Cats are Obligate Carnivores

To understand what food is best for our cats, we need to know what type of food their anatomy is designed to best utilize. Dr. Bruce Cauble, D.V.M., puts it very succinctly when he says "Animals, just like people, are still using digestive systems that evolved thousands of years ago, systems designed to provide them with nutrients derived from whole foods. You need only look at the digestive system of an organism to determine its appropriate diet." Simply put, we need to consider the physiological structure of the cat. When you have the chance, take a good look at your cat. It has needle-sharp teeth and claws meant to catch, hold, and tear flesh. It has ears that face forward and eyes in the front of its head to better focus on that mouse or bird it wants to chase. That mouse or bird is built differently, being a prey animal, they usually have eyes more to the side of their heads to better see who's trying to make them into lunch! Your cat's digestive system is brilliantly designed to eat specific foods. It's short and acidic; proper foods are utilized swiftly and efficiently. That snuggly-wuggly kitty in your house is a well-developed predator.
Officially, the cat is a carnivore. In fact, your cat is actually an obligatory carnivore, which means it must eat meat. Just because it resides in your home and enjoys your company doesn't change its physical make-up and its nature. Obviously, it should not eat the same food as a cow or a chicken because that's not how its body is designed. Cows and chickens are herbivores and are structurally quite different from your cat.
Now, I'm not saying that you must place a live mouse in your cat's dinner bowl (although some cats would love it), but we do need to feed our cats as closely as we can to what they were built to eat. That's the responsible thing to do when you've chosen to live with a carnivore. Your feline friend's health will benefit greatly if you keep its natural dietary needs in mind when reading cat food product labels or choosing what type of food to put in its dish.(http://cats.about.com)