In a new study, researchers found that there appears to be no association between type 2 diabetes and developing breast cancer overall.
This may be because most women in the study with type 2 diabetes were taking metformin, a medication widely used to treat type 2 diabetes.
The medication may help reduce the risk of developing estrogen-positive (ER-positive) breast cancer.
The research was conducted by a team at the National Institute of Environmental Health Sciences.
ER-positive breast cancer (cancer that has receptors on cell surfaces for the hormone estrogen) accounts for about 80% of breast cancer diagnosed in the USA.
In the study, the team tested 44,541 women.
Over the 8 years of the study, the researchers found that type 2 diabetes was linked to a 40% increased risk of triple-negative breast cancer when compared with women who did not have diabetes.
In contrast, there was a small (8%) decrease in risk for ER-positive breast cancer.
When the researchers considered women according to the type of treatment they received, type 2 diabetes treated with metformin was associated with a 14% decreased risk of developing ER-positive breast cancer but a 25% increased risk of developing ER-negative breast cancer.
However, there was a 74% increased risk of developing triple-negative breast cancer among those treated with metformin.
The team also found that having type 2 diabetes for 15 years or more seemed to be linked to a 39% reduced risk of ER-positive breast cancer, and this is most likely to be because of long-term use of metformin.
Taken together, these findings suggest that having type 2 diabetes may increase the risk of developing breast cancer, but that taking metformin may protect against developing ER-positive breast cancer, the most common type of breast cancer.
Metformin did not appear to protect against ER-negative or triple-negative breast cancer.
One author of the study is Professor Dale Sandler.
The study is published in the Annals of Oncology.
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