Wholesome nutrition is the key to maintaining a healthy immune system and resistance to disease. Nutritional deficiencies or imbalances as well as exposures to various chemicals, drugs and toxins present an ongoing immunological challenge which can suppress immune function, especially in those animals genetically susceptible to immune dysfunction (immune deficiency, autoimmunity, allergies).

Genetic differences between individuals lead to quantitative variations in dietary requirements to maintain health. Genetic defects also may result in inborn errors of metabolism that affect one or more pathways involving nutrients or their metabolites. While minimal and maximal nutrient requirements have been established for most vitamins and trace mineral elements, optimum amounts for every individual should not be assumed. Examples of important vitamin and mineral requirements in this regard include vitamin C, vitamin E and selenium, vitamin A, copper and vitamin B-12. Similarly, a wide variation occurs in the energy needs of dogs depending on their breed, age, sex, and size.

Nutritional factors that play an important role in immune function include zinc, selenium and vitamin E, vitamin B-6 (pyridoxine), and linoleic acid. Deficiency of these compounds impairs both humoral as well as cell-mediated immunity. The requirement for essential nutrients increases during periods of rapid growth or reproduction and also may increase in geriatric individuals, because immune function and the bioavailability of these nutrients generally wanes with aging. As with any nutrient, however, excessive supplementation can lead to significant clinical problems, many of which are similar to the respective deficiency states of these ingredients.

Synthetic antioxidants like butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate, and ethoxyquin have been used as preservatives in human and animal foods for many years, although the safety of these chemicals when chronically fed at permitted amounts in dog and cat foods has been questioned. As a result, naturally occurring antioxidants (vitamins E and C) are also commonly used in animal foods.

Nutritional influences can have a deleterious effects on thyroid metabolism. The classical example is the iodine deficiency that occurs in individuals eating cereal grain crops grown on iodine-deficient soil. Iron and zinc also are important minerals in regulating thyroid metabolism. Another link has recently been shown between selenium deficiency and hypothyroidism. Cereal grain crops grown on selenium-deficient soil will contain relatively low levels of selenium. While commercial pet food manufacturers compensate for variations in basal ingredients by adding vitamin and mineral supplements, it may be difficult to determine optimum levels for so many different animal breeds having varying genetic backgrounds and metabolic needs. The selenium-thyroid connection has significant clinical relevance, because blood, but not tissue, levels of thyroid hormones rise in selenium deficiency. Thus, selenium-deficient individuals showing clinical signs of hypothyroidism could be overlooked on the basis that blood levels of thyroid hormones appear normal.