A prospective study of hemoglobin A1c concentrations and risk of breast cancer in women

Impaired glucose metabolism and hyperinsulinemia have been hypothesized to increase breast cancer risk. However, findings from observational studies relating blood concentrations of hyperinsulinemia markers to breast cancer risk have been inconsistent. We prospectively evaluated whether hemoglobin Ale (HbAlc) concentrations predict breast cancer risk in a large female cohort. We included 27,110 female participants of the Women's Health Study who were, at baseline, free of cancer and had usable blood specimens as well as sufficient information on potential risk factors for breast cancer. Relative risks (RR) and 95% confidence intervals (95% CI) were estimated from Cox proportional hazards regression models. All Ps were two sided. During an average of 10 years of follow-up, 790 incident cases of invasive breast cancer were confirmed. Higher baseline HbAlc levels were not associated with an increased risk of breast cancer. The multivariate RR for the highest relative to the lowest quintile of HbAlc levels was 0.87 (95% CI, 0.69-1.10; P-trend = 0.22). Higher HhAlc levels were also not associated with an increased risk of breast cancer according to alternative clinical cutoff points for HbAlc or in the analyses stratified by body mass index or according to certain tumor characteristics. However, a weakly inverse association was noted among postmenopausal women, especially among those who had never used hormone therapy. There was also a weakly inverse association between HbAlc levels and estrogen receptor-negative breast tumors. These data suggest that higher HbAlc concentrations do not seem to increase risk of breast cancer among apparently healthy women.