The research is the first to find a link between this natural substance, called insulin-like growth factor, or IGF-1, and the chance of breast cancer in pre-menopausal women. It raises the possibility that doctors someday might screen for this hormone to help identify those at higher risk of the disease.
American and Canadian researchers, led by Dr. Susan Hankinson of the Brigham and Women's Hospital in Boston, Massachusetts, say further studies using more women are needed to better define the risk before action can be recommended.
The results, published in this week's issue of The Lancet, a British medical journal, could help doctors determine which women to monitor most closely in order to catch the disease early, said Dr. Debu Tripathy, a cancer specialist at the University of California in San Francisco.
The results also could help prevent breast cancer, if doctors determine whether lowering hormone levels really does help and how best to lower them, he said.
The scientists found that among the 76 pre-menopausal women, those with IGF-1 concentrations in the highest category had almost three times the risk of those with levels in the lowest category.
And among pre-menopausal women younger than 50, the risk of breast cancer for those with the highest levels of the hormone was about seven times more than for their counterparts with the lowest levels.
"The up to sevenfold increase ... suggests that the relation between IGF-1 and risk of breast cancer may be greater than that of other established breast cancer risk factors, with the exception of a strong family history of breast cancer or a high-density mammographic profile," the study said.
"For the last 50 years, the main research into the causes of cancer has focused on damage to genes," said Jeff Holly, Director of Research at Bristol Royal Infirmary in England. "This is powerful evidence that the incidence of cancer is not just due to genetics but also due to the hormonal balance in the body and that those hormones could be your defense."
In previous studies of the link between IGF-1 and breast cancer, scientists tested the blood only after the women were diagnosed, which made it difficult to tell whether the onset of cancer could have increased the concentration of the hormone, the researchers said.
Studies have shown that the widely used cancer drug tamoxifen, which scientists suggested last month can prevent the onset of the disease lowers IGF-1 levels in the blood of women with breast cancer.
But tamoxifen has some severe side effects, Holly said, adding that nutrition also seems to regulate levels of the IGF-1 hormone.
"All the genetic work is not making headway in finding a cure," he added.
By Emma Ross. 1998 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed