Phytoestrogens: potential benefits and implications for breast cancer survivors.

Phytoestrogens are a group of plant-derived substances that are structurally or functionally similar to estradiol. There has been much interest in the potential role of phytoestrogens in cancer prevention and treatment of estrogen-deficient states. This review summarizes the evidence for phytoestrogen risks and benefits relevant to the breast cancer survivor, including prevention of a second primary breast cancer or metastatic disease, reduction in menopausal symptoms, and interactions with tamoxifen. Epidemiological data suggest a breast cancer protective role for phytoestrogens, and there is some supporting clinical data, but they are far from conclusive. In addition, there is some evidence that genistein, the most prevalent isoflavone in soy, can stimulate estrogen receptor-positive (ER+) breast cancer growth and interfere with the antitumor activity of tamoxifen at low levels. Given current knowledge, women who have ER+ tumors should not increase their phytoestrogen intake. Several studies suggest an inhibitory effect on ER- breast cancer cell growth, and it may be reasonable for women with ER- tumors to safely consume soy and possibly other phytoestrogens. However, the optimal amount and source are not clear. More research is needed to clarify the role of phytoestrogens in breast cancer prevention and in treating estrogen-deficient diseases in women who have had breast cancer.