Similar to healthy dogs, sick dogs need to eat dog food in order to supply energy and nutrients for growth, for replacement and repair; as well as to meet an ever changing need. Dog nutrition still remains the same for a sick dog and usually do not differ greatly from those of a healthy dog. A sick dog's dietary needs, however, can become substantially different from those of a healthy dog.

Diseases in which a dog's diet will need to be changed are often due to true nutritional diseases, in which the diet itself is responsible for the disease. Most of the true nutritional diseases are deficiency diseases. What this means is that they are diseases caused by a dog food diet that contains an insufficient amount of one or more needed nutrients. Most of these diseases were the result of inadequate or improperly balanced home-made dog food.

Since cost-effective commercial dog food became more and more nourishing, most of these deficiency diseases gradually disappeared. Vitamin or mineral deficiencies are rarely seen as a primary disease any more. Both vitamins and minerals are inexpensive and are needed in such small amounts that today, few commercial dog foods fail to contain them in adequate amounts.

Although energy and protein continue to be a problem with some dog foods, the number of dog food brands that still contain insufficient fat or poor quality, indigestible protein become less and less every year. When a deficiency of fat occurs, it most often appears as an insufficient amount of total energy in the diet which results in weight loss, sluggishness, dry and dull hair coat, poor physical condition and, in some extreme cases, emaciation and uncontrolled diarrhea.

A deficiency of essential fatty acids may also occur, although it is unlikely. The total amount of the fats most often used in commercial foods can drop to as low as 1% of the diet and that diet will still contain sufficient fatty acids. The only exception to this might be in cases of dry dog foods where larger quantities of fat have turned rancid.