Chapter 12. Protein Requirements
A requirement for dietary protein actually represents the need for essential amino acids and adequate nonessential amino acids to maintain body protein and to supply nitrogen for the synthesis of nonessential amino acids and other essential nitrogen-containing compounds (see Section 1, p. 21). This requirement is commonly expressed as a protein requirement because amino acids and nitrogen-containing compounds are most typically supplied in the diet in the form of intact protein. Adult animals require dietary protein to maintain whole-body protein turnover. 1 This turnover represents the synthesis and breakdown of proteins in all tissues of the body, including skin, hair, skeletal muscle, digestive enzymes, hormones, serum transport proteins, and mucosal cells. It is the sum of the losses of all of the body’s individual proteins and nitrogen-containing compounds that ultimately determines an individual’s daily protein (amino acid) requirement. Young animals have the same maintenance requirements as adult animals, plus an added requirement for the deposition or growth of new tissue.
Protein in the diets of adult dogs, adult cats, puppies, and kittens is necessary for the replacement of protein losses in the skin, hair, digestive enzymes, and mucosal cells, as well as amino acid losses from normal cellular protein catabolism. Puppies and kittens also require protein for growth.
DETERMINING PROTEIN REQUIREMENTS
Historically, the response criteria that have been used to determine protein requirements in dogs and cats are nitrogen balance and growth rate. Nitrogen balance studies are based upon the fact that protein, on the average, contains 16% nitrogen. The nitrogen contents of food, feces, and urine are commonly measured using analytical tests, the Kjeldahl method and the Leco Nitrogen Protein Analyzer.2 Measuring nitrogen intake and excretion provides a rough estimate of the body’s protein status. Nitrogen balance is calculated as:
The nitrogen in the feces is comprised of unabsorbed dietary protein and nitrogen from endogenous sources, such as intestinal cells and gut microflora. Urinary nitrogen is composed primarily of urea, which is the end product of amino acid catabolism. Further nitrogen losses occur from desquamated cells of the skin surface, hair, and nails. However, these losses are very difficult to measure and are usually not considered when measuring nitrogen balance in experimental studies.
Requirement studies with growing animals use maximum positive nitrogen balance and growth rate as response variables to indicate an adequate level of protein in the diet. Studies of adult companion animals at maintenance use zero nitrogen balance to indicate dietary protein adequacy. Zero nitrogen balance provides an indirect measure of whole-body protein turnover and suggests that the body’s daily loss of protein is replaced by intake, without a net gain or loss in total body protein. Although the majority of requirement studies have used zero nitrogen balance to assess the protein requirement of adult animals during maintenance, it is important to recognize that there are certain limitations to the use of the nitrogen balance technique. First, nitrogen balance does not provide information regarding the adequacy of (or requirement for) individual amino acids. Therefore amino acid requirement studies have been conducted independently of protein requirement studies (see pp. 95-101). Second, the minimum level of protein needed to maintain zero nitrogen balance may not be adequate to promote optimal performance and health. For example, in an early study, when adult dogs were fed diets containing just enough protein to attain zero nitrogen balance, they were found to be more susceptible to the toxicity of certain drugs. 3 In addition, higher levels of protein in the diet may be needed to obtain adequate individual amino acid intakes to maintain lean body mass and protein reserves. (Although protein is not stored in the body as is fat and, to a lesser degree, carbohydrate, the term reserves refers to the ability of the body to mobilize protein from prioritized body tissues during periods of stress.) For example, many diseases cause a loss of muscle mass, which is generally considered the largest protein depot. Therefore it is prudent to consider that estimates obtained using zero nitrogen balance may represent a minimum protein requirement for adult animals. Adequate intake (AI) estimates for dogs and cats are expected to be slightly higher than this minimum amount for healthy adults, especially during periods of physiological stress. AI estimates also account for variations in the population of interest.
