1. The immune systems of athletes and nonathletes when measured in the resting state are more similar than disparate. Even when significant reductions in resting immune function have been observed in athletes, investigators have had little success in linking these to a higher incidence of infection and illness. 2. Many components of the immune system exhibit change after prolonged, heavy exertion. During this "open window" of altered immunity (which may last between 3 and 72 hours, depending on the measure of immune function), viruses and bacteria may gain a foothold, increasing the risk of subclinical and clinical infection. 3. The influence of some nutritional supplements on acute immune response to prolonged exercise has been evaluated in endurance athletes. Though vitamin C and glutamine have received the most attention, the data thus far are inconclusive as to a beneficial effect. 4. In contrast to micronutrient and amino acid supplementation, the ingestion of fluids that contain carbohydrate reduce perturbations in the immune system (less disturbance in blood immune cell counts, lower granulocyte and monocyte phagocytosis and oxidative burst activity; diminished pro- and anti-inflammatory cytokine response). 5. By maintaining higher plasma glucose levels and attenuating the cortisol and growth hormone responses during heavy exercise, carbohydrate-beverage ingestion may reduce stress to the immune system of athletes.
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