Although a dog will not die from a diet exclusively made up of cat food, cats and dogs have different dietary needs. According to The Daily Cat, cat food is formulated to have much higher levels of fat, protein and calories than is healthy for most dogs. A cat's metabolism does well on such a diet, but dogs will gain weight and can experience gastrointestinal issues such as vomiting or diarrhea.
Cats are obligate carnivores; that is, they need animal-based protein to survive, but they cannot digest vegetable matter. Cat food, consequently, is very heavy on animal protein and animal fat. Kitten food is even higher in animal-based nutrients than is food meant for adult cats. Dogs can digest both meat and vegetable matter, but prefer meat. According to PetMD, this preference for meat can make cat food very tempting to a dog, but it is not designed with a canine's health in mind. The high animal fat content can even cause acute pancreatitis in dogs, as flare-ups of inflammation in the pancreas are associated with high fat levels in blood serum. Cat food can serve as a treat to an especially insistent dog, or as a last-ditch measure to keep a hungry dog fed when dog food is not available, but it is not recommended as a primary diet.