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Black women with type 2 diabetes at greater risk for breast cancer subtype
Black women with type 2 diabetes demonstrated a 40% increased risk for ER-negative breast cancer, according to results of a study published in Cancer Research.
Type 2 diabetes did not appear to be associated with incidence of ER-positive breast cancer, researchers reported.
“We already know from different levels of research that ER-negative breast cancer has more of an inflammatory component to the ideology,” Julie R. Palmer, ScD, associate director of Boston University’s Slone Epidemiology Center, professor of epidemiology at Boston University School of Public Health, and associate director for population sciences at the BU-BMC Cancer Center, told HemOnc Today. “We also know type 2 diabetes is associated with inflammation.
“When we started this analysis, we planned to look separately at the two major subtypes — ER-positive and ER-negative — but my hypothesis was that I might find more of an association with ER-negative breast cancer because of the information we had,” Palmer added.
Approximately 75% of all breast cancers are ER-positive, which carries the highest survival rate. ER-negative breast cancers are less common and have a 5-year survival rate about 10% less than ER-positive.
Previous studies have shown that diabetes is a risk factor for breast cancer and that type 2 diabetes is twice as prevalent among black women as white women. White women with type 2 diabetes have an estimated 20% increased risk for developing any type of breast cancer. However, little is known about associations by breast cancer subtype among black women.
Palmer and colleagues analyzed data collected from the Black Women’s Health Study, which was established in 1995 when 59,000 black women in the United States aged 21 to 69 years began completing biennial health questionnaires that address — among other things — new diagnoses of breast cancer, type 2 diabetes and BMI.
In this study, the prevalence of type 2 diabetes in the overall cohort of 54,337 women was 19% (n = 6,694). During 870,358 person-years of follow-up, researchers reported 1,851 incidents of invasive breast cancers, including 914 ER-positive cancers and 485 ER-negative cancers.
Women with type 2 diabetes tended to be older (mean age, 56.1 years vs. 47 years), to have a higher recent BMI (34.9 kg/m2 vs. 29.5 kg/m2) and higher BMI when aged 18 years (23.7 kg/m2 vs. 21.3 kg/m2), and to be younger at menarche (18% at 11 years or younger vs. 11%). A smaller proportion of women with diabetes had 16 years or more of education (47% vs. 53%) and reported 3 or more hours per week of vigorous physical activity (13% vs. 20%).
Researchers reported a positive association between type 2 diabetes and risk for ER-negative breast cancer (multivariable HR = 1.43; 95% CI, 1.03-2), but not ER-positive cancer (HR = 1.43; 95% CI, 0.8-1.31).
The positive association of type 2 diabetes with ER-negative cancer occurred for stage I (HR = 1.47; 95% CI, 0.84-2.48), stage II (HR = 1.35; 95% CI, 0.76-2.38), and stage III and IV (HR = 2.34; 95% CI, 1.15-4.76) disease.
Additionally, type 2 diabetes appeared associated with a 92% increased risk for ER-negative breast cancer among nonobese women and a 55% increased risk among women with waist-hip ratios less than 0.85.
“Our results showed statistically significant evidence of an increased risk for ER-negative breast cancer in black women who had type 2 diabetes before they ever had breast cancer, primarily in the women who had diabetes for at least 5 years,” Palmer said.
“Having type 2 diabetes is bad for you and plays a much bigger role in other bad outcomes as well, such as vision loss, cardiovascular complications and peripheral vascular disease that can result in loss of lower limbs,” Palmer added. “That’s why there’s so much effort now to have intervention programs focused around physical activity and diet in populations at greater risk of diabetes.”
Limitations of the study included researchers’ dependence on self-reports rather than medical records for classification of type 2 diabetes status.
Black women could reduce their chances of getting ER-negative breast cancer by monitoring blood sugar levels that identify prediabetes, Palmer noted, adding that diabetes medication, such as metformin, could lower the risk for ER-negative breast cancer.
Researchers’ findings do not establish a causal connection between diabetes and breast cancer and Palmer said more studies are needed to explain why mortality from breast cancer is higher in black women than white women.
“It takes more information than just this one epidemiologic study,” Palmer said. “I would like to see other researchers do biologic experiments to try to explain the mechanisms. There just aren’t that many large studies of breast cancer in African American women and some never ask about type 2 diabetes because it’s only been in the last 10 years that people really started to wonder more about connections between diabetes and cancer.” – by Chuck Gormley
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Julie R. Palmer, ScD, can be reached at jpalmer@bu.edu.
Disclosures: The NIH and the Dahod Breast Cancer Research Program at Boston University School of Medicine funded this study. The authors report no relevant financial disclosures.