Nov. 1, 2010— -- Being fit — or at least feeling fit — appears to be associated with a reduction in upper respiratory tract infections, researchers found.
During a 12-week period, people who said they exercised at least five days a week had 43 percent fewer days with an upper respiratory tract infection than those who exercised no more than one day a week, according to David Nieman, a researcher at Appalachian State University in Kannapolis, N.C., and colleagues.
Similarly, those who rated themselves as highly fit had 46 percent fewer days with a respiratory infection than those who reported low fitness, the researchers reported online in the British Journal of Sports Medicine.
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The findings are consistent with previous epidemiologic and randomized studies, they wrote. Although the underlying mechanism remains unclear, the relationship might be explained by the effect of exercise on the body's immune response.
"Each aerobic exercise bout causes a transient increase in the recirculation of immunoglobulins, and neutrophils and natural killer cells, two cells involved in innate immune defenses. Animal data indicate that lung macrophages play an important role in mediating the beneficial effects of moderate exercise on lowered susceptibility to infection," Nieman and his colleagues wrote.
"Stress hormones, which can suppress immunity, and pro- and anti-inflammatory cytokines, indicative of intense metabolic activity, are not elevated during moderate aerobic exercise," they continued.
"Although the immune system returns to pre-exercise levels within a few hours after the exercise session is over, each session may improve immunosurveillance against pathogens that reduce overall upper respiratory tract infection incidence and symptomatology."
The researchers followed 1,002 adults up to age 85 during two 12-week periods in 2008; half participated in the fall and half participated in the winter.