Nutrition--mechanisms of immunosuppression.

Nutritionists must formulate diets that supply adequate amounts of nutrients from five major groups. These are carbohydrate, protein (amino acids), fat, minerals and vitamins. Carbohydrate is usually a cheaper source of energy than fat, but fat is often used to increase the caloric concentration of the diet. Variations in energy intake which may effect immunocompetence usually result from management practices rather than diet formulation. Feed restriction for broiler breeders and withholding feed in forced molting practices may affect immunocompetence. Feed restriction causes higher plasma corticosterone levels, which are known to decrease the immune response, possibly through effects on cytokines. Excessive feed, through forced feeding, may also have short-term effects on indicators of humoral immunity. Protein and amino acid nutrition have been studied in relation to immunocompetence. The level of dietary amino acid needed to maximize growth and feed efficiency will also generally maximize measures of immunocompetence. The level of amino acids needed for maximum growth is lower in chicks which have been immunologically stressed than in chicks which have not. An immune response changes metabolism so that less growth occurs, thereby decreasing the need for amino acids. Dietary levels of minerals can affect immunocompetence. While deficient levels of sodium and chloride decrease humoral immunity, levels of these nutrients which supported maximum growth also supported maximal humoral immunity. Low dietary zinc levels did not affect indicators of immunocompetence in the chick. The effect of fat soluble vitamin levels on the immune system has been studied. Vitamin A is needed to maintain epithelial tissue and prevent infection. Cellular immune response is decreased when the chick is deficient in this nutrient. Several indicators of immune responsiveness are depressed when chicks are vitamin E and/or selenium deficient. Since these nutrients serve as antioxidants, cellular integrity may be affected by a deficiency. Cellular integrity is very important for receiving, and responding to the messages needed to coordinate an immune response. High levels of vitamin E (greater than 10 times the required level) have been found to be immunostimulatory.