Moderate to vigorous physical activity and risk of upper-respiratory tract infection

Purpose: A "J"-shaped model has been proposed to describe the relationship between physical activity and risk of upper-respiratory tract infection (URTI). However, little epidemiologic evidence is available to support the contention that moderately active individuals are at lowest risk of URTI. This investigation examined differences in URTI risk between physically inactive and moderately active adults. Methods: Observational study of 547 healthy adults (49% women) aged 20-70 yr reported URTI events at 90-d intervals over 12-month of follow-up (5 evaluations). Three 24-h physical activity recalls per evaluation were obtained and averaged to quantify total moderate-vigorous activity (greater than or equal to3.0 metabolic equivalents [MET]). Associations between URTI and physical activity levels were estimated using incidence rate ratios (IRR) derived using Poisson regression while adjusting for a number of potential confounders including age, education, anxiety, cynicism, and selected dietary factors. Results: Men and women reported 1.2 (1.4) and 1.2 (1.2) URTI events per year, respectively (mean [(SD]). Adjusting for gender and potential confounders, the IRR for less than 3.93, 3.94-7.15, 7.16-11.95, and greater than or equal to 11.96 MET-h(.)d(-1) among men, and less than 2.38, 2.39-4.09, 4.10-6.24, and greater than or equal to 6.25 MET-h(.)d(-1) among women, were 1.00 (referent), 0.87 (95% confidence interval [CI], 0.70-1.07), 0.88 (95% CI, 0.71-1.09), and 0.77 (95% CI, 0.62-0.95), respectively (P-trend = 0.03). This effect was stronger in men (P-trend = 0.03) than women (P-trend = 0.17), although at similar expenditure levels (6-7 MET-h(.)d(-1)), risk was reduced by about 20% in men and women. Risk reduction was most pronounced in the fall of the year (P-trend = 0.02). Conclusions: These data support the hypothesis that moderate levels of physical activity are associated with a reduced risk for URTI.