Cats: obligate carnivore (Proceedings)
Protein metabolism in cats is unique; this is apparent because of their unusually high maintenance requirement for protein in the diet as compared to dogs or other omnivores. Cats have both a higher basal requirement for protein and an increased requirement for essential amino acids. Cats depend on protein not only for structural and synthetic purposes but also for energy. They will continue to use protein in the form of gluconeogenic amino acids for production of energy, even when inadequate protein is consumed in the diet. Although these changes impair the cat's ability to conserve protein when dietary sources are limited, on their natural diet it ultimately conserves energy by eliminating the cost of enzyme synthesis and degradation. In 1986, National Research Council recommended a minimum of 240 grams of protein/kg in the diets of growing kitten and 140 grams of protein/kg in the diets of adult cats. This is equivalent to 26% of metabolizable energy in the diet for kittens and 23% of metabolizable energy for adult maintenance. Keep in mind, these are minimum recommendations, and they assume a highly digestible protein source is provided in the diet.
TaurineTaurine, which is an essential amino acid for cats, is not incorporated into proteins or degraded by mammalian tissues, but is essential for conjugation of bile salts, vision, cardiac muscle function, and proper function of the nervous, reproductive and immune systems. Cats can only conjugate bile acids with taurine to make bile salts. Taurine continues be lost in the gastrointestinal tract through this conjugation with bile, this coupled with a low rate of synthesis contributes to the obligatory requirement for cats. Carnivorous diet supplies abundant taurine; however cereal and grains supply only marginal or inadequate levels of taurine for cats. Therefore, diets based on these types of protein sources may be lacking or limiting in taurine. Taurine is either more available or better retained by cats fed dry food diets. Because of the wide spread uses of taurine within the body, changes from deficiency can be seen in virtually all body systems. Three syndromes have been identified related strictly to taurine deficiency; feline central retinal degeneration, reproductive failure and impaired fetal development and feline dilated cardiomyopathy. Clinical signs of taurine deficiency occur only after prolonged periods of depletion (from 5 months to 2 years).
Methionine and Cystine
Methionine is an essential amino acid for cats; this species has a higher requirement than do dogs or other omnivores. Cystine is also required for production of hair and felinine, an amino acid found in cat urine. Felinine is found in largest amounts in intact male cats and is thought to be used for territorial marking. Can replace up to half of the methionine requirement in cats, methionine tends to be the first limiting amino acid in many food ingredients.
Nutritional deficiencies are possible, especially in cats fed home-made, vegetable based diets or human enteral diets. Clinical signs of methionine deficiency include poor growth and a crusting dermatitis at the mucocutaneous junctions of the mouth and nose.
The cat is unable to convert beta-carotene to retinol (vitamin A) because of a lack of intestinal enzymes necessary for the conversion, and therefore this species requires a dietary source of pre-formed vitamin A. Vitamin A is necessary for the maintenance of vision, bone and muscle growth, reproduction and healthy epithelial tissues. Because vitamin A is a fat soluble vitamin and is stored in the liver, deficiencies are slow to develop, and are only seen in cats with severe liver failure or gastrointestinal disease resulting in fat malabsorption. Cats also are unable to convert sufficient amounts of the fatty acid linoleic acid to meet the requirements for arachidonic acid.
Cats also lack sufficient enzymes to meet the metabolic requirements for vitamin D photosynthesis in the skin; therefore they require a dietary source of vitamin D. The primary function of vitamin D is calcium and phosphorus homeostasis, with particular emphasis on intestinal absorption, retention and bone deposition of calcium. As with vitamin A, deficiency is rare and slow to develop.
Vitamin A, vitamin D and arachiodonic acid are found in plentiful amounts in animal fats. This fat is important not only for provision of fuel for energy, but also for increasing palatability and acceptance of food.
Cats require increased amounts of many dietary water-soluble B vitamins, including thiamin, niacin, pyridoxine (vitamin B6), and in certain circumstances cobalamin (vitamin B12). The requirement for niacin and pyridoxine is four times higher than that for dogs. Because most water-soluble B vitamins are not stored (except cobalamin, which is stored in the liver), a continually available dietary source is required to prevent deficiencies. Deficiencies are rare in cats eating appropriate diets because each of the B vitamins is found in high concentrations in animal tissue.
The cat may be seen as one of our most visible "specialists". As an obligate carnivore, they have evolved to such a point that many of the redundant systems that we have are no longer required. Instead of seeing cats as "inferior", I think we need to acknowledge that they have surpassed both us and dogs, and have streamlined their lives. We need to appreciate this unique and wonderful creature that continues to enrich our lives and protect our houses and yards.
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