Associations between weight in early adulthood, change in weight, and breast cancer risk in postmenopausal women.

BACKGROUND Adult weight is positively associated with postmenopausal breast cancer but few studies have investigated whether there are associations with weight and body mass index (BMI) in early adulthood, or subsequent weight change. METHODS A total of 14,441 postmenopausal women from the Melbourne Collaborative Cohort Study (MCCS) were followed for 16.5 years (mean) and 668 incident breast cancers were identified. Hazard ratios (HRs) were estimated using Cox regression. RESULTS Weight and BMI at 18 to 21 years were not associated with risk of any type of breast cancer and there was no variation by age. Women with the greatest increase in weight and BMI had higher risk at older ages [HR per 5 kg/m(2) gain in BMI = 1.24; 95% confidence interval (CI), 1.11-1.40], although the test for homogeneity by age was not significant. At older ages, the association was stronger for progesterone (PR) positive disease compared with PR negative disease (HR per 5 kg/m(2) gain in BMI, 1.43; 95% CI, 1.23-1.66; test for homogeneity by PR status, P < 0.01) and for diseases that were positive for both estrogen (ER) and PR (HR per 5 kg/m(2) gain in BMI, 1.45; 95% CI, 1.24-1.69; test for homogeneity by ER/PR status, P = 0.02). HRs were also greater for HER2- and luminal A tumors, but the P values for homogeneity by tumor subgroups were not significant. CONCLUSION Early adulthood weight is not associated with risk of postmenopausal breast cancer. Greater weight gain during adulthood might be associated with increased risk for older women (>69 years) and this association might vary by tumor hormone receptor status. IMPACT Further studies need to investigate the impact of increase in weight during adulthood on postmenopausal breast cancer risk and the potential variation by age or tumor characteristics.