To determine whether physical inactivity and obesity increase risk for colon cancer and adenomas, which are precursors of cancer, and whether the abdominal distribution of obesity is an independent risk factor for these events.

Prospective cohort study.

United States.

47 723 male health professionals, 40 to 75 years of age, who responded to a questionnaire mailed in 1986.

Questionnaires in 1986 about physical activity level and body mass index, and questionnaires in 1987 (31 055 respondents) about waist and hip circumferences. Between 1986 and 1992, 203 new patients were diagnosed with colon cancer and 586 were diagnosed with adenomas.

Physical activity was inversely associated with risk for colon cancer (high compared with low quintiles of average energy expenditure from leisure-time activities: relative risk, 0.53 [95% CI, 0.32 to 0.88], P for trend = 0.03) after adjustment for age; history of colorectal polyp; previous endoscopy; parental history of colorectal cancer; smoking; body mass; use of aspirin; and intake of red meat, dietary fiber, folate, and alcohol. Body mass index was directly associated with risk for colon cancer independently of physical activity level. Waist circumference and waist-to-hip ratio were strong risk factors for colon cancer (waist-to-hip ratio ≥ 0.99 compared with waist-to-hip ratio < 0.90: multivariate relative risk, 3.41 [CI, 1.52 to 7.66], P for trend = 0.01; waist circumference ≥ 43 inches compared with waist circumference < 35 inches: relative risk, 2.56 [CI, 1.33 to 4.96], P for trend < 0.001). These associations persisted even after adjustment for body mass and physical activity. Similar associations were seen between obesity and physical inactivity and adenomas of 1 cm or more, but no association was observed for smaller adenomas. Height was also associated with a higher risk for colon cancer (height ≥ 73 inches compared with height ≤ 68 inches: multivariate relative risk, 1.76 [CI, 1.13 to 2.74], P for trend = 0.02).

The results support an inverse association between physical activity and risk for colon cancer, whereas height and obesity, particularly abdominal adiposity, are associated with an elevated risk.