Exercise and cancer: linkages with obesity?

A growing number of well-controlled studies suggest that both hard physical work and an active leisure can reduce the overall incidence of cancer. Benefit is demonstrated most clearly for colonic tumors. Here, benefit persists after control of the data for body mass index, and a likely mechanism is the speeding of colonic transit. Occasional reports suggest protection against tumors of the breast, reproductive tract, and other body organs. In some of these latter sites, potential mechanisms of protection include a suppression of sex hormone secretion, and a reduction of body fat depots, where androgens are converted to toxic estrogen derivatives; vigorous athletic participation may well modulate hormone output, but a reduction of body fat is a more likely explanation of any benefit from moderate physical activity.