Nitrogen equilibrium (zero balance) is the normal state for healthy adult animals during maintenance. An animal is described as being in a state of positive nitrogen balance when protein intake exceeds excretion. Positive nitrogen balance occurs when new tissue is being synthesized by the body, such as during the physiological stages of growth and gestation or the recovery phase following a prolonged illness. Positive nitrogen balance cannot occur when there is insufficient protein intake or a significant imbalance of the essential amino acids. Negative nitrogen balance results when protein excretion exceeds intake. An animal that exhibits negative balance is losing nitrogen from tissues more rapidly than it is being replaced. This loss of nitrogen may occur for several reasons. If the animal is consuming an insufficient amount of energy or is anorectic, body tissues must be catabolized to provide energy to the body. If inadequate levels of available protein and/or amino acids or an inappropriate ratio of amino acids are fed, tissue replacement cannot occur. Severe or prolonged illness or injury results in a catabolic state in animals that is evidenced by excessive breakdown of the body’s tissues and negative nitrogen balance. Excess losses of nitrogen from the urine during renal failure or from the gastrointestinal tract during some types of gastrointestinal disease can also cause negative nitrogen balance to occur (Table 12-1).
| N, Nitrogen. | ||
| S tate | B alance | P hysiological stage |
|---|---|---|
| Zero | N intake = N excretion | Maintenance |
| Positive | N intake > N excretion | Growth, gestation, recovery from illness |
| Negative | N intake < N excretion | Inadequate nutrition, severe illness or injury, urinary N loss during renal failure, gastrointestinal tract loss during certain diseases |
FACTORS AFFECTING PROTEIN REQUIREMENT
The determination of the exact protein requirements for dogs and cats is a difficult task because many factors affect an individual animal’s need for protein. Dietary factors that affect protein turnover (nitrogen balance) include protein quality and amino acid composition, protein digestibility, and the energy density of the diet. In addition, an animal’s activity level, physiological state, and prior nutritional status can all influence protein requirement as determined by nitrogen balance, whole-body protein turnover, or growth rate (Box 12-1).
BOX 12-1
Protein quality: As protein quality increases, protein requirement decreases.
Amino acid composition: As amino acid composition improves, protein requirement decreases.
Protein digestibility: As protein digestibility increases, protein requirement decreases.
Energy density: As energy density increases, protein requirement as a percentage of the diet increases.
An animal’s protein requirement varies inversely with the protein source’s digestibility and with its ability to provide all of the essential amino acids in their correct quantities and ratios. As protein digestibility and quality increase, the level of protein that must be included in the diet to meet the animal’s needs decreases. Early protein requirement studies with dogs and cats used either purified or semipurified diets; the protein and amino acids in these types of diets are highly digestible and available. In contrast, most of the protein sources used in commercial pet foods have comparatively lower digestibility coefficients. For example, protein digestibility in a semipurified diet approaches 95%, but that of high-quality, commercial diets range between 80% and 90%. On the other hand, low-quality, commercial pet foods can have protein digestibilities of less than 75%. Because of these differences, requirement studies with purified or semipurified diets generally underestimated the protein requirements of animals consuming mixed protein diets that contained less available or inappropriate ratios of the essential amino acids.
Protein quality also influences an animal’s protein requirement. The higher the biological value of a protein, the less the amount that is needed to meet all of an animal’s essential amino acid needs (see Section 1, pp. 23-25). Therefore, as the quality of the protein in the test diet increases, the estimated requirement decreases. Again, the diets that were used in early amino acid and protein requirement studies had amino acid contents that were adjusted to carefully fit the needs of the experiment. Few, if any, naturally occurring protein sources have amino acid compositions that specifically fit the requirements of the animal consuming it. Most practical sources of dietary protein contain excesses of some amino acids and slight or severe deficiencies of others relative to an animal’s requirement. Commercial pet foods correct for these inadequacies by using mixtures of protein sources that have complimentary profiles of essential amino acids and individual amino acids.
The caloric density of the diet used in a requirement study significantly affects the estimated protein requirement. This effect occurs because the presence of nonprotein calories has a protein-sparing effect. A diet must first meet an animal’s energy needs before the energy-containing nutrients can be used for other purposes. Therefore, adequate nonprotein calories in the form of carbohydrate or fat spare the protein in the diet from being metabolized for energy. If sufficient nonprotein calories are not provided, at least a portion of the dietary protein will be metabolized as an energy source. At caloric intakes that are less than the animal’s energy requirement, protein will not be available for the building or replacing of body tissues because it will all be used for energy. Therefore, when a diet is limiting in both energy and protein, weight loss and a loss of lean body tissue result. Nitrogen balance studies have shown that when the dietary protein level is held constant, nitrogen retention increases as caloric intake increases and approaches the animal’s energy requirement. 4
A second aspect of the relationship between protein and energy must also be examined. Assuming that adequate nonprotein energy is present in the diet, as the energy density of the diet increases, a higher proportion of protein is required for maximal nitrogen retention; however, the protein-to-energy ratio remains the same. The most important factors that affect the energy density of commercial pet foods are dietary fat concentration and diet digestibility. The relationship between energy density and protein content is illustrated by the results of one of the first requirement studies of growing dogs. 5 When a diet containing 25% crude protein and 20% fat was fed, maximal growth rate resulted. However, when the fat content of the diet was increased to 30%, 29% crude protein was necessary to support maximal growth. The reason for this change relates to an animal’s tendency to eat to satisfy its energy needs. Provided that these controls are in place, an animal will naturally consume less of a more energy-dense ration. Pet owners who use portion-controlled feeding regimens usually adjust quantity according to their pet’s body weight and/or growth rate. Therefore portion-controlled feeding schedules are still regulated according to a pet’s energy requirements. When lower quantities of food are fed because of greater energy density, protein must contribute a higher proportion of the diet so that the animal is still able to meet its total protein needs. Although protein is the most commonly used example, this relationship with energy also applies to all other essential nutrients.
