Physical Activity and Risk of Breast Cancer

In a prospective cohort study 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 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 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 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.

References

• 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 postmenopausal women: the Women's Health Initiative Cohort Study. JAMA 2003 Sep 10;290(10):1331-6. [PubMed]
• 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. [PubMed]
• 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):322-344. [PubMed]
• 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 Cancer 2018;18(4):e601-e612 [PubMed]