Objective — There is growing evidence of a link between type 2 diabetes and breast cancer, possibly through insulin resistance and/or hyperinsulinemia. Because insulin levels are at their highest prior to the development of diabetes, breast cancer risk may be even greater during the pre-diabetes period.
Research Design and Methods — In this cross-sectional study, women aged 55–79 years living in Ontario, Canada, with newly diagnosed diabetes from 1994 to 2002 were identified from a validated, population-based database (N = 82,390). Prior history of breast cancer in this group was recorded from 1964 until their diabetes diagnosis from a linkable cancer registry, and was compared to a similarly aged comparison group without diabetes (N = 411,950).
Results — Prior breast cancers were identified in 3.7% of women with diabetes and in 3.1% women without diabetes (odds ratio, OR 1.22, 95% confidence interval, CI 1.17–1.27, p < 0.0001). The mean time from breast cancer diagnosis to diabetes diagnosis was 7.9 years. The likelihood of a breast cancer history remained significantly higher in women with diabetes after adjustment for age, income and physician visits (OR 1.13, 95% CI 1.09–1.18, p < 0.0001).
Conclusions — These results suggest that breast cancer risk may be increased in the pre-diabetes phase and may have implications for screening and prevention strategies. Further studies are required to better characterize the processes that link insulin resistance, diabetes and breast cancer.