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Given the current data, women with a history of estrogen receptor-positive (ER+) tumors should not increase their intake of soy or other phytoestrogens, according to a review of studies on the topic. The review was supported in part by the Agency for Healthcare Research and Quality (T32 HS00011).
Chemotherapy for breast cancer may induce or accelerate ovarian failure, often causing severe menopausal symptoms. In addition, the drug tamoxifen, used as additional therapy to prevent breast cancer proliferation or recurrence, often causes hot flashes. Reducing menopausal symptoms with hormone replacement therapy is not an option for breast cancer survivors, since the hormone estrogen is linked to breast cancer development. Thus, some women (up to 12 percent in one study) increase soy in their diets to minimize these symptoms.
Soy contains plant-based phytoestrogens (weak estrogens), mainly isolavones, of which genistein is the most prevalent. For instance, there is some limited clinical evidence that genistein can stimulate breast cancer growth. In addition, in women with ER+ tumors who are likely to be taking tamoxifen, genistein may interfere with the drug's antitumor activity.
For women with tumors that are estrogen receptor-negative (ER-), genistein may inhibit breast cancer cell growth, and it may be reasonable for these women to safely consume soy and possibly other phytoestrogens. However, the optimal amount and source are unknown. In addition, ER status is not absolute, and patients who have ER- tumors may have some element of ER positives or could develop ER+ tumors. They should be informed of this possibility, according to the researchers who conducted the study. They reviewed the current clinical evidence on the relationship between phytoestrogens and breast cancer and its implications for phytoestrogen consumption by breast cancer survivors.
See "Phytoestrogens: Potential benefits and implications for breast cancer survivors," by Christine Duffy, M.D., M.P.H., and Michele Cyr, M.D., in the September 2003 Journal of Women's Health 12(7), pp. 617-631.
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