Is Infection Risk Linked to Exercise?
ABSTRACT & COMMENTARY
Synopsis: Excessive, strenuous exercise appears to increase infection risk, while moderate exercise appears protective.
Source: Nieman DC. Is infection risk linked to exercise workload? Med Sci Sports Exerc 2000;32(7):S406-S411.
A common perception is that elite athletes subject to prolonged and intense exertion have decreased resistance to upper respiratory tract infections (URTI). Much of the data regarding this association are anecdotal and obtained from surveys. The reverse association has also been suggested—that is, a recent URTI can result in a sudden deterioration of athletic performance. In fact, following a viral infection, a clinical syndrome of decreased athletic performance, lethargy, easy fatigability, and myalgia termed the "post-viral fatigue syndrome" has been described. However, the majority of endurance athletes, as Nieman stresses, do not experience URTIs after competitive events, and in fact, regular moderate exercise is said to confer resistance against infection.
There appears to be a link between exercise workload and infection with the risk of URTI increasing during periods of excessive, intense exercise. Changes in the immune system occur after heavy, but not necessarily moderate, exercise. These include increased neutrophils and decreased lymphocytes in the blood, increased blood granulocyte and monocyte phagocytosis, but decreased nasal neutrophil phagocytosis, increased plasma concentration of inflammatory cytokines, and decreased nasal and salivary IgA concentrations. These and other changes imply that the immune system is stressed following prolonged endurance exercise. The stress to the immune system can be further accentuated by lack of sleep, severe mental stress, malnutrition, or weight loss. Therefore, to decrease the risk of infection, athletes (since most will not alter their intense vigorous training schedules) should decrease the confounding factors just listed.
There is also some evidence that nutritional supplements, including vitamin C, glutamine, and carbohydrates may decrease the negative effects on the immune system from intense exercise. For example, Neiman notes that ". . .carbohydrate compared with placebo supplementation during prolonged exercise is associated with higher plasma glucose levels, an attenuated rise in plasma stress hormone concentrations (in particular, cortisol), and reduced stress to the immune system. . ." Such data would imply that athletes might benefit from carbohydrate sports drinks before, during, and after intense exercise bouts. Levels of glutamine, a nonessential amino acid and fuel for lymphocytes and monocytes, appear to decrease with prolonged exercise. Therefore, some feel that a decrease in glutamine could impair immunity by altering the proliferation rates of lymphocytes, but this theory has not been scientifically substantiated. Further research to more clearly define the association of exercise and immunity, as well as factors influencing this relationship, is warranted.
Comment by Letha Y. Griffin, MD, PhD
Research in the area of decreased immunity in athletes has been stimulated by the observation that athletes appear to experience a higher number of URTIs during periods of intense activity. However, research data regarding this alteration of immune function in athletes is conflicting. Although the numbers of circulating immune cells do not seem to vary with exercise, the cell function may, as might immunoglobulin concentration. However, no conclusive evidence has been gathered to date.
Nonetheless, the take-home message in this article and others in this special supplement of Medicine and Science in Sports and Exercise is that there is some relationship between exercise and immunity. Although the data are somewhat conflicting, it does appear that regular fitness exercise enhances the immune system, whereas overexertion, especially if combined with other stresses such as not eating or sleeping properly, can decrease the immune response. The cause of this suggested decrease in immunity is not well understood.
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