Finally, protein requirement studies must take into account an animal’s prior nutritional status and physiological state. The amount of absorbed protein needed to produce nitrogen equilibrium depends on the degree of protein depletion. Although it seems paradoxical, dogs with depleted body protein reserves require lower levels of nitrogen to achieve nitrogen balance than do dogs with normal reserves. 6 This effect may be the result of an increased efficiency of absorption and use of dietary protein when in a depleted state, and to the metabolic down-regulation of protein catabolism. Ensuring that all dogs are in nitrogen balance and have adequate body protein reserves by feeding a high-protein diet before the onset of a requirement study can help to eliminate this discrepancy. Conversely, correcting for lean body mass (a measure of protein reserves) can also at least partially account for differences in protein requirements. However, nitrogen balance studies still cannot provide information regarding the presence or degree of change in protein synthesis, breakdown, and oxidation that can occur in response to differing levels of protein or essential amino acids in the diet. In recent years the use of 13 C-labeled leucine infusion as a measure of whole-body protein metabolism has been used to further elucidate these changes. 7. and 8. Physiological state also directly affects the body’s need for protein and will therefore affect the outcome of requirement studies that use nitrogen balance. For example, in growing puppies and kittens, the rate of growth and, subsequently, the protein requirement decrease slightly with age. 9.10.11. and 12.
PROTEIN REQUIREMENTS
Dogs
Numerous studies have been conducted on the minimum protein requirement of the adult dog. However, differences in the protein sources, energy densities, and amino acid ratios of the experimental diets have led to a great deal of confusion regarding this requirement. Early studies showed that when diets containing very high-quality protein sources are fed, adult dogs require between 4% and 7% of their metabolizable energy (ME) calories to be supplied as protein. 13.14. and 15. The current National Research Council (NRC) recommends a minimum protein requirement of 80 g of crude protein per kg diet in foods with an energy density of 4.0 kilocalories (kcal) ME/g, when proteins that are of high quality (both bioavailable and with the correct amounts of the essential amino acids) are fed. 16 This is equivalent to just 7% of the diet’s ME. The NRC’s recommended allowance is slightly higher (8.75% of ME), presumably to account for lower digestibility coefficients of protein sources used in practical diets. It is important to consider that when lower-quality protein sources are fed, protein requirement estimates will increase significantly, typically as high as 20% of the ME calories. 17 For this reason, the current American Association of Feed Control Officials’ (AAFCO’s) Nutrient Profiles for dogs recommends that adult maintenance dog foods contain at least 18% of ME calories as protein (see below). 18
The protein requirement of growing puppies is significantly higher than that of adult dogs. Early studies using mixed protein sources reported minimum protein requirements of between 17% and 22% of ME for growing dogs. 5.19. and 20. These experiments used maximum weight gain as an indicator of minimum protein needs. More recent studies, which also used weight gain as the major response criterion, reported minimum requirement estimates for recently weaned puppies of approximately 180 g crude protein/kg diet in a food containing 4.0 kcal/g. 17.21. and 22. This is equivalent to just 16% of ME. However, the protein sources used in all of these studies were either highly digestible protein or supplied as free amino acids. Interestingly, weight gain in growing dogs is maximized at lower protein intakes than is nitrogen retention. For example, nitrogen balance data with young puppies fed a highly digestible protein source provided a slightly higher protein requirement estimate of 20% of ME. 21 The current NRC recommends that a minimum of 16% of a diet’s calories should be supplied as high-quality protein to maximize nitrogen retention in newly weaned puppies between the ages of 8 and 14 weeks. 16 After 14 weeks, the minimum requirement decreases to about 12.25% of ME. However, just as with adult maintenance diets, these estimates increase substantially when feeding practical diets that contain less available protein sources. The NRC recognizes this and recommends minimum levels of 21% (250 grams/kilogram [g/kg]) for puppies less than 14 weeks of age and 17.5% for puppies over 14 weeks of age when fed practical diets. 16 The current AAFCO Nutrient Profiles recommend a minimum level of 22% protein ME for growth and reproduction and do not distinguish between newly weaned and adolescent puppies (Table 12-2).
