Cancer specialists have been concerned about soy foods in the diet of breast cancer patients. The growth and spread of many breast cancers are stimulated by estrogens. For this reason one of the goals of therapy in breast cancer is to block, or eliminate estrogen from the body.

Soy foods are rich in phytochemicals called isoflavones which have an estrogen-like effect. Isoflavones bind to the estrogen receptors in humans. Tamoxifen is one highly effective treatment for breast cancer that works by blocking the effects of estrogen by binding to estrogen receptors on cancer cells and throughout the body.

The isoflavones of soy foods compete with tamoxifen for estrogen receptors. Researchers feared that the isoflavones in soy foods might reduce the effectiveness of cancer therapy. They worried that results would include increased risks of death from breast cancer and higher recurrence of breast cancer among women receiving treatment.

This question was settled by an analysis of multiple studies of breast cancer survival. Results were presented at the American Association of Cancer Research meetings in Orlando. The study involved 16,000 women in the United States and China. Soy intake came from tofu products and soymilk.

Survival was compared between women whose diet contained less than half a milligram of isoflavones and women who had more than 23 mg of isoflavones in their diet. The women who had highest intake of soy foods had a 9% reduction in the risk of dying from breast cancer and a 15% reduction in the risk of recurrence of breast cancer.

This study shows that it is safe for women with breast cancer to eat soy foods. Soy will not interfere with standard breast cancer therapy, and may actually help reduce the risk of dying from breast cancer. Soy may also reduce the likelihood of breast cancer recurrence among women who have been successfully treated.