High Insulin Levels Linked To Breast Cancer

By Margarita Nahapetyan

Scientists at Albert Einstein College of Medicine of Yeshiva University in Bronx, NY, have discovered that higher circulating insulin levels in the blood appear to double the risk of developing breast cancer in postmenopausal women.

Previous studies have demonstrated that extreme weight and diabetes are associated with breast cancer in postmenopausal women. Both conditions involve insulin resistance, which leads to higher circulating levels of insulin. Insulin is a hormone that keeps levels of glucose under control, and normalizes levels of blood sugar in the body. The hormone also helps to digest food. Every time we eat something, insulin is released by the pancreas. The amount of insulin in the blood depends on what type of food is being consumed. Sticking to a healthy diet keeps levels of the hormone from becoming higher, now believed to be a risk factor for breast cancer.

"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," said Geoffrey Kabat, Ph.D., senior epidemiologist in the department of epidemiology and population health at Einstein and the principal investigator of the new study. Dr. Kabat added that previous studies were based on just a single baseline measurement of the hormone, whereas the new study examined repeated measurements of insulin taken over several years. This allowed researchers to see a more accurate picture of the possible link between levels of insulin in the blood and the risk of breast cancer.

During their trial, Dr. Kabat and his team analyzed data on nearly 5,500 women who took part in the Women's Health Initiative, a large multicenter study that examined the influence of different factors on women's health. The investigators split all the women into three groups, separated by measurements of insulin levels. The majority of the participants were involved in the clinical trial portion of the study and were asked to provide fasting blood samples at the baseline of the study, and then after one year, three years and six years. Other women, who participated in a separate "observational" portion of the study, were asked to provide fasting blood samples at the start of the study and then three years later.

The results revealed that among all these female participants, 190 cases of breast cancer were developed over 8 years of follow-up period. Researchers found that women in the upper third for insulin level had a more than three-fold increased risk for breast cancer, when compared to their counterparts in the bottom third for insulin level. The link between insulin level and breast cancer incidence was even stronger for the participants in the control group, who had not received the treatment in the clinical trial or were in the observational component.

The experts noted that the association between elevated levels of insulin and breast cancer was highest among lean women and weakest among women with obesity. In general, heavy women have higher insulin levels when compared with skinny 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," Dr. Kabat said. However, because the number of lean women in the study was small, this finding is preliminary.

While more investigation on the matter is needed as the new findings still require confirmation from other studies, Dr. Kabat says that the current recommendations for reducing breast cancer risk in postmenopausal women, such as maintaining a healthy weight and engaging in regular physical activity, can help to bring down insulin levels.

The new findings are published in the online version of the International Journal of Cancer.