Effects of soy phytoestrogens genistein and daidzein on breast cancer growth

OBJECTIVE: To determine whether genistein and daidzein, the major phytoestr ogens in soy, can stimulate breast cancer growth. DATA SYNTHESIS: Systematic search through primary English-language literatu re on MEDLINE (1966-January 2001), EMBASE (1982-January 2001) and Current C ontents (1998-January 2001). DATA SOURCES: Genistein and daidzein at low concentrations were found to st imulate breast tumor growth in in vitro and in vivo animal studies, and ant agonize the antitumor effect of tamoxifen in vitro. At high concentrations, genistein inhibited tumor growth and enhanced the effect of tamoxifen in v itro. CONCLUSIONS: Genistein and daidzein may stimulate existing breast tumor gro wth and antagonize the effects of tamoxifen. Women with current or past bre ast cancer should be aware of the risks of potential tumor growth when taki ng soy products.