Data from the Women's Health Initiative Observational Study show that higher than normal insulin levels are 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 breast cancer and blood 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 Journal of the National Cancer Institute, the researchers report a positive association between insulin levels and breast cancer risk, with women with the highest levels of insulin having a 46 percent greater risk than women with the lowest levels.
The association between insulin and breast cancer risk varied depending on whether or not the women took hormone therapy. Upon further analysis, insulin level was linked with breast cancer risk only in those who did not use hormones.
For women who did not use hormones, those with a body mass index of 30 (obese) compared with those with a BMI between 18 and 25 (normal weight), had a two-fold greater risk of breast cancer. Adjusting for insulin levels attenuated this association, however.
Fasting levels of estrogen were also linked with 59 percent higher risk among women with the highest versus the lowest estrogen levels.
Given that high levels of insulin and estrogen 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.