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.
Comment: The quality of evidence is upgraded by a clear dose-response gradient.
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