Dual Effects of Weight and Weight Gain on Breast Cancer Risk

Context. —Breast cancer is a major cause of mortality among women. It is important to identify modifiable risk factors for this disease. Objective. —To examine body mass index (BMI) at the age of 18 years and at midlife and adult weight change in relation to breast cancer incidence and mortality. Design. —Cohort study. Setting. —A cohort of 95256 US female nurses aged 30 to 55 years who were followed up for 16 years. Main Outcome Meausure. —Incident and fatal breast cancer. Results. —During 1 203498 person-years, 2517 incident breast cancers (60% postmenopausal) were documented. Higher current BMI was associated with lower breast cancer incidence before menopause and was minimally associated with incidence after menopause. However, a stronger positive relationship was seen among postmenopausal women who never used hormone replacement (relative risk=1.59 for BMI >31 kg/m2vs ≤20 kg/m2; 95% confidence interval, 1.09-2.32;Pfor trend Conclusions. —Avoiding adult weight gain may contribute importantly to the prevention of breast cancer after menopause, particularly among women who do not use postmenopausal hormones.