Physical activity and cancer: Progress and opportunities.

PL02-01 The potential for physical activity, and particularly recreational exercise activity, to reduce cancer risk continues to be of great interest to epidemiologists, cancer prevention researchers and health care providers. A large body of evidence indicates that physical activity reduces the risks of colon cancer and breast cancer, yet a number of questions remain to be answered. For example, for colon cancer, it is not yet clear whether women derive less benefit than men or whether the greatest benefit is seen for distal vs. proximal colon. For both cancers, we still have no specific exercise prescription to recommend: what types of activity are most beneficial, at what ages, and at what intensity and duration. Further, we have yet to define the mechanisms by which physical activity reduces risk. Physical activity may lower cancer risk by altering hormone profiles, growth factors, immune function and oxidative stress. Physical activity is part of the energy balance equation and thus may influence cancer risk indirectly through its impact on body size measures. Intriguing data are now emerging on the impact on survival of physical activity after diagnosis of breast and colon cancer. Although data for endometrial cancer and lung cancer are accumulating and are suggestive of an etiologic relationship, further research must be conducted to define these associations and exclude alternative explanations for study findings. Many studies of prostate cancer have been conducted; however, results are still inconclusive. Physical activity may protect against more advanced or higher grade disease. Other cancer sites have been studied in relation to physical activity, but generally the body of evidence is small and inconclusive. Ideally, large scale intervention efforts would help to clarify many of the remaining questions in this field; however, these are unlikely to be conducted and would be limited by our inability to study a range of alternative exercise interventions. To move forward in this field we need more small scale intervention studies to establish mechanisms and to assess the impact of exercise programs on biomarkers of risk.