Among African-American women, those with type 2 diabetes may have a higher risk of developing oestrogen receptor (ER)-negative breast cancer.
The majority of breast cancers are ER-positive; this type of breast cancer is the easiest to treat and has the highest survival rate.
African-American women are disproportionally affected by ER-negative breast cancers, with double the incidence as compared to white women, said 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.
"We are still trying to understand the basic biological processes that lead to ER-negative breast cancer. One way to do this is to study factors that are more common in an African-American population," added Palmer.
Several studies suggest that diabetes is a risk factor for breast cancer, and type 2 diabetes is twice as prevalent in African-American women as compared to white women, commented Palmer.
One previous study analysed the association with type 2 diabetes and breast cancer in African- American women, but did not report results separately for ER-negative and ER-positive breast cancer.
The analyses conducted by Palmer and colleagues were based on information provided by participants in the Black Women's Health Study (BWHS).
The BWHS was established over 20 years ago and utilizes biennial questionnaires to obtain information from 59,000 African- American women from across the United States.
Using this large data set, Palmer and colleagues could control for many factors, including age and body mass index (BMI).
"Our results showed statistically significant evidence of an increased risk of 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 five years," said Palmer.
They did not find an association with ER-positive breast cancer in the same cohort.
Importantly, the association between type 2 diabetes and ER-negative breast cancer was observed only among women with lower BMI (<30).
This suggests that abnormal metabolic status may play a larger role in ER-negative breast cancer than obesity, Palmer said.
"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 Palmer.
She stressed that further work is needed to corroborate these relatively new findings.
"If these results are confirmed, type 2 diabetes would be a modifiable risk factor for ER-negative breast cancer," noted Palmer.
"Women could reduce their chances of getting ER-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."
Palmer's future work includes testing if diabetes medication, such as metformin, could lower the risk for ER-negative breast cancer.