Cats are obligate carnivores: their physiology has evolved to efficiently process meat, and they have difficulty digesting plant matter. In contrast to omnivores such as rats, which only require about 4% protein in their diet, about 20% of a cat's diet must be protein.
Cats are unusually dependent on a constant supply of the amino acid arginine, and a diet lacking arginine causes marked weight loss and can be rapidly fatal. Arginine is an essential additive in cat food because cats have low levels of the enzymes aminotransferase and pyrroline-5-carboxylate which are responsible for the synthesis of ornithine and citruline in the small intestine. Citruline would typically go on to the kidneys to make arginine, but because cats have a deficiency in the enzymes that make it, citruline is not produced in adequate quantities to make arginine. Arginine is essential in the urea cycle in order to convert the toxic component ammonia into urea that can then be excreted in the urine. Because of its essential role, deficiency in arginine results in a build up of toxic ammonia and leads to hyperammonemia. The symptoms of hyperammonemia include lethargy, vomiting, ataxia, hyperesthesia and can be serious enough to induce death and coma in a matter of days if a cat is being fed a arginine free diet. The quick onset of these symptoms is due to the fact that diets devoid in arginine will typically still contain all of the other amino acids, which will continue to be catabolized by the body producing mass amounts of ammonia that very quickly build up with no way of being excreted.
Another unusual feature is that the cat cannot produce taurine, with taurine deficiency causing macular degeneration, wherein the cat's retina slowly degenerates, causing irreversible blindness. This is due to the hepatic activity of cystinesulfinic acid decarboxylase being low in cats. This limits the ability for cats to biosynthesize the amino acid taurine from its precursor amino acids ethionine and cysteine, which ultimately results in inadequate taurine production needed for normal function. Deficiencies in taurine result in compensated function of feline cardiovascular and reproductive systems. These abnormalities can also be accompanied by developmental issues in the central nervous system along with degeneration of the retina.