Strenuous prolonged exertion and heavy training are associated with depressed immune function. Furthermore, improper nutrition can compound the negative influence of heavy exertion on immunocompetence. Dietary deficiencies of protein and specific micronutrients have long been associated with immune dysfunction. An adequate intake of iron, zinc, and vitamins A, E, B6 and B12 is particularly important but excess intakes can also impair immune function. Immune system impairment has also been associated with excess intake of fat. To maintain immune function, athletes should eat a well balanced diet sufficient to meet their energy requirements. An athlete exercising in a carbohydrate-depleted state experiences larger increases in circulating stress hormones and a greater perturbation of several immune function indices. Conversely, consuming carbohydrate during exercise attenuates rises in stress hormones such as cortisol and appears to limit the degree of exercise-induced immunosuppression, at least for non-fatiguing bouts of exercise. Strong evidence that high doses of antioxidant vitamins, glutamine supplementation or echinacea extracts can prevent exercise-induced immunosuppression is lacking.