The findings of three studies, funded by the National Institutes of Health, are detailed in the Journal of Nutrition (November), Carcinogenesis (October) and Cancer Research (July).

The results demonstrate that genistein in various forms stimulates tumor growth. They also suggest that women with estrogen-dependent breast cancer or a predisposition to it may want to reduce their consumption of soy products with a high isoflavone content, says William G. Helferich, a UI professor of food science and human nutrition.

Many soy isoflavone-containing products are marketed to women over age 50 for the relief of menopausal symptoms.

"Our pre-clinical laboratory animal data suggest that caution is warranted regarding the use of soy supplements high in isoflavones for women with breast cancer, particularly if they are menopausal," says Helferich, who was the principal researcher on the papers.

For most people, soy is a healthy food and can be used as part of a healthy diet, he says. Isolated soy protein had been found in previous UI studies to effectively lower cholesterol. Studies elsewhere have shown potential relief of menopausal symptoms and protection against cancer.

In the Journal of Nutrition, Helferich and colleagues show that the estrogen-dependent tumors implanted into experimental mice models grow at a rate in proportion to the levels of genistein consumed. Researchers used athymic mice that lack the ability to reject human cancer cells.

After inserting breast cancer cells, researchers were able to closely monitor the dietary estrogen to stimulate tumor growth. Genistein at or above 250 parts per million, a dosage that produces blood levels similar to what is observed in women consuming soy diets, was enough to stimulate tumor growth.

In the paper in Carcinogenesis, the researchers compared the isoflavone in its two forms, as a glycoside (genistin, as it appears in plants) and aglucone (genistein). They found that both forms produced similar tumor growth rates, and that the conversion of genistin to genistein in the body begins with contact with saliva in the mouth.

In Cancer Research, Helferich compared soy protein isolates containing varying levels of isoflavones. The researchers found that estrogen-dependent tumor growth increased as the isoflavone content increased in the soy-containing diet.