Objective: To 1) review the epidemiologic literature on physical activity and breast cancer risk examining the effect of the different parameters of activity and effect modification within different population sub-groups and 2) review the biologic mechanisms whereby physical activity may influence breast cancer risk.
Methods: A review of all published literature to September, 2007 was conducted using online databases. A total of 34 case-control and 28 cohort studies were included. The impact of the different parameters of physical activity on the association between activity and breast cancer risk was examined by considering the type of activity performed, the timing of activity over the life course, and the intensity of activity. Effect modification of this association by menopausal status, body mass index (BMI), racial group, family history of breast cancer, hormone receptor status, energy intake and parity were also considered.
Results: Evidence for a risk reduction associated with increased physical activity was found in 47 (76%) of 62 studies included in this review with an average risk decrease of 25-30%. A dose-response effect existed in 28 of 33 studies. Stronger risk decreases were observed for recreational activity, lifetime or later life activity, vigorous activity, among postmenopausal women, women with normal BMI, non-white racial groups, hormone receptor negative tumours, women without a family history of breast cancer and parous women.
Conclusions: The effect of physical activity on breast cancer risk is stronger in specific population sub-groups and for certain parameters of activity that can be further explored in future intervention trials.