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.
Nieman and his colleagues captured the effects of the common cold using the Wisconsin Upper Respiratory Symptom Survey, a reliable and valid daily logging system.
The participants reported how many days they exercised during their leisure time per week and rated their physical fitness on a 10-point Likert scale. They were then divided into tertiles based on their responses.
After adjustment for age, sex, years of education, marital status, mental stress level, body mass index, and fruit intake, individuals who reported exercising at least five days a week spent significantly fewer days, on average, with an upper respiratory tract infection, compared with those who exercised the least.
In addition to the number of days spent with an upper respiratory tract infection, the severity and symptomatology of such infections was reduced as well, by up to 41 percent in those who reported high aerobic activity and physical fitness compared to those who reported low exercise and fitness levels.
In a model looking at perceived fitness level, several factors were associated with fewer days spent with an upper respiratory tract infection, including older age, high or medium fitness level, lower education, being married or male, having high versus low BMI, and eating three or more servings of fruit per day.
With the exception of fruit intake, the same factors were significant in a model of exercise frequency as well.
Nieman and his colleagues acknowledged that the study was limited by the lack of adjustment for all potential confounders — in particular, the study did not adjust for exposure to pathogens at work and in the home.