Change in body mass index and physical activity and postmenopausal breast cancer risk in a cohort of Dutch nurses.

Participants completed a questionnaire at enrollment and up to two follow-up questionnaires. Body mass index (BMI, in kg/m2) and physical activity were assessed at age 18, at enrollment and prospectively around menopause. Associations with postmenopausal BC risk were assessed using multivariable Cox models.

Weight gain between age 18 and menopause is associated with increased postmenopausal BC risk, irrespective of physical activity. Being overweight/obese and developing overweight/obesity at menopause increases postmenopausal BC risk, however higher sports activity at the time of enrollment may be associated with a lower BC risk among overweight women.

With a median follow-up of 13.2 years, 1,776 incident breast cancers occurred among 43,127 postmenopausal women. A BMI ≥25 at menopause was associated with increased BC risk. A high level of sports activity was associated with decreased BC risk (ptrend 0.04) only among overweight (BMI = ≥25 to 30) women. The Hazard Ratio (HR) for postmenopausal BC increased 1.06-fold (95% Confidence Interval (CI)=1.04-1.09) with every 5kg weight gain between age 18 and menopause. Physical activity did not modify this association. Women with normal weight at enrollment, who developed overweight/obesity, had increased risk of postmenopausal BC (HR = 1.39, 95% CI = 1.06-1.83). We observed no clear associations between changes in physical activity since enrollment and BC risk, irrespective of BMI changes.

Body mass index and physical activity are among the best established risk factors for breast cancer (BC). We examined changes in these factors and postmenopausal BC risk in a cohort of registered Dutch female nurses.