Type II Diabetes and Incidence of Estrogen Receptor Negative Breast Cancer in African American Women.

White women with type II diabetes (T2D) have an estimated 20% increased risk of developing breast cancer. Little is known about associations by breast cancer subtype or among African American (AA) women, who are disproportionately affected by T2D and estrogen receptor negative (ER-) breast cancer. We assessed the relation of T2D to incidence of ER- and ER+ breast cancer in data from the Black Women's Health Study, a prospective cohort of AA women enrolled in 1995 and followed biennially. During 847,934 person-years of follow-up, there were 1,851 incident invasive breast cancers, including 914 ER+ and 468 ER- cases. Multivariable Cox proportional hazards models were used to compute HRs for breast cancer incidence associated with T2D relative to no T2D, controlling for body mass index (BMI) and other potential confounders. The HR for T2D relative to no T2D was 1.18 [95% confidence interval (CI) 1.00-1.40] for overall breast cancer incidence, with the increase accounted for by ER- cancer: HRs were 1.02 (95% CI, 0.80-1.31) for ER+ and 1.43 (95% CI, 1.03-2.00) for ER- cancer. The HR for T2D and ER- breast cancer was highest among nonobese women (1.92; 95% CI, 1.22-3.04). The findings suggest that AA women with T2D are at increased risk of developing ER- breast cancer and that poor metabolic health may be more important than obesity for this subtype. Given the high prevalence of T2D in AA women, the observed association could, in part, explain racial disparities in incidence of ER- breast cancer. Cancer Res; 77(22); 6462-9. ©2017 AACR. ©2017 American Association for Cancer Research.