A prospective cohort study of the combined effects of physical activity and anthropometric measures on the risk of post-menopausal breast cancer

Although keeping a healthy weight and being physically active are among the few modifiable risk factors for post-menopausal breast cancer, the possible interaction between these two risk factors remains to be established. We analyzed prospectively a cohort of 19,196 women who provided detailed self-report on anthropometric measures, physical activity and possible confounders at enrollment in 1997. We achieved complete follow-up through 2010 and ascertained 609 incident cases of post-menopausal invasive breast cancer. We calculated metabolic energy turnover (MET h/day) per day and fitted Cox proportional hazards models to estimate hazard ratios (HRs) with 95 % confidence intervals (CIs). The incidence of post-menopausal breast cancer among obese women (BMI ≥ 30 kg/m²) was 58 % higher (HR 1.58, CI 1.16–2.16) than in women of normal weight (18.5 ≤ BMI < 25). Women in the lowest tertile of total physical activity (< 31.2 MET h/day) had 40 % higher incidence of post-menopausal breast cancer (HR 1.40, CI 1.11–1.75) than those in the highest tertile (≥ 38.2 MET h/day). The excess incidence linked to these two factors seemed to combine in an approximately additive manner; the incidence among the most obese and sedentary women was doubled (HR 2.07, CI 1.31–3.25) compared with the most physically active women with normal weight. No heterogeneity of the physical activity-linked risk ratios across strata of BMI was detected (p value for interaction = 0.98). This prospective study revealed dose-dependent, homogenous inverse associations between post-menopausal breast cancer incidence and physical activity across all strata of BMI, and between post-menopausal breast cancer incidence and BMI across all strata of physical activity, with no evidence of additive or multiplicative interaction between the two, suggesting independent effects.