Of course, there is protein in some plants (think about tofu), but not in high concentrations. And the proteins are incomplete. In addition, there are nutrients in meat that can’t be found in plants, or that most mammals can synthesize in their bodies from incomplete proteins, but that cats can’t synthesize. Cats cannot live without these nutrients, and they need to get them from their diet.
For example, taurine, an essential amino acid, is found only in animal proteins. Without enough taurine, cats develop vision and heart problems. Arginine is an amino acid that is found most abundantly in animal proteins. This nutrient clears the blood of ammonia, which is a by-product of protein metabolism. Since cats use so much protein, just one meal without arginine is enough to drive ammonia in the blood to toxic levels. Choline and the other B vitamins, many of which are important to protein metabolism, are also found in the highest concentrations in meat, and your carnivore cat needs lots of those vitamins.
As efficient as cats are at metabolizing protein, they can’t metabolize vitamin A from the betacarotene in plants like carrots, and need vitamin A in a different form that is found only in meat. There are a variety of fatty acids found only in meat that cats need for proper function of the immune system, healthy skin, control of inflammation and other body processes-your cat can’t synthesize these nutrients either. And while we can sit in the sun and convert the 7-dehydrocholesterol in our skin into vitamin D, cats cannot.
Face it: Kitty is a little tiger. If she were bigger, she’d be out hunting antelopes. In your house, maybe she hunts mice. Maybe she just hunts cat food. No problem, as long as it’s meat.