But when they added genistein (a plant estrogen or "phytoestrogen" present in many dietary supplements) to the mix, the researchers observed a dose-dependent reduction in the effectiveness of the breast cancer drug. Specifically, the tumors began to grow again. They grew fastest at the highest dietary doses of genistein.
"To think that a dietary supplement could actually reverse the effects of a very effective drug is contrary to much of the perceived benefits of soy isoflavones, and unsettling," said William Helferich a professor of food science and human nutrition at Illinois and principal investigator on the study. "You have women who are taking these supplements to ameliorate post-menopausal symptoms and assuming that they are as safe as consuming a calcium pill or a B vitamin."
Many women take genistein supplements to control hot flashes and other symptoms of menopause. The researchers found that the doses commonly available in dietary supplements were potent enough to negate the effectiveness of aromatase inhibitors.
"These compounds have complex biological activities that are not fully understood," Helferich said. "Dietary supplements containing soy-based phytoestrogens provide high enough dosages that it could be a significant issue to breast cancer patients and survivors."
Plant estrogens from soy are not the only ones of concern, Helferich said. In a recent study, he and his colleagues found that certain mixtures of estrogenic botanical components and extracts marketed as supplements to assist "female libido enhancement" and sold without a prescription appeared to spur breast cancer tumor growth at low doses, while having no effect on tumors at high doses.
That study appeared last year in Food and Chemical Toxicology.
"We are just starting to understand the complex effects of the
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign