Soy-rich diets have been credited with the low incidence of breast, stomach, and prostate cancers in Asian populations compared with their much higher prevalence in the Western hemisphere, where soy consumption is low.139 There has been a multifold increase in research studies correlating consumption of soybean products to prevention of breast, colon, uterine, and prostate cancers. Soy was found to be one of the potent natural supplements with cancer-prevention properties.140 It is a host of two important isoflavones, namely genistein and daidzein. These substances are shown to have various anticarcinogenic properties, including prevention of oxidation, inhibition of protein tyrosine phosphorylation, triggering of apoptosis, regulation of gene transcription, modulation of transcription factors, antiangiogenesis, and inhibition of DNA topoisomerage.141 Daidzein and genistein down-regulate the expression of prostate androgen-regulated transcript-1 (PART-1) gene, a possible prostate cancer tumor marker.142 These isoflavones are known as phytoestrogens because of their estrogenic property, and they are being used as a natural alternative to hormone replacement therapy (HRT). Recent studies indicate that soy isoflavones are cardioprotective and help to reduce menopausal hot flashes and increase bone density in women.143 The U.S. Food and Drug Administration recently approved a health claim that soy protein as part of a diet low in saturated fats and cholesterol may reduce the risk of heart disease.144
A community-based prospective cohort study of Japanese men and women in Takayama, Japan, established the inverse relationship between risk of death from stomach cancer and soy intake.145 Epidemiological studies indicated that women who consume a traditional Japanese diet excrete 1000 times more genistein than those from societies with high breast cancer rates, supporting the role of genistein in lowering the risk factor. These isoflavones inhibit the growth of estrogen-dependant tumor cells by blocking the cell uptake of this hormone. However, recent in vivo studies on animals indicate that genistein and soy diets containing varying amounts of genistein can stimulate the growth of estrogen-dependent (MCF-7) tumors in a dose-dependent manner.146 The effect of phytoestrogens on breast cancer risk thus seems to be complex, and the outcome may depend on the dose and time of administration. A review of epidemiological studies and studies involving immigrant women in Western countries demonstrates that a soycontaining diet may be beneficial against breast cancer if consumed before puberty and during adolescence.147 148 These studies found no negative effects of soy on breast cancer and suggested that consumption of soy only during the adult life had little or no effect on the risk of breast cancer. Thus postmenopausal women who have breast cancer or those at high risk of developing breast cancer should be extra cautious until the results from well-documented clinical trials are available.
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