Are African American Women With Type 2 Diabetes at Increased Risk for ER-Negative Breast Cancer?

Key Points

  • African American women with type 2 diabetes are at increased risk of developing estrogen receptor–negative breast cancer, and poor metabolic health may be more important than obesity for this breast cancer subtype.
  • Given the high prevalence of type 2 diabetes in African American women, the observed association could, in part, explain racial disparities in the incidence of estrogen receptor–negative breast cancer.

A prospective study by Palmer et al assessing the relationship of type 2 diabetes and the incidence of estrogen receptor–negative and estrogen receptor–positive breast cancer among African American women has found statistically significant evidence of an increased risk of estrogen receptor–negative breast cancer—but not estrogen receptor–positive breast cancer—in these women. The study also found that poor metabolic health may be more important than obesity in the development of this breast cancer subtype, suggesting that abnormal metabolic status may play a larger role in estrogen receptor–negative breast cancer than obesity. The study was published in Cancer Research.

Study Methodology

The study researchers analyzed data from women enrolled in the Black Women’s Health Study, a nationwide prospective cohort study established in 1995 when 59,000 African American women, aged 21 to 69, completed mailed health questionnaires followed by biennial questionnaires. Study participants were asked a series of questions about weight, height, waist circumference, hip circumference, number of births, lactation, age at menarche, oral contraceptive use, menopausal status, age at menopause, female hormone use, breast cancer in first-degree relatives, vigorous physical activity, alcohol consumption, smoking habits, years of education, diagnosis of type 2 diabetes, among others. Follow-up questionnaires ascertained occurrences of incident breast cancer and updated information on type 2 diabetes, weight, and other variables.

During 847,934 person-years of follow-up, there were 1,851 incidences of invasive breast cancers, including 914 estrogen receptor–positive and 468 estrogen receptor–negative cases. Multivariable Cox proportional hazards models were used to compute hazard ratios (HRs) for breast cancer incidence associated with type 2 relative to no type 2 diabetes, controlling for body mass index (BMI) and other potential confounders.

Study Findings

The researchers found the hazard ratio for type 2 diabetes relative to no type 2 diabetes was 1.18 [95% confidence interval (CI) = 1.00–1.40] for overall breast cancer incidence, with the increase accounted for by estrogen receptor–negative cancer: hazard ratios were 1.02 (95% CI = 0.80–1.31) for estrogen receptor–positive cancer and 1.43 (95% CI = 1.03–2.00) for estrogen receptor–negative cancer. The hazard ratio for type 2 diabetes and estrogen receptor–negative breast cancer was highest among non-obese women (1.92; 95% CI = 1.22–3.04).

The findings suggest that African American women with type 2 diabetes are at increased risk of developing estrogen receptor–negative breast cancer and that poor metabolic health may be more important than obesity for this subtype.

“Findings from the present study suggest that African American women with [type 2 diabetes] are at increased risk of developing [estrogen receptor–negative] breast cancer and that poor metabolic health may be more important than BMI for [estrogen receptor–negative] disease. Given that the prevalence of [type 2 diabetes] is twice as high in non-Hispanic blacks as in non-Hispanic whites, the observed association, if confirmed, may explain in part why African American women have a disproportionately high incidence of [estrogen receptor–negative] breast cancer compared to U.S. white women. Whether adequate treatment with metformin or other medications can ameliorate the increased risk associated with diabetes requires further research,” concluded the study authors.

“Our findings may account for some of the racial disparity in breast cancer and could partly explain why mortality from breast cancer is so much higher in black women than white women,” said Julie R. Palmer, ScD, Associate Director of Boston University’s Slone Epidemiology Center and the lead author of this study. “If these results are confirmed, type 2 diabetes would be a modifiable risk factor for [estrogen receptor–negative] breast cancer. Women could reduce their chances of getting [estrogen receptor–negative] breast cancer if they could avoid developing type 2 diabetes. Monitoring of blood sugar levels to identify pre-diabetes may allow for early interventions to prevent diabetes.”

Funding for this study was provided by the National Institutes of Health and the Dahod Breast Cancer Research Program at Boston University School of Medicine.

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.


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