Objective: The purpose of this paper is to update epidemiological research on relations between physical activity and cancer risk, including physical activity measurement and potential mechanisms of prevention of cancer. Design: Review of recent systematic reviews, meta-analyses and studies on the topic that have been published in the recent literature. Results: Convincing epidemiological evidence exists that physical activity reduces colon and breast cancers. The evidence is weaker for prostate (classified as probable), lung and endometrial cancers (classified as possible), and insufficient for cancers at all other sites. Hypothesized biological mechanisms for the physical activity – cancer association include changes in hormone level, reduced percentage of body fat, enhancement of the immune system, and alteration in free radical damage by scavenger systems. The available data indicate that 30-60 minutes per day of moderate-to vigorous physical activity is needed to be protective against breast and colon cancers. Conclusion: A greater understanding of the biological mechanisms operating in the physical activity – cancer relation, complete measurements of physical activity through a subject’s life, assessment of all potential confounders and association modifiers are needed to confirm a protective role of physical activity in cancer development and allow specific exercise prescriptions for prevention in particular cancer sites.
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