Puppy food. A good puppy food satisfies the needs of a baby animal whose every bodily system, from their bones to their nerves to their viscera to their immune system, is developing very quickly. Because of this, puppy food is not only high in protein, vitamins and minerals but has much more fat than food for a grown dog. If puppy food was fed to an adult dog, the animal would probably become obese. Owners of puppies wonder how long they should feed their dog puppy chow, especially if they have something like a Great Dane who is already larger and heavier than kindergardener at only eight months old. The rule of thumb is that a puppy should be fed puppy food for about a year.