Abstract
BACKGROUND: Although obesity is an established risk factor for postmenopausal breast cancer, the results of weight loss and breast cancer studies are inconsistent. Therefore, we evaluated associations between weight change and breast cancer risk in postmenopausal women in the Women's Health Initiative Observational Study. METHODS: Postmenopausal women (n = 61,335) who had no prior breast cancer and a normal mammogram had body weight and height measured and body mass index (BMI) calculated at baseline and year 3. Weight change at year 3 was categorized as stable (<5%), loss (≥5%), or gain (≥5%) with further assessment of weight loss intentionality by self-report. Multivariable Cox proportional hazard regression models were used to evaluate relationships between weight change and subsequent breast cancer incidence. RESULTS: During a mean follow-up of 11.4 years with 3061 incident breast cancers, women with weight loss (n = 8175) had a significantly lower risk of breast cancer compared with women whose weight remained stable (n = 41,139) (hazard ratio [HR], 0.88; 95% confidence interval [CI], 0.78-0.98; P = .02) with no interaction by BMI. Adjustment for mammography did not alter findings (HR, 0.88; 95% CI, 0.78-0.99) with no significant difference by weight loss intentionality. Weight gain (≥5%) (n = 12,021) was not associated with breast cancer risk (HR, 1.02; 95% CI, 0.93-1.11) but was associated with higher triple-negative breast cancer incidence (HR, 1.54; 95% CI, 1.16-2.05). CONCLUSIONS: Postmenopausal women who lose weight have lower breast cancer risk than those with stable weight. These findings suggest that postmenopausal women who lose weight may reduce their breast cancer risk.