Increased breast cancer risk in postmenopausal women has previously been linked to obesity and diabetes. Both conditions involve insulin resistance, which causes increases in circulating levels of insulin.
Since insulin is known to promote cell division and enhance breast tumor growth in animal models, the Einstein scientists reasoned that relatively high insulin levels might contribute to breast cancer risk in 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," Kabat added.
In the new study, Dr. Kabat and his colleagues analyzed data on 5,450 women enrolled in the Women's Health Initiative, a large multicenter study investigating the influence of a number of factors on women's health. Most of the women had participated in the clinical trial portion of the study and provided fasting blood samples at the start of the study (i.e., at baseline) and then at years one, three and six.
The remaining women, who were enrolled in a separate "observational" component of the study, provided fasting blood samples at baseline and at year three of the study. Among all these women, 190 cases of breast cancer were identified over eight years of follow-up.
The results showed a strong association between elevated insulin levels and increased risk for breast cancer.
"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," Dr. Kabat said.
Their findings are published in the online version of the International Journal of Cancer.