Physical activity and cancer prevention: a review of current evidence and biological mechanisms.

Objective. The main aim of this paper is to review the evidence available from the date of PubMed's inception to May 2011 for a link between cancer and physical activity (PA) in both animal models and humans. Methods. We decided to select studies that comply with the scheme proposed by the American College of Sports Medicine/American Heart Association (ACSM/AHA) that distinguish occupational physical activity (OPA) and leisure-time physical activity (LT-PA), further classified in three levels of intensity (low, moderate and heavy) based on the Metabolic Equivalent of Task (MET) index. Results. Considering animal models, there was strong evidence for an inverse association between voluntary wheel exercise and the risk of colon and breast cancer. Regarding human studies, we identified the following main results: 1) colorectum: LT-PA provided an overall colon risk reduction of 13-14%; 2) breast: significant reduction in the frequency of post-menopausal (PMP) cancers in women that practiced heavy and moderate LT-PA; 3) prostate: heavy OPA and LT-PA seemed to reduce the risk of advanced prostate cancers; 4) endometrium: strong protective effect of heavy/moderate LT-PA among overweight/obese women; 5) lung: inverse relationship between heavy LT-PA and lung cancer in former or current smokers across all histologies. Conclusion. Increased LT-PA is associated with cancer prevention in several organs, but strong biases, such as body mass index (BMI), gender and age, make it difficult to assess which aspects of PA contribute most strongly to the reduced risk. Furthermore, we found few studies that indicated a protective role for OPA in cancer prevention when compared with LT-PA.