Cats should not eat dog food. Dogs can eat cat food just fine, but cats should not eat dog food. Cats and dogs are more different than meets the eye, and they have vastly different nutritional needs.
Dogs have developed side by side with humans to a point where they can eat most of the things we eat and digest. They are considered omnivores, which means that they are able to digest carbohydrates, and they can survive on plant based foods. That doesn’t mean that dogs prefer meat to vegetables and other plants, or that a diet based completely on plants would be nutritionally adequate, but they can survive on it. A cat cannot.
Cats are carnivores. That means that they cannot live unless they eat meat in some form. One major difference between dog food and cat food is the percentage of meat.
Other differences between cats and dogs include the ability to handle vitamins and amino acids. Dogs, for instance, have the ability to break down organic pigments from plants called carotenoids and convert them into active Vitamin A. Cats can’t do that.
Cats have to get Vitamin A already converted by some other animal. That is, they have to eat the meat from some animal who already converted the carotenoids to active Vitamin A. This is one of the reasons most carnivores don’t eat other carnivores; they need to eat animals who live off plants in order to survive.
Arginine is another big difference between cats and dogs; it is a building block for proteins and cats have to get it with every meal. Dogs produce an enzyme that helps them produce Arginine, and they only need to get a little with their food.
Taurine is an amino acid distributed through the heart, retina, and other organs. Dogs produce their own, but cats must eat it, and it is not present in any plants.
There are more examples of substances cats must get through their food, such as niacin and arachidonic acid.