There are two main groups of vitamins: water soluble and fat soluble. The water soluble vitamins are not stored to any great extent in the body, whereas fat soluble vitamins are stored in fatty tissue and in the liver. The vitamins are:

The fat soluble vitamins have very specific functions. Deficiencies, while rare, produce very distinct signs. The primary function of vitamin A is in helping to maintain proper eyesight and helping in maintenance of normal healthy skin. Vitamin D helps the body utilize calcium and phosphorus and is critical in the formation and maintenance of bones and teeth. Vitamin E functions as an anti-oxidant.

Unlike the fat soluble vitamins, most of the water soluble vitamins function as cofactors in various metabolic reactions, which enable the animal to extract energy from the ingested fats, proteins and carbohydrates. For this reason, the signs of a B-vitamin deficiency are vague, but are usually characterized by a loss of appetite and poor skin and coat. Vitamin C is not required by the normal healthy dog or cat as it can make vitamin C from glucose. Some of the vitamins shown above, like biotin, may not be included in a commercial pet food since the microorganisms in a pet's intestinal tract make these vitamins, which are then absorbed by the pet. Only in cases when a dog or cat might be on a prolonged antibiotic therapy might these vitamins need to be included in a food.

Choline, while not officially a vitamin, is often included with vitamins because it shares some of the same traits, although its requirement is 100 to 1000 times that of most vitamins. Choline functions in a variety of metabolic reactions and also plays an important structural role in cell membranes and nerve transmission.

Much is often made about the detrimental effect that processing (either extrusion or canning) has on vitamins. While a small number of vitamins are destroyed by the heat and pressure used in making pet food, the reactions are well known and extra vitamins are added to pet foods to more than compensate for these losses. This has not always been true. During the early stages of the pet food industry, diets, which were deficient in vitamins due to processing losses, were shown to cause health problems in pets, which could be corrected by vitamin supplementation. Today there is more of a likelihood that a pet would suffer from an excess of vitamins because of an overly concerned owner who has unnecessarily added a vitamin supplement to the diet.

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