Soy estrogens: Too much of a good thing?

Estrogen has a mixed reputation. It wards off heart disease and sustains bone mass in women, but it appears to promote breast and ovarian cancers. Two studies using female mice now suggest that a form of estrogen found in soybeans and generally considered a safe dietary supplement also could contribute to the risk of cancer.

The chemical analogs of estrogen found in soybeans and other plants belong to the class of chemicals called isoflavones and appear to share some of the hormone's beneficial qualities. In contrast to estrogen, the research on soy isoflavones has been overwhelmingly positive. This has led to the marketing of capsules of genistein, the primary isoflavone in soy.