The evidence for a protective influence of physical activity is most clear for colon cancer, although exercise does not appear to protect against rectal cancers. Cancers of the colon make up about three-quarters of total colorectal cancers. A decrease in colon cancer risk has been observed in individuals who work at occupations that require high degrees of physical exertion. The evidence regarding colon cancers is quite strong and has been observed consistently in numerous studies in different countries. There are at least two plausible ways in which exercise may help prevent colon cancer.
Exercise is required for the proper functioning of the large bowel.
For example, people who are bedridden frequently suffer from constipation. Exercise increases the rate in which stool transits through the intestine and, as a result, contact with the potential carcinogens in the stool may be minimized. Some evidence indicates that high circulating levels of insulin may promote the growth of tumors. Although a reduction in colon cancer risk through physical activity is very likely, the magnitude of the benefit is not well quantified. Much evidence suggests that even moderate levels of activity, such as brisk walking three hours per week, could substantially lower risk.