A study published in the International Journal of Cancer confirms previous research showing that diabetes increases the risk of breast cancer. In this study elevated insulin levels in the blood appeared to raise the risk of breast cancer in postmenopausal women.

"Up to now, only a few studies have directly investigated whether insulin levels are associated with breast cancer risk, and those studies have yielded conflicting results," says Geoffrey Kabat, Ph.D., senior epidemiologist in the department of epidemiology and population health at Einstein and the lead author of the paper. "Those other studies were based on just a single baseline measurement of insulin, while our study involved analyzing repeated measurements of insulin taken over several years--which provides a more accurate picture of the possible association between insulin levels and breast cancer risk."

According to the American Diabetes Association, breast cancer is the most common type of malignancy in women and the second leading cause of cancer death. It's three times more common than all gynecologic cancers put together, and it's been steadily increasing since 1960.

Obesity and diabetes have been linked to increased breast cancer risk in postmenopausal women. Both of these conditions involve insulin resistance, which elevates the circulating levels of insulin. Since insulin promotes cell division and enhances breast tumor growth in animal models, the Einstein scientists determined that relatively high insulin levels may contribute to breast cancer risk in women.

In the study, Dr. Kabat and his colleagues analyzed data on 5,450 women enrolled in the Women's Health Initiative, a large multi-center study investigating how a number of factors affect women's health. When the researchers divided the participants into three groups based on their insulin levels, they discovered that women in the upper third for insulin level were more than twice as likely to develop breast cancer compared with women in the bottom third for insulin level.

The link between elevated insulin level and breast cancer was even stronger for lean women and the weakest for obese women (who generally have higher insulin levels compared with lean women). "This finding is potentially important because it indicates that, in postmenopausal women, insulin may be a risk factor for breast cancer that is independent of obesity," says Dr. Kabat. However, the number of lean women in the study was small, so this finding is preliminary.

While these results require confirmation from other studies, Dr. Kabat notes that the current recommendations for reducing breast cancer risk in postmenopausal women - including maintaining a healthy weight and engaging in regular physical exercise - can help to reduce insulin levels.

A Potential New Way to Lower Breast Cancer Risk from Diabetes

In an interesting twist, a diabetes drug may have the potential to fight breast cancer. In a small study presented at the 2008 American Society of Clinical Oncology meeting, diabetic patients with breast cancer were three times more likely to respond to chemotherapy before radiation or surgery if they were taking metformin (Fortamet® or Glucophage®). They also had a 50 percent higher response rate than non-diabetic patients, says Sao Jiralerspong, M.D., of the University of Texas MD Anderson Cancer Center in Houston.

The findings add to previous evidence that diabetic patients treated with metformin have a lower risk of cancer and cancer-related death. In laboratory studies, metformin inhibited the growth of breast cancer cells by activating an enzyme, AMP kinase, and inhibiting the mTOR pathway, which plays a role in several diseases, including cancer.

Speak to your doctor to find out if metformin is suitable for treating your diabetes to help reduce your risk of breast cancer and other types diabetes-related cancer.

Albert Einstein College of Medicine of Yeshiva University news release, "Einstein Scientists Link Elevated Insulin to Increased Breast Cancer Risk."
Journal: International Journal of Cancer
Date: Published online June 2, 2009

Study: Repeated measures of serum glucose and insulin in relation to postmenopausal breast cancer.
Website: http://www3.interscience.wiley.com/journal/122414334/abstract?CRETRY=1&SRETRY=0
: Geoffrey C. Kabat et. al.
Journal: Journal of Clinical Oncology, Vol. 27 No. 20, pp. 3297-3302
Date: July 2009

Study: Metformin and Pathologic Complete Responses to Neoadjuvant Chemotherapy in Diabetic Patients With Breast Cancer
Website: http://jco.ascopubs.org/cgi/content/abstract/27/20/3297
: Sao Jiralerspong, Shana L. Palla, et. al.