The relation of body mass index (BMI) and weight gain to breast cancer risk is complex, and little information is available on Black women, among whom the prevalence of obesity is high. We assessed BMI and weight gain in relation to breast cancer risk in prospective data from the Black Women's Health Study. In 1995, 59,000 African American women enrolled in the Black Women's Health Study by completing mailed questionnaires. Data on anthropometric factors were obtained at baseline and every 2 years afterwards. In 10 years of follow-up, 1,062 incident cases of breast cancer occurred. Incidence rate ratios (IRR) were computed in multivariable Cox proportional hazards regression. BMI at age 18 years of >/=25 relative to <20 was associated with a reduced risk of breast cancer among both premenopausal women (IRR, 0.68; 95% confidence interval, 0.46-0.98) and postmenopausal women (IRR, 0.53; 95% confidence interval, 0.35-0.81). There was an inverse association of current BMI with premenopausal breast cancer but no association with postmenopausal breast cancer, either overall or among never-users of hormone therapy. Weight gain was not associated with postmenopausal breast cancer risk. In analyses restricted to breast cancers that were estrogen and progesterone receptor positive, IRRs for current BMI and weight gain were elevated but not statistically significant. The findings indicate that being overweight at age 18 years is associated with a reduced risk of both premenopausal and postmenopausal breast cancer in African American women. Understanding the reasons for the association may help elucidate the pathways through which adolescent exposures influence breast cancer risk. The lack of association of obesity with receptor-negative tumors in postmenopausal African American women may partially explain why breast cancer incidence in older Black women is not high relative to other ethnic groups in spite of the high prevalence of obesity in Black women.
