PHYSICAL-ACTIVITY AND RISK OF COLON-CANCER - THE PHYSICIANS HEALTH STUDY (UNITED-STATES)

Previous studies indicate that physical activity is related inversely to colon cancer risk, However, details regarding that association - wh ether a dose-response relation exists, whether the relation differs be tween non-obese and obese persons, the effect of long-term physical ac tivity - are unclear. We examined these issues in the Physicians' Heal th Study (United States). Physical activity was assessed at baseline a mong 21,807 men, aged 40 to 84 years, and again 36 months later. Men w ere followed for an average of 10.9 years (from baseline) during which 217 developed colon cancer. After adjusting for potential confounders (including age, obesity, and alcohol intake), the relative risks for colon cancer associated with vigorous exercise in times per week (< 1, 1, 2-4, 5+, at baseline) were 1.0 (referent); 1.1 (95% confidence int erval [CI] = 0.7-1.7); 1.2 (CI = 0.8-1.6); and 1.1 (CI = 0.7-1.6), res pectively; P trend = 0.6. Physical activity was not associated signifi cantly with colon cancer risk either among non-obese or obese men. Whe n we used physical activity assessments at baseline as well as at 36 m onths, physical activity again was unrelated to colon cancer risk. The se data do not support the hypothesis that physical activity reduces t he risk of colon cancer. Plausible alternate explanations for the null finding include misclassification of physical activity and the potent ial for increased surveillance for colon cancer ('screening effect') a mong those physically active.