Immunology, nutrition, and the athlete—part I

Townsend Letter for Doctors and Patients,  June, 2004  by Jason Barker,  Chris Meletis

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With the ever-increasing popularity of endurance-type sporting events (marathons, triathlons, and "ultra" versions of these and other combined sports), the study of immunology in the athlete continues to grow as a burgeoning field. More practically, preventative and natural medicine-oriented physicians continue to provide a role in serving this sector of the population as research continues to explore the role of nutraceutical and botanical medicines and their role in athletic performance and immunologic function. The ever-increasing number of such grueling competitions that challenge the limits of human endurance continues to push the boundaries of current dietary recommendations for these athletes. The selection of diet is dependent on factors such as the type and duration of the event, recovery time, and total energy expended.

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Another interesting new challenge involves exploring the effects of endurance exercise on the athlete's immune system. Well-established in the literature, moderate, near daily exercise leads to positive changes in the immune system that correlates with less frequent upper respiratory tract infections. Therefore, a commonly held belief is that moderate (near-daily physical activity of 30 minutes or more) exercise benefits the immune system. An example of this was detailed in a study during which a group of people that walked briskly on a near-daily basis were compared to another group of completely sedentary individuals; the walkers experienced a greater than 50% decrease in sick days over a period of 3-4 months; these changes were noted without observed immune function changes. (1) It is hypothesized that the improvement in host protection and immunosurveillance that is concordant with moderate exercise is related to an additive effect from the acute positive immunologic changes that occur with individual exercise sessions. Additionally, the benefits of moderate exercise may be applied to numerous disease processes as both a preventative and treatment therapy. Over time the most notable benefit of a regular, moderate exercise program equates to less sick days with the common cold or other upper respiratory tract infections. (2) This is not to negate the other far-reaching effects of exercise on several disease processes; epidemiologic studies demonstrate a beneficial effect from exercise at decreasing risk from heart disease, stroke, hypertension, diabetes mellitus (type 2), osteoporosis, falls and fractures in the elderly, dementia, anxiety, and depression. In addition, this research shows a link, albeit weak, between sedentary lifestyles and higher risks of colon, breast, and lung cancers. (3) Exercise is without a doubt perhaps the single greatest contributor to one's state of health; nearly every person can benefit from some form of physical activity.

On the other hand, intensive, prolonged exercise causes several changes in immune function that are a reflection of the physiologic stress and suppression endured in such circumstances; training of this type leads to an increased predisposition to upper respiratory tract infections (URTIs) and excessive physical exertion that extends beyond roughly 90 minutes is correlated with adverse immune system changes in several areas including the skin, upper respiratory tract mucosa, blood, lung tissue, and muscle. (2)

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The risk of exercise-related infection seems to follow a Jshaped curve, with non-exercisers experiencing more infections in comparison to regular, moderate exercisers while those that exercise regularly at a strenuous level have the highest risk. This risk of infection continues to grow when factors such as travel (exposure to new pathogens), inadequate sleep, weight loss, excessive mental stress and inadequate nutrition are added to an aggressive exercise regimen. Following strenuous exercise, athletes enter a period of time in which they experience impaired immune resistance in which they are more susceptible to URTIs. However a definitive causal relationship has not yet been clearly defined. (4) According to Neiman, anecdotal, survey, and epidemiologic data all correlate with an increased risk of URTI among endurance athletes both during and following (perhaps up to 1-2 weeks) periods of intensive training. (5) The period of time in which the athlete is more susceptible to infection is referred to as the open window; the time appears to vary from as little as three hours to seventy-two hours. (6)

In a review of the literature focusing on the effects of chronic exercise training on human immune function, it was revealed that although immune cell numbers remain at normal levels during the training episodes, other evidence shows slight impairment in immune parameters such as neutrophil function, serum and mucosal immunoglobulin levels, plasma glutamine concentration, and cytotoxicity of natural killer cells. (7) Whether an athlete is clinically immunosuppressed or not, the possibility exists that the combined effects of these small changes in immune function may contribute to a compromised resistance to minor illnesses thereby having a detrimental effect on performance and by preventing the athlete from competing at their maximal level.