The Carnivore Connection to Nutrition in Cats
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1202ttn.qxd 11/6/2002 11:13 AM Page 1559 Timely Topics in Nutrition The carnivore connection to nutrition in cats Debra L. Zoran, DVM, PhD, DACVIM The JAVMA welcomes contributions to this feature. nutritional biochemistry of cats. In addition, informa- Articles submitted for publication will be fully reviewed tion is included on possible roles of nutrition in the with the American College of Veterinary Nutrition (ACVN) development of obesity, idiopathic hepatic lipidosis acting in an advisory capacity to the editors. Inquiries (IHL), inflammatory bowel disease, and diabetes melli- should be sent to Dr. John E. Bauer, Department of Small tus in cats. Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX Protein 77843-4474. The natural diet of cats in the wild is a meat-based regimen (eg, rodents, birds) that contains little CHO; n another time long ago, Leonardo da Vinci said, thus, cats are metabolically adapted to preferentially I“The smallest feline is a masterpiece.”1 And for those use protein and fat as energy sources (Appendix 1). of us who marvel at the wonder that is a cat, there is no This evolutionary difference in energy metabolism doubt that his statement was remarkable for its sim- mandates cats to use protein for maintenance of blood glucose concentrations even when sources of protein plicity as well as its truth. Cats are amazing creatures, 2 unique and interesting in almost every way imaginable. in the diet are limiting. The substantial difference in Despite this, it has been common for veterinarians to protein requirements between cats and omnivores, consider cats and dogs as similar beings for anesthesia such as dogs, serves to illustrate this important meta- protocols, clinical diseases, and treatments. However, it bolic distinction. For example, whereas the protein is quite clear that cats are unique in all conceivable requirement of kittens is 1.5 times that of the young of other species, adult cats require 2 to 3 times more pro- ways, particularly in their nutritional biochemistry. 2,4 Cats are strict carnivores that rely on nutrients in ani- tein in their diet than adults of omnivorous species. mal tissues to meet their specific and unique nutrition- The fact that cats have such a greater dietary protein al requirements. This statement is news to few, yet the requirement, compared to dogs, necessitates that cats importance of these nutritional differences is often must have a higher basal requirement for nitrogen underestimated, especially during periods when cats (protein) or an increased requirement for essential are ill or have prolonged anorexia. In their natural amino acids. In the case of adult cats, the increased habitat, cats consume prey high in protein with mod- protein requirement is attributable to both; however, the requirement for essential amino acids in kittens is erate amounts of fat and minimal amounts of carbohy- 5 drate (CHO); thus, they are metabolically adapted for similar to that of the young of other species, so a high- higher metabolism of proteins and lower utilization of er basal requirement for nitrogen is suggested to play CHOs (starch, not soluble or insoluble fiber) than dogs the largest role in kittens. or other omnivores. Although cats can use CHOs as a Several possible reasons exist for the increased source of metabolic energy, they have limited ability to need for protein, but the fact that cats depend on pro- spare protein utilization by using CHOs instead. tein for energy as well as structural and synthetic pur- Nevertheless, commercial diets are formulated with a poses is a major component. When fed a low-protein mixture of animal- and plant-derived nutrients, most diet, most omnivores conserve amino acids by reduc- commonly in dry kibble form that requires CHOs for ing the activities of aminotransferases and other enzymes involved in protein catabolism.6,7 However, in the expansion and cooking process, to provide easy-to- 8 use food for domestic cats. And although cats have a classic study, cats were fed diets low (170 g/kg [77 adjusted to most manufactured diets, the limitations of g/lb] of body weight) and high (700 g/kg [318 g/lb] of substituting animal-origin nutrients with plant-origin body weight) in protein to determine whether they nutrients in foods formulated for cats are being responded to low-protein conditions in a manner sim- increasingly realized. ilar to that of omnivorous species. Regardless of The information reported here is an attempt to whether cats were fed low or high amounts of dietary protein, there was little adaptation in the activities of describe what it means metabolically and nutritionally 8 to be a strict carnivore, with a focus on differences in