Breast cancer cells seem to be more sensitive than normal cells to the hormone insulin, according to a new study by researchers at the University of California at San Francisco.

Numerous studies have shown an association between obesity and an increased risk of developing breast cancer, a link researchers have surmised was due to a high fat diet.

This report, published in November in the Journal of Clinical Investigation, may help explain why.

Ira Goldfine, director of the Diabetes and Endocrine Research Laboratory at the Mount Zion Medical Center in San Francisco, and his colleagues analyzed breast tissue and cells from 159 women with breast cancer and compared it to breast tissue from 33 healthy women.

They found that breast cancer cells had seven times the number of receptors for insulin -- the hormone that enables cells to metabolize sugar. This suggests, Goldfine said, that some breast cancer cells may use insulin as a growth promoter.

It also helps explain why obese women have a higher risk of breast cancer. "Previous studies have shown that obese individuals have twice the level of insulin as normal weight people," Goldfine said. "In some women, overeating could contribute to breast cancer via insulin." That is because overeating stimulates the pancreas, which must produce more insulin.

Goldfine and other researchers say they believe that the high levels of insulin could allow breast tumors to grow more easily and more quickly than tumors in women who are not overweight.

Knowing that some breast cancer cells are sensitive to insulin could also help treat it. According to Goldfine, a greater number of insulin receptors may suggest a more virulent tumor. In cases where a doctor found that a woman's tumor had an increased number of insulin receptors, "more aggressive treatment, including chemotherapy and radiation, could be recommended," he said.