NEW YORK (Reuters Health) – Genistein, a popular soy-based dietary supplement, can wipe out the effectiveness of a mainstay of breast cancer treatment known as aromatase inhibitor therapy, researchers warn based on animal experiments.

Roughly two-thirds of women have breast tumors that are “estrogen-dependent” — meaning that the tumors grow more rapidly in the presence of estrogen. These women are often treated with an aromatase inhibitor, like letrozole (Femara), which blocks production of estrogen in the body that can promote breast cancer growth.

By mimicking the effects of estrogen in the body, genistein, a plant or “phyto” estrogen found in many dietary supplements, can negate the effect of aromatase inhibitor therapy, researchers have found.

“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,” study chief Dr. William Helferich, a professor of food science and human nutrition at University of Illinois at Urbana-Champaign said in a written statement.

In mice with estrogen-dependent breast tumors, Helferich and colleagues found that letrozole worked significantly less well in the presence of genistein — and the higher the concentration of genistein, the less effective letrozole proved to be.

According to the researchers’ report in the researcher journal Carcinogenesis, the doses of genistein commonly found in dietary supplements used by many women to curb hot flashes and other symptoms of menopause, were potent enough to block the effectiveness of letrozole.

In the presence of letrozole alone, breast tumors stopped growing; the tumors began to grow again when genistein was present with letrozole.

“These studies draw attention to interactions of estrogenic dietary supplements and breast cancer therapies and the potential to negate the positive effects of an effective treatment such as aromatase inhibitors,” Helferich noted in comments to Reuters Health.

“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.”

SOURCE: Carcinogenesis, September 2008.