High insulin level is a risk factor for postmenopausal breast cancer

NEW YORK (Reuters Health) - Data from the Women's Health Initiative Observational Study show that hyperinsulinemia is an independent risk factor for breast cancer.Study investigators, led by Dr. Marc J. Gunter at Albert Einstein College of Medicine in the Bronx, New York, examined the association between incident breast cancer and serologic parameters at study entry in 835 women who later developed breast cancer and 816 who did not. All of the women were postmenopausal and none were diabetic at baseline.In the January 7 issue of the Journal of the National Cancer Institute, the researchers report a positive association between insulin levels and breast cancer risk, with a hazard ratio of 1.46 for the highest versus the lowest quartile of insulin level.The association between insulin and breast cancer risk varied depending on whether or not the women took hormone therapy. On multivariate analysis, insulin level was linked with breast cancer risk only in those who did not use exogenous hormones, with a hazard ratio of 2.40 for the highest versus the lowest quartile of insulin level. In hormone nonusers, obesity was also associated with breast cancer risk, with a hazard ratio of 2.12 for a body mass index (BMI) of at least 30 versus 18 to less than 25. Adjusting for insulin level attenuated this association, however.Fasting levels of endogenous estradiol were also linked with cancer risk, with a hazard ratio of 1.59 for the highest versus the lowest tertile.Given that high levels of insulin and endogenous estradiol raise the risk for postmenopausal breast cancer -- and largely account for the link between obesity and breast cancer -- efforts to bring down fasting insulin or circulating estrogen levels "through weight loss or increased physical activity or via pharmacologic approaches" might be indicated, the researchers conclude.