Cancer and the protective effect of physical activity: the epidemiological evidence

Studies of activity and all-cancer mortality have inconsistent findings and are difficult to interpret, largely because cancer refers not to one disease but to many distinct, site-specific diseases. However, mounting evidence suggests that physical activity may be associated with decreased mortality from and incidence of certain types of cancers. In 15 of 18 studies, higher levels of occupational and/or recreational activity were inversely related to colon cancer incidence and mortality. One major study found activity to be negatively related to occurrence of breast cancer, and conflicting findings exist regarding the association between activity and prostatic cancer. Given the consistency in the direction and magnitude of the findings regarding activity and colon cancer, the presence of appropriate temporal relationships between measured exposure and outcome, the suggestion of dose-response relationships and the existence of plausible biological mechanisms, including increased transit time and gut motility, the evidence supports the conclusion that activity is protective against colon cancer. Although that protective effect may be small, the attributable risk of colon cancer associated with inactivity may be quite high given the prevalence of inactivity in Western societies.