundamental knowledge of the idiosyncrasies of metabolism in dogs and cats is essential for guiding their feeding and care, as well as food manufacturing. Of particular note, current commercial vegan petfoods may be nutritionally inadequate for dogs (low content of calcium, potassium, sodium, and methionine) and cats (low content of protein, arginine, taurine, and potassium, as well as an improper Ca/P ratio). Animal-sourced foods contain nutritionally significant amounts of AAs, lipids, and minerals and therefore play an important role in balancing AAs in diets for dogs and cats play an important role in optimizing the nutrition and health of these companion animals.

Table 4.4 Recommended allowances of dietary fatty acids, minerals, and vitamins for dogs, cats, and pigs