Adiposity is an established risk factor for postmenopausal breast cancer. Recent data suggest that high insulin levels in overweight women may play a major role in this relationship, due to insulin’s mitogenic/anti-apoptotic activity. However, whether overweight women who are metabolically healthy (i.e. normal insulin sensitivity) have elevated risk of breast cancer is unknown. We investigated whether overweight women with normal insulin sensitivity (i.e., homeostasis model assessment of insulin resistance [HOMA-IR] index, or fasting insulin level, within the lowest quartile [q1]) have increased breast cancer risk. Subjects were incident breast cancer cases (N=497) and a subcohort (N=2,830) of Women’s Health Initiative (WHI) participants with available fasting insulin and glucose levels. In multivariate Cox models, metabolically healthy overweight women, defined using HOMA-IR, were not at elevated risk of breast cancer compared to metabolically healthy normal weight women (hazard ratio [HR]HOMA-IR=0.96; 95% confidence interval [CI],0.64-1.42). In contrast, the risk among women with high (q3-4) HOMA-IRs was elevated whether they were overweight (HRHOMA-IR=1.76; 95% CI,1.19-2.60) or normal weight (HRHOMA-IR=1.80; 95% CI,0.88-3.70). Similarly, using fasting insulin to define metabolic health, metabolically unhealthy women (insulin q3-4) were at higher risk of breast cancer regardless of whether they were normal weight (HRinsulin=2.06; 95% CI,1.01-4.22) or overweight (HRinsulin=2.01; 95% CI,1.35-2.99), whereas metabolically healthy overweight women did not have significantly increased risk of breast cancer (HRinsulin=0.96; 95% CI,0.64-1.42) relative to metabolically healthy normal weight women. Metabolic health (e.g., HOMA-IR or fasting insulin) may be more biologically relevant and more useful for breast cancer risk stratification, than adiposity per se.
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