Having high levels of insulin in the blood when a patient has breast cancer is a bad sign, according to doctors from Mount Sinai Hospital in Toronto.

The researchers, led by Pamela Goodwin, MD, found that the cancer came back sooner and was more likely to cause death in women with high blood insulin levels. Their report appeared in the Journal of Clinical Oncology (Vol. 20: 42-51).

Doctors know that obese women with breast cancer have a poorer outcome than those who are a regular weight. Obese people also have high blood levels of insulin. Goodwin wanted to know if the high insulin levels were the reason for the poor outcome.

They studied 512 women who were diagnosed with breast cancer between 1989 and 1996 and followed them for an average of four years. At the time of their initial treatment, the women had their height and weight recorded and blood taken for insulin levels.

It is important to note that the researchers excluded any woman with diabetes. The results do not apply to women with diabetes whether or not they are taking insulin.

By the end of the study period, the cancer had come back in an area distant from the breast in 76 women, and 46 women had died of their cancer.

The doctors discovered that the highest recurrence and death rates were in the most obese women and those with the highest insulin levels. Women with the highest blood insulin levels were three times more likely to die of their breast cancer than women with lower levels.

But the high insulin levels and high death rate did not occur only in obese women. Some of the women with high insulin levels were slim, and they too had higher death rates. It wasn’t clear why these slim women had high insulin levels, but the high levels were related to a poorer outcome.

What does this mean for women with breast cancer?

Experts say overweight women with breast cancer should try to lose weight. But aside from that, there is little a woman can do about high insulin levels.

Goodwin and her colleagues think that this is an area that deserves more research. They hope that a way can be found to help lower insulin levels. If that can’t be done, perhaps a drug can be developed that blocks the cancer cell from reacting to insulin, they suggested.