Vitamin E was first discovered in 1922 when it was determined that female rats required vitamin E in order to maintain a normal pregnancy. Vitamin E is a natural fat-soluble vitamin and is stored in the liver. It is found in most tissues of the body. Vitamin E is a potent antioxidant that neutralizes harmful molecules called "oxygen-free radicals" that can damage cells. It has also been shown to have reduce inflammation, enhance the immune system, and help cells fight infection.

The requirements for vitamin E in a diet depend on a number of factors, including the dietary levels of polyunsaturated fatty acids and selenium, with which it is closely linked.

In commercial pet foods, vitamin E is used to prevent the oxidation of fatty acids which leads to rancidity. It accomplishes this by "neutralizing" the free radicals which cause rancidity. However, during this process, vitamin E is used up. Therefore, an increase in the amount of fatty acids in a diet, especially unsaturated, requires a corresponding increase in the amount of vitamin E and also increases an animal's requirement for vitamin E. For this reason, feeding a diet that has been improperly prepared or stored, or supplemented with large amounts of unsaturated fatty acids, can lead to a vitamin E deficiency.

The effects of vitamin E deficiency, while uncommon in dogs and cats, are well-recognized. In dogs, vitamin E deficiency has been associated with impaired immunological response, retinal atrophy, muscle dystrophy, testicular problems, and failure of gestation. In cats fed marginal or low levels of vitamin E and high amounts of unsaturated fatty acids, pansteatitis or "yellow fat disease" can result.

Vitamin E supplementation has been shown to have some anti-inflammatory effect on certain skin disorders in dogs. It is used in the treatment of certain immune-mediated diseases, including such diseases as dermatomyositis, discoid lupus erythematosus, pemphigus erythematosus, and epidermolysis bullosa, with varying degress of success.

Vitamin E at high doses ( four to ten times the normal canine daily intake) has been shown to be effective in the treatment of a disease called primary acanthosis nigricans in dachshunds, with reductions in inflammation, crusting, and itchiness. No toxic effects were reported. In fact, high levels of vitamin E are considered to be non-toxic to animals, although an excess intake may increase the requirements for vitamins A and D.

Studies have shown vitamin E to be ineffective on its own in the treatment of itchiness and inflammation in dogs with allergic disease Decreased blood levels of vitamin E were thought to cause suppression of the immune system leading to demodicosis, a mite infestation in dogs that is the result of a faulty immune system. Dogs with demodicosis were reported to show significant improvement when treated with supplemental vitamin E . However, these findings were contradicted by other studies which demonstrated no significant improvements in dogs with demodicosis when supplemented with vitamin E.

Vitamin E plays a role in both canine and feline nutrition, both in preventing certain deficiency-related disorders and as a form of adjunctive therapy in certain skin diseases . It can also play an important role, along with vitamin C, as an effective antioxidant in commercial pet foods.