Physical activity and risk of colon cancer: the Physicians‘ Health Study (United States)

Previous studies indicate that physical activity is related inverselyto colon cancer risk. However, details regarding that association - whether adose-response relation exists, whether the relation differs between non-obeseand obese persons, the effect of long-term physical activity - are unclear.We examined these issues in the Physicians‘ Health Study (United States).Physical activity was assessed at baseline among 21,807 men, aged 40 to 84years, and again 36 months later. Men were followed for an average of 10.9years (from baseline) during which 217 developed colon cancer. Afteradjusting for potential confounders (including age, obesity, and alcoholintake), the relative risks for colon cancer associated with vigorousexercise in times per week (< 1, 1, 2-4, 5+, at baseline) were 1.0(referent); 1.1 (95% confidence interval [CI] = 0.7-1.7); 1.2 (CI = 0.8-1.6);and 1.1 (CI = 0.7-1.6), respectively; P trend = 0.6. Physical activity wasnot associated significantly with colon cancer risk either among non-obese orobese men. When we used physical activity assessments at baseline as well asat 36 months, physical activity again was unrelated to colon cancer risk.These data do not support the hypothesis that physical activity reduces therisk of colon cancer. Plausible alternate explanations for the null findinginclude misclassification of physical activity and the potential forincreased surveillance for colon cancer (‘screening effect’) among thosephysically active.