David C. Nieman, Dr.P.H.
Appalachian State University
Prolonged and intensive exertion causes numerous changes in immunity in multiple body compartments. These exercise-induced immune changes occur at the same time the human body is experiencing physiologic and oxidative stress, inflammation, and suppressed function against foreign pathogens. Risk of upper respiratory tract infection (URTI) is 2-6 times higher in endurance athletes compared to controls during the 1-2 week period following competitive race events. URTI risk may be compounded when the endurance athlete goes through repeated cycles of unusually heavy exertion, has been exposed to novel pathogens, and experienced other stressors to the immune system including lack of sleep, severe mental stress, malnutrition, or weight loss.
Although endurance athletes are at increased infection risk during heavy training or competitive cycles, they must exercise intensively to contend successfully. Can athletes use nutrient supplements to counter exercise-induced inflammation and immune alterations? Supplements studied thus far include zinc, dietary fat, plant sterols, antioxidants (e.g., vitamins C and E, beta-carotene, N-acetylcysteine, and butylated hydroxyanisole), glutamine, and carbohydrate. Antioxidants and glutamine have received much attention, but the data thus far do not support their role in negating immune changes after heavy exertion. Most of the focus on nutritional countermeasures has been on carbohydrate.
Research during the 1980s and early 1990s established that a reduction in blood glucose levels was linked to hypothalamic-pituitary-adrenal activation, an increased release of adrenocorticotrophic hormone and cortisol, increased plasma growth hormone, decreased insulin, and a variable effect on blood epinephrine levels. Given the link between stress hormones and immune responses to prolonged and intensive exercise, carbohydrate compared to placebo ingestion should maintain plasma glucose concentrations, attenuate increases in stress hormones, and thereby diminish changes in immunity. Carbohydrate supplementation may also alter immunity following exercise by increasing the availability of energy substrate to immune cells. Glucose is the major energy substrate for immune cells.
Several studies with runners and cyclists have shown that carbohydrate beverage ingestion plays a role in attenuating changes in immunity when the athlete experiences physiologic stress and depletion of carbohydrate stores in response to high-intensity (~75-80% VO2max) exercise bouts lasting longer than two hours. In particular, carbohydrate ingestion (about one liter per hour of Gatorade) compared to a placebo has been linked to significantly lower blood cortisol and epinephrine levels, a reduced change in blood immune cell counts, lower pro- and anti-inflammatory cytokines, and diminished gene expression for IL-6 and IL-8 (two important cytokines) in the muscle. These data demonstrate that the endurance athlete ingesting carbohydrate during the race event experiences a much lower perturbation in hormonal and immune measures compared to the athlete avoiding carbohydrate. Overall, the hormonal and immune responses to carbohydrate compared to placebo ingestion indicate that physiologic stress is diminished.
