Having type 2 diabetes appears to give post-menopausal women a 27% higher risk of developing breast cancer, according to a new study published in the British Journal of Cancer this week. While the link may be indirectly associated with being overweight, a known risk factor for both diseases, the researchers don't rule out that type 2 diabetes may affect breast cancer risk directly.

The study, from researchers at the International Prevention Research Institute (i-PRI), Lyon, France, is a comprehensive review of 40 separate studies into possible links between type 2 diabetes and breast cancer. Altogether, the data covers more than 56,000 breast cancer cases from four continents.

Lead author and president of i-PRI, Peter Boyle, tells the press in a statement:

"Our study found a significantly increased risk of breast cancer in women who had diabetes, which was restricted to those of post-menopausal age."

The study found no evidence to support such a link in women of pre-menopausal age, or those with type 1 diabetes.

High BMI Could Be a Reason

Boyle and colleagues suggest having a high BMI could be a factor. (BMI is Body Mass Index, a measure of obesity where the square of the person's height in metres is divided by their weight in kilograms).

Carrying too much weight, especially around the waist, is a known risk factor for type 2 diabetes.

Obesity is also a risk factor for breast cancer. Figures from Cancer Research UK, suggest it raises the risk of postmenopausal breast cancer by up to 30%, and that excess body weight is tied to more than 4,000 cases of breast cancer each year in the UK.

There May Be a Direct Biological Link

Boyle says they don't know what the underlying biological mechanism might be to explain the link between type 2 diabetes and breast cancer. It could be indirect, or it could be direct.

Being overweight is often tied to type 2 diabetes, and the biological mechanism to breast cancer from that would come via the effect on hormone activity, which in turn affects processes that lead to cancer growth.

"But it's also impossible to rule out that some factors [directly] related to diabetes may be involved in the process," he adds.

Healthy Weight, Exercise, Less Alcohol, Help Reduce Breast Cancer Risk

Martin Ledwick, Head Information Nurse at Cancer Research UK, says the study does not show whether diabetes directly raises breast cancer risk in post-menopausal women.

"But as we know that having a high BMI can contribute to an increased risk of both type II diabetes and breast cancer, it makes sense for women to try and maintain a healthy weight," he urges, and advises anyone with concerns to talk to their GP about their lifestyle, for example diet and physical activity, to try and minimize the risk of both diseases.

According to the World Cancer Research Fund, every year in the UK, 89.1 of every 100,000 women develop breast cancer, making it the country with the 11th highest breast cancer rate in the world.

They also suggest about 42% of breast cancers in the UK could be prevented through drinking less alcohol, being physically active and maintaining a healthy weight.

Other factors that affect breast cancer risk include: age, reproductive history, endogenous hormones, exogenous hormones, breast density, history of some types of benign breast disease, and family history.

Written by Catharine Paddock PhD