| AAFCO, Association of American Feed Control Officials; NRC, National Research Council. | ||
| ∗National Research Council: Nutrient requirements of dogs and cats, Washington, DC, 2006, National Academy Press. | ||
| †Association of American Feed Control Officials (AAFCO): Official publication, 2008, AAFCO. | ||
| NRC∗ | AAFCO† | |
|---|---|---|
| Dogs | ||
| Adult maintenance | 8.75% of ME | 18% of ME |
| Growth and reproduction | 21% of ME (Puppies ≤ 14 weeks) 17.5% of ME (Puppies > 14 weeks) | 22% of ME |
| Cats | ||
| Adult maintenance | 17.5% of ME | 22.75% of ME |
| Growth and reproduction | ∼20% of ME | 26.25% of ME |
Cats
Early studies of the cat’s nutrient requirements showed that it has a protein requirement substantially higher than that of other mammals, including the dog. 23. and 24. When growing kittens were fed varying levels of dietary protein, supplied as minced herring and minced liver, growth was reported to be satisfactory only when protein exceeded 30% of the dry weight of the diet. 24 In comparison, growing puppies fed mixed diets required only 20% protein for adequate growth and development. One of the first studies of the protein requirement of the adult cat reported that 21% dietary protein was necessary to maintain nitrogen balance when cats were fed a mixed diet containing liver and whitefish as the primary protein sources. 25
Subsequent experimentation using crystalline amino acids and protein isolates allowed more precise definition of the minimum protein requirements of growing kittens and adult cats. One study reported a protein requirement of 18% to 20% (by weight) in growing kittens fed either crystalline amino acid diets or casein diets supplemented with methionine. 26 Another study reported requirements as low as 16% of ME calories when growing kittens were fed a purified diet containing all of the essential amino acids in their assumed correct concentrations and ratios. 27 Using a similar semipurified diet, the protein requirement of adult cats was determined to be 12.5% of ME. 28 The profound effect that protein digestibility, amino acid balance, and amino acid availability have on determining an animal’s dietary protein requirement is illustrated by the substantially lower values that were obtained when semipurified and purified diets were used to determine requirements. However, the comparison of these figures with the ideal minimum protein requirements of other mammals still demonstrates that the cat, together with other obligate carnivores such as the fox and the mink, has a higher requirement for dietary protein (see pp. 94-95 for a complete discussion). For example, although the cat requires 20% of a 100%-available, well-balanced protein for growth and 12% for maintenance, the dog requires only 12% and 4%, respectively. It should be noted that these values are substantially lower than the protein requirement of a cat fed a practical diet containing protein sources that are not balanced for individual amino acids or highly bioavailable.
The current NRC recommendations use nitrogen balance data from a study reporting on a group of 18 adult cats to provide a recommended minimum requirement of protein for adult cats of 160 g crude protein/kg food in a diet containing 4 kcal/kg. 16. and 29. This is equivalent to a minimum requirement of 14% of ME. Adding a safety allowance to this minimum provides an estimate of 200 g/kg food (17.5% of ME) as a recommended allowance (RA) (see Table 12-2). The NRC’s minimum requirement for kittens after weaning is 180 g/kg, equivalent to 15.75% of ME and the RA is 225 g/kg (∼20% of ME). Once again, it is important to recognize that all of these values assume highly available and well-balanced protein sources that contain all of the necessary amino acids. 30. and 31. Conversely, the AAFCO Nutrient Profiles provide nutrient estimates for use in the actual formulation of pet foods. Therefore, it is not surprising that the AAFCO Nutrient Profiles for cat foods, as with dog foods, suggest a higher level of protein for inclusion in commercially prepared foods. 18 A level of 30% of the diet (dry matter [DM]) is suggested for growth and reproduction in foods containing 4 kcal of ME/g of food. This value is equivalent to 26.25% of ME calories. A level of 26% of the diet, equivalent to 22.75% ME, is suggested for adult maintenance (see Table 12-2).
