Women who have high insulin levels have a higher risk of breast cancer, according to a new study.

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Doctors know that obesity puts women at a greater risk of breast cancer after menopause, and some had thought that estrogen was the cause, because its levels rise in obese women. But researchers from the Albert Einstein College of Medicine said that they also have higher insulin levels, and that insulin stimulates the growth of cancer cells in the lab.

So they evaluated 1,600 women from a much larger, long-term study of 93,000 patients called the Women's Health Initiative.

About half the women in the sample had had breast cancer. They found that the women in the quarter that had the highest insulin levels were nearly twice as likely to have had breast cancer than those with the lowest levels.

They also were able to factor out estrogen levels in their analysis.

Dr. Howard Strickler, the senior author of the paper, said it has important implications for the prevention and treatment of breast cancer.

"It is ... possible that screening non-diabetic postmenopausal women for high insulin levels could prove useful in identifying individuals at high risk for breast cancer," he said.

The findings were published in the January 7 issue of the Journal of the National Cancer Institute.