​Assessing Type 2 Diabetes and Breast Cancer Risk

Study: Non-obese diabetic African-American women may have a higher risk of developing ER-negative breast cancer.

A recent study published in Cancer Research, a journal of the American Association for Cancer Research (AACR), showed that adult-onset diabetes – type 2 diabetes – was associated with an increased risk of estrogen receptor (ER)-negative breast cancer in African-American women.

"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 Palmer, ScD, associate director of Boston University’s Slone Epidemiology Center and the lead author of the study.

About two-thirds of breast cancers are ER-positive. This means that the malignant tumor cells grow more quickly in response to the hormone estrogen. ER-negative breast cancers, on the other hand, are not fueled by estrogen and are more challenging to treat.

"We are still trying to understand the basic biological processes that lead to ER-negative breast cancer," she added. "One way to do this is to study factors that are more common in an African-American population."

One of those factors is type 2 diabetes. African-Americans are about twice as likely to have diabetes compared to the non-Hispanic white population, according to data from the Centers for Disease Control and Prevention.

Diabetes has long been believed to be a risk factor for many types of cancer. Indeed, studies have shown that patients with type 2 diabetes have a greater risk for developing certain cancers, including liver, pancreatic, endometrial, and breast cancer. Researchers are studying whether there is a causal link between diabetes and cancer or whether the two share common risk factors, such as being a smoker or having a high body mass index (BMI).   

Obesity – defined as having a BMI of 30 or more – is a leading cause of type 2 diabetes and obesity has been linked to many types of cancer. However, about 10 percent of type 2 diabetics are not overweight.

To investigate what factors in African-American women increase their breast cancer risk, Palmer and her colleagues used data provided by the Black Women’s Health Study (BWHS). This large data set uses biennial questionnaires from 59,000 African-American women over the past 20-plus years. Participants were asked a wide variety of questions, including age, height, weight, type 2 diabetes diagnosis, medication used to treat diabetes, and others. Follow-up questions established the development of breast cancer and provided up-to-date information on weight and other factors.

Palmer found that African-American women with type 2 diabetes had an increased risk of developing ER-negative breast cancer, but not ER-positive breast cancer. Intriguingly, this increased risk was associated with women who were not obese – those with BMIs of less than 30. This suggests that factors other than obesity, such as an abnormal metabolic status, may play a role in the development of ER-negative breast cancer.

This is an initial report and further studies are needed to substantiate these findings, Palmer noted. However, if confirmed, type 2 diabetes could be a modifiable risk factor for ER-negative breast cancer.

"Women could reduce their chances of getting ER-negative breast cancer if they could avoid developing type 2 diabetes," said Palmer. "Monitoring of blood sugar levels to identify pre-diabetes may allow for early interventions to prevent diabetes."

This study provides further evidence of the link between diabetes and cancer. As rates of diabetes continue to increase worldwide, our risk for developing certain types of cancer may increase, too.

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