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WESTERLIND, K. C. Physical Activity and Cancer Prevention—Mechanisms. Med. Sci. Sports Exerc., Vol. 35, No. 11, pp. 1834–1840, 2003.This paper presents potential mechanisms by which exercise or physical activity may affect cancer development.Analysis of published and unpublished experimental and epidemiological data from the cancer-activity literature and from other fields of study are compiled to provide a summary of potential mechanisms by which exercise may mediate cancer development.Exercise appears to have a beneficial effect relative to cancer development, and the reader is referred to other sections of this symposium. To date however, the mechanism(s) remains unknown. Potential mechanisms influenced by exercise include alterations in steroid hormones or insulin/insulin-like growth factors, immune modulation, alterations in free radical generation, changes in body composition or weight, and direct effects on the tumor. Cancer is a complex process. It is clear that multiple mechanisms may be operative and that the characteristics of the individual, type of exercise, as well as type of cancer and stage of carcinogenesis will affect which mechanisms may affect the disease. More experimental research in both animal models and in human clinical studies is needed to understand the basic biological mechanisms underlying the effect of physical activity on cancer.In general, physical activity is associated with reduced risk of cancer development, yet to date, the mechanisms remain unknown.