THE CAT’S HIGH PROTEIN REQUIREMENT
The cat’s comparatively high dietary requirement for protein is the result of increased needs for the maintenance of whole-body protein turnover, rather than increased needs for growth. Approximately 60% of the growing kitten’s protein requirement is used for the maintenance of body tissues; only 40% is used for growth. The opposite is true in most of the other species that have been studied. For example, the growing rat requires only 35% of its dietary protein for maintenance and 65% for growth; similarly, the growing dog uses only 33% of its protein requirement for maintenance and 66% for growth. 32 The higher growth rate of puppies compared with kittens causes this increased need for dietary protein during growth; when compared during adult maintenance, cats have almost twice the requirement for bioavailable protein compared with dogs. 16
The elevated protein requirement for maintenance results from the inability of the amino acid and amino acid nitrogen catabolic enzymes in the cat’s liver to effectively down-regulate in response to reduced dietary protein intake. When most mammals are fed diets high in protein, the enzymes involved in amino acid catabolism, nitrogen disposal, and gluconeogenesis increase in activity to use the carbon backbone of surplus amino acids and convert excess nitrogen to urea for excretion. Conversely, when low-protein diets are fed, the activity of these enzymes declines (i.e., is down-regulated), which allows for the conservation of amino acids for whole-body protein synthesis and results in lower amounts of nitrogen to be produced via the urea cycle. 33. and 34. This adaptive mechanism is a distinct advantage because it allows animals to conserve amino acids while consuming low-protein diets. The ability to up-and down-regulate these catabolic enzymes provides a mechanism by which potentially toxic amino acids can be catabolized when animals are consuming high-protein diets. For example, an early study fed adult cats either high-protein (70%) or low-protein (17.5%) diets for 1 month. 35 The activities of three urea cycle enzymes and seven nitrogen catabolic enzymes in the liver were then measured. With the exception of one transaminase enzyme, no significant differences in enzyme activity were found between the cats fed the low-protein diet and the cats fed the high-protein diet. Several gluconeogenic and lipogenic enzymes were also measured, none of which exhibited any change in activity in response to the changes in dietary protein level. On the other hand, similar rat hepatic enzymes decrease in activity from 2.75-fold to 13-fold after rats are changed from a high-protein diet to a low-protein diet. 34
In addition to the inability of the cat’s protein-catabolizing enzymes to down-regulate in response to reduced dietary protein, the enzymes involved in nitrogen catabolism function at relatively high rates of activity. This metabolic state causes the cat to catabolize a substantial amount of amino acids after each meal, regardless of its protein content. Thus the cat does not have the capability to conserve nitrogen from the body’s general nitrogen pool and has a higher urinary obligatory nitrogen loss when fed low-protein (or protein-free) diets. 36 This difference is of important significance when considering the nutritional care of cats that are ill or anorectic, as long-term food deprivation causes a much higher loss of urinary nitrogen in cats compared with other species (see Chapter 33, pp. 432-434 for a complete discussion). 37 The only alternative that ensures adequate conservation of body protein stores is the consistent consumption of a diet containing high levels of protein. Recent studies have shown that although cats do not adapt to low-protein diets, they do efficiently adapt to medium- and high-protein diets. 38. and 39. This appears to occur via increased liver mass, increased delivery of substrate to urea cycle enzymes, and to allosteric regulation of rate-controlling enzyme activities. It can be theorized that because of the cat’s strict adherence to a carnivorous diet, it experienced little selective pressure throughout its evolutionary history to develop metabolic adaptations to low-protein diets. Additionally, a high rate of gluconeogenesis from amino acid catabolism would provide endogenous glucose to an animal that evolved to ingest a low-carbohydrate diet, thus having selective advantage. 40
Another factor that contributes to an animal’s dietary protein requirement is its need for essential amino acids. When the protein nutrition of the cat was first studied, it was postulated that its high dietary protein requirement might be the result of an unusually high requirement for one or more of the essential amino acids. However, results of several experimental studies have shown that with the exception of slightly higher requirements for leucine, threonine, methionine, and arginine, as well as a unique dietary requirement for taurine (see below), the cat’s requirements for specific essential amino acids are not significantly higher than those of other species such as the rat, dog, or pig. 41 More recently, studies have shown that the growing kitten appears to be less sensitive to the amino acid imbalances that are seen in omnivores and herbivores when one or more amino acid is limiting and total protein level of the diet is increased. 42. and 43. In other animals, but not in cats, increasing dietary crude protein causes a concomitant increase in essential amino acid requirements.