the aminotransferases or urea cycle enzymes. In another study,9 it was suggested that cats have a limit- From the Department of Small Animal Medicine and Surgery, ed ability to adjust protein utilization to the amount of College of Veterinary Medicine, Texas A&M University, College protein in their diets; however, the primary finding in Station, TX 77843-4474. that study was that protein oxidation increased in cats JAVMA, Vol 221, No. 11, December 1, 2002 Vet Med Today: Timely Topics in Nutrition 1559 1202ttn.qxd 11/6/2002 11:13 AM Page 1560 fed high-protein diets. Protein oxidation did not taurine in healthy cats is > 300 nmol/mL, with con- decrease in cats fed diets with moderate amounts of centrations < 160 nmol/mL consistent with deficiency.2 protein (low-protein diets were not evaluated).9 Arginine is an essential amino acid in dogs and Nevertheless, those studies document that cats contin- cats; however, in contrast to dogs, cats are unable to ue to use protein (eg, dispensable nitrogen in the form synthesize sufficient amounts of ornithine or citrulline of gluconeogenic amino acids) for production of ener- for conversion to arginine. Thus, it must be available in gy and in other metabolic pathways (eg, urea cycle), their diet.13,14 In addition, cats continually use large even in the face of low availability of proteins. These amounts of arginine in the urea cycle, because this increased protein requirements are an important rea- cycle is not down-regulated in cats during periods son why protein malnutrition can occur more quickly when food is withheld or in cats consuming low-pro- in sick, injured, or anorectic cats. tein diets.4,7 Cats and kittens fed a diet devoid of argi- In addition to their increased need for dispensable nine have clinical signs of hyperammonemia (eg, sali- protein, cats also have need for increased amounts of vation, neurologic abnormalities, hyperesthesia, eme- specific amino acids in their diet: taurine, arginine, sis, tetany, and coma) within hours, and the condition methionine, and cysteine.2 These specific amino acid may progress to death.13,14 Fortunately, arginine and cit- requirements of cats have likely been determined on rulline are abundant in animal tissues; thus, arginine the basis that their natural diet contains an abundance deficiency is rare in cats consuming appropriate foods. of each of these specific amino acids (in addition to 11 However, arginine supplementation must be used to essential amino acids; Appendix 2). The likely reason avoid a deficiency in cats fed diets with plant-origin that synthetic pathways for these amino acids, which protein sources. Arginine supplements should also be are found in omnivorous species, are not found in cats considered in anorectic cats with IHL, because a defi- is that they are redundant and, thus, energy inefficient. ciency of arginine may be responsible for some of the Furthermore, even though cats do not have the ability clinical signs observed in cats with this disease. The to synthesize these amino acids, the amino acids are dose of arginine that will benefit cats with IHL is not conserved. In fact, utilization of these amino acids unknown, but 250 mg of arginine/d is recommended. (taurine, arginine, methionine, and cysteine) is higher Cats also have a higher requirement for methio- in cats than in dogs or other animals. nine and cysteine in their diet than dogs or other omni- Taurine is a sulfur-containing β amino acid that is vores.2,15 Although there are numerous explanations for not incorporated into proteins or degraded by mam- this increased requirement, 1 major reason is that malian tissues; however, it is essential for vision, car- methionine and cysteine are gluconeogenic amino diac muscle function, and proper function of the ner- acids in cats that are catabolized to pyruvate and then vous, reproductive, and immune systems.2 Taurine is subsequently oxidized to provide energy (Appendix 2). essential in cats because they cannot synthesize ade- In dogs and other animals, methionine and cysteine quate quantities from the typical precursors (ie, have many uses but are primarily converted to taurine, methionine or cysteine). Enzymes required for synthe- homocysteine, and S-adenosyl-methionine and its sis of taurine (eg, cysteine dioxygenase and cysteine metabolites (eg, glutathione), which are important sulfinic acid decarboxylase) are only minimally active antioxidants and scavengers of free radicals in cats.10 Furthermore, cats have a constant and obli- (Fig 1).2,4,16,17 In addition to the aforementioned path- gate loss of taurine into bile, because they conjugate ways, the requirement for cysteine is high in cats for bile acids only with taurine.10 Complicating matters, production of hair and felinine, a sulfur-containing the requirement