Q. Why is it a no-no to give dog food to cats?
A. As we often say, "Cats are not small dogs." In the case of diet, it is important to realize a cat's nutritional requirements are much different than those of a dog. For example, cats require higher levels of protein than dogs. Cats must have the amino acid called 'taurine' in their diet; dogs can actually make their own taurine. A cat eating food deficient in taurine can develop severe heart disease and other health problems. Almost all cat foods now contain added taurine.
Cats require a different form of Vitamin A than dogs do. Dogs can use beta-carotene as a source of Vitamin A; cats cannot. Cats can not manufacture the fatty acid called 'arachidonic acid' and must have it supplemented in their diet; it is not essential for dogs to have this fatty acid in their food.
So, you see, if a cat is allowed to eat a significant amount of dog food, the cat would be eating a diet deficient in many of the cat's required nutrients. For your cat's health, be sure she is eating quality cat food.