The protein requirement of the cat is higher than that of the dog as a result of the cat’s greater need for the maintenance of normal body tissues rather than from increased need for growth. This is because of the inability of certain catabolic enzymes in the cat’s liver to down-regulate in response to changes in dietary protein intake.
ESSENTIAL AMINO ACIDS IN DOG AND CAT NUTRITION
The following 10 amino acids have been identified as being essential for growing puppies and kittens: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Although both the dog and the cat have a dietary requirement for arginine, the cat is unusual in its immediate and severe reaction to the consumption of an arginine-free meal. The other amino acids that are of special concern in the feeding of dogs and cats are lysine, the sulfur amino acids (SAAs) methionine and cysteine (and the production of felinine), and the amino-sulfonic acid, taurine. Of less practical concern, but of academic interest, is the cat’s inability to convert the amino acid tryptophan to the B vitamin niacin.
Arginine
The amino acid arginine is not considered a dietary essential for many adult animals because most species can synthesize adequate amounts to meet their metabolic needs. However, arginine has been shown to be essential for both dogs and cats throughout life. 44.45. and 46. Arginine is needed by the body for normal protein synthesis and as an essential component of the urea cycle. Arginine functions in the urea cycle as an ornithine and urea precursor. In this capacity arginine allows the large amounts of nitrogen generated from amino acid catabolism to be converted to urea for excretion from the body. If nitrogen cannot be liberated through the urea cycle by the presence of arginine, then both free urea and ammonia will begin to rise in the blood. A lack of arginine in the diet causes an immediate and severe deficiency response in the cat. Cats will develop hyperammonemia within several hours of consuming a single arginine-free meal. 47 Clinical signs include emesis (vomiting), muscle spasms, ataxia, hyperesthesia (sensitivity to touch), and tetanic spasms. These signs can eventually lead to coma and death. Dogs show similar, but less severe, clinical signs of arginine deficiency following consumption of an arginine-free meal, suggesting a low level of endogenous arginine production. 48
The metabolic basis for the cat’s extreme sensitivity to arginine deficiency is related to an inability to synthesize de novo ornithine. In most animals, the amino acids glutamate and proline act as precursors for ornithine synthesis in the intestinal mucosa. However, the cat’s intestinal mucosal cells have extremely low levels of active pyrroline-5-carboxylate synthase, an essential enzyme in this pathway. 49 The cat also has low activity of a second essential enzyme, ornithine aminotransferase. 50 In addition to being unable to synthesize ornithine, the cat is also unable to synthesize citrulline from ornithine for use by extrahepatic tissues, even if dietary ornithine is provided. Studies in the rat demonstrated that the normal route of arginine synthesis for use by extrahepatic tissues involves both the liver and the kidneys. Arginine cannot leave the liver to provide for extrahepatic tissues because high activity of liver arginase prevents its accumulation to a concentration above that of the bloodstream. However, citrulline, which is produced from ornithine either in the intestinal mucosa or in a urea cycle intermediate in the liver, can travel to the kidneys where it is then converted to arginine. This arginine provides the kidneys and other tissues of the body with their needs for normal growth and tissue maintenance in most animals. In the cat, however, citrulline is not produced in the intestinal mucosa (because of the inability to produce ornithine), and the citrulline produced in the liver appears to be unable to leave the hepatocyte to be converted to arginine by the kidneys. As a direct result of these metabolic deficiencies, arginine becomes an essential amino acid for both urea cycle function and for normal growth and maintenance in the cat (Table 12-3). The importance of arginine for normal functioning of the urea cycle, coupled with the cat’s high and inflexible rate of protein catabolism, causes the cat to be extremely sensitive to arginine deficiency. Like the cat, the growing dog also has a dietary requirement for arginine. However, the response of the growing dog to an arginine-deficient diet is not as severe as that observed in the immature cat. 48
| R eaction | M ost mammals | C ats |
|---|---|---|
| Glutamate + proline → ornithine (intestine) | Normal | Low |
| Ornithine → citrulline (intestine) | Normal | Little activity |
| Citrulline travels to the kidney | Does occur | Does not occur |
| Citrulline → arginine (kidney) | Does occur | Does not occur |
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