To paraphrase feminist dogma, a cat needs carbohydrates like a fish needs a bicycle. They lack the physical capacity to utilize carbs effectively and, because of this, cats on a high-carb diet run the risk of becoming obese and even diabetic.
Like their wild relatives, large and small, domestic cats are obligate carnivores or pure meat eaters. Wild felines of approximately the size of Felis domesticus normally catch small birds and mammals such as mice several times a day and eat the whole carcass (except the feathers). They get Vitamin A from the liver, calcium from the bones and other vital nutrients from eating organs, skin and both the tender and chewy bits. The best diet advocated for healthy cats is a high-quality meat-based (beef, lamb, chicken, turkey) canned food that contains not more than 3 to 5 percent carbohydrates.
Wait a minute -- why isn't fish on that list? Everybody knows cats love fish -- it's in all the cartoons. It seems the cartoons are wrong. Since cats are basically a desert species, fish would not be a normal part of their environment or their diet. Only one species of cat-size wild feline -- the Asian fishing cat -- is known to fish, and its diet also includes birds and small mammals, plus snakes and even snails. Many cats lust after the taste of fish and some even become addicted to it, to the exclusion of better food, but it should not be the basis of a cat's diet, since it does not supply enough of the essential amino acid taurine. If your cat craves fish, save it for an occasional treat.
How Dry I Am
Domestic cats, like their wild cousins, retain the desert-born characteristics of high thirst tolerance and concentrated urine. The first means they tend to visit the water bowl seldom and sparingly, and the smell of the litter box and sprayed furniture testifies to the latter. Dry cat food, high in carbohydrates, does nothing to benefit either. A nice, juicy mouse, on the other hand, is something like 75 percent water and provides a cat with a significant portion of its need for fluid intake. Fluffy not interested in hunting? Canned cat food contains about the same amount of water.
If you yourself are not an eater of meat for moral, ethical, religious or any other reasons, imposing this restriction on your cat is a hotly debated topic. While highly specialized non-meat diets touted as nutritionally complete are available for cats, they can be pricey or limited in availability, and some cats will not eat them. The cat is designed by nature, from teeth to bowels, to eat meat, and "it's not nice to fool Mother Nature."