Diabetes and breast cancer among women with BRCA1 and BRCA2 mutations.

BACKGROUND Hyperinsulinemia and the metabolic syndrome are both risk factors for breast cancer. It is not clear if diabetes is associated with the risk of breast cancer in women with a BRCA1 or BRCA2 mutation. METHODS The authors reviewed the medical histories of 6052 women with a BRCA1 or BRCA2 mutation, half of whom had been diagnosed with breast cancer. They estimated the odds ratio for breast cancer, given a self-report of diabetes. They then estimated the hazard ratio for a new diagnosis of diabetes associated with a history of breast cancer. RESULTS There was no excess of diabetes in the period before the diagnosis of breast cancer, compared with controls with no diagnosis of breast cancer. The risk of diabetes was doubled among BRCA carriers in the 15-year period after the diagnosis of breast cancer (relative risk, 2.0; 95% confidence interval [CI], 1.4-2.8; P = .0001), compared with carriers without cancer. The risk was particularly high for women with a body mass index (BMI) >25.0 kg/m(2) (odds ratio, 5.8; 95% CI, 4.0-8.6; P = .0001). CONCLUSIONS After a diagnosis of breast cancer, women with a BRCA1 or BRCA2 mutation face a 2-fold increase in the risk of diabetes, which is exacerbated by a high BMI.