In a prospective cohort study «McTiernan A, Kooperberg C, White E, Wilcox S, Coates R, Adams-Campbell LL, Woods N, Ockene J, Women's Health Initiative Cohort Study. Recreational physical activity and the risk of breast cancer in po»1 including 74171 women aged 50 to 79 years, 1780 newly diagnosed breast cancers were documented over a mean follow-up of 4.7 years. Compared with less active women, women who engaged in regular strenuous physical activity at age 35 years had a 14% decreased risk of breast cancer (RR 0.86, 95% CI 0.78 to 0.95). Women who engaged in the equivalent of 1.25 to 2.5 hours of brisk walking had an 18% decreased risk of breast cancer (RR 0.82, 95% CI 0.68 to 0.97).
A systematic review and meta-analysis «Neil-Sztramko SE, Boyle T, Milosevic E et al. Does obesity modify the relationship between physical activity and breast cancer risk? Breast Cancer Res Treat 2017;166(2):367-381. »2 included 18 cohort and 11 case-control studies assessing the relationship between physical activity and breast cancer and how it may be modified by body mass index (BMI). Overall, a significant reduction in the relative risk of breast cancer was found in postmenopausal women with high versus low levels of physical activity for women with a BMI <25 kg/m2 (RR 0.85, 95% CI 0.79, 0.92) and ≥25 kg/m2 (RR 0.87, 95% CI 0.81, 0.93) but not ≥30 kg/m2 (RR: 0.93, 95% CI 0.76, 1.13). Physical activity was not associated with a significant reduction in risk of breast cancer in premenopausal women in any BMI group.
Another meta-analysis «Neilson HK, Farris MS, Stone CR et al. Moderate-vigorous recreational physical activity and breast cancer risk, stratified by menopause status: a systematic review and meta-analysis. Menopause 2017;24»3 assessed moderate-vigorous recreational physical activity and breast cancer risk, stratified by menopause status. Recreational activity decreased the risk: Pooled relative risks for women with higher versus lower levels of moderate-vigorous recreational activity were RR 0.80, 95% CI 0.74 to 0.87 (43 studies) and RR 0.79, 95% CI 0.74 to 0.84 (58 studies) for premenopausal and postmenopausal breast cancer, respectively, with high heterogeneity.
A meta-analysis «Hardefeldt PJ, Penninkilampi R, Edirimanne S, et al. Physical Activity and Weight Loss Reduce the Risk of Breast Cancer: A Meta-analysis of 139 Prospective and Retrospective Studies. Clin Breast Cance»4 included 139 prospective and retrospective studies woth a total of 236 955 cases and 3 963 367 controls. Physical activity reduced the risk of breast cancer (odds ratio [OR] 0.78; 95% CI 0.76 to 0.81; P < .001), with high-intensity physical activity being slightly more protective (OR 0.73; 95% CI 0.65 to 0.81; P < .001) than low-intensity exercise. The effect size for general exercise was similar in both premenopausal and postmenopausal women. Additionally, weight loss reduced the risk of breast cancer incidence (OR 0.82; 95% CI 0.67 to 0.97).
Comment: The quality of evidence is upgraded by a clear dose-response gradient.