Exercise in the prevention and treatment of cancer. An update.

Physical activity potentially encourages a healthy lifestyle and it could have a more direct preventive effect on certain forms of carcinogenesis (for instance, by speeding gastrointestinal transit, or by moderating sex hormone levels). However, there are also potential negative effects, particularly an excessive exposure to ultraviolet light in certain water sports. The many types of neoplasm and the equally varied sources of physical activity militate against finding any simple relationship between the risk of malignancy and the individual's physical activity history. Nevertheless, evidence that physical activity protects against certain forms of cancer can be deduced from studies of experimental animals, former athletes, people employed in active occupations, and those with an active recreational lifestyle. Many occupational surveys and a number of studies of recreational activity show an association between sedentary living and a risk of colon cancer, both in men and in women. Moreover, an application of Bradford Hill's criteria gives some support to the causal nature of the association. More limited data suggest that a history of active leisure is associated with a reduced risk of all-cause cancer and in women of breast and reproductive system cancers. The last observation must still be reconciled with an apparent increase in the risk of prostatic cancer in active men. Since moderate exercise elevates mood and helps to conserve lean tissue, it may finally be a helpful component of treatment after a neoplasm has been diagnosed.