High Insulin Level Is An Independent Risk Factor For Breast Cancer

Obesity is a known risk factor for postmenopausal breast cancer. This relationship has been attributed to the high estrogen levels in many obese postmenopausal women. Obesity is also associated with high insulin levels. However, it is not known whether high levels of insulin, which stimulates the growth of breast cells in tissue culture, are also associated with breast cancer. The current investigation was the first study to prospectively examine the role of insulin in breast cancer while controlling for estrogen levels.

In the current study, Marc Gunter, Ph.D., and Howard Strickler, M.D., of the Albert Einstein College of Medicine in New York, and colleagues, examined the association between incident breast cancer and baseline fasting insulin, insulin-like growth factor-1 (a related hormone), and estradiol levels in 835 women enrolled in the Women's Health Initiative Observational Study who developed breast cancer and a randomly-selected sample of 816 women in the study who did not develop breast cancer.

When the researchers divided the women into four groups based on their fasting insulin levels, they found that women with the highest insulin levels had nearly a 1.5-fold higher risk of developing breast cancer than the women with the lowest insulin levels. Further, the investigators separately analyzed women who were not using hormone therapy and found that, in these women, those individuals with the highest insulin levels had a 2.4-fold increased risk of developing breast cancer compared to those with the lowest levels, even after accounting for multiple other breast cancer risk factors, including estrogen levels.

"These data suggest that hyperinsulinemia is an independent risk factor for breast cancer and may have a substantial role in explaining the obesity-breast cancer relationship," the authors conclude.

This research is published in the December 30 issue of the Journal of the National Cancer Institute.