I have specific concerns about deliberately increased soy consumption for the purpose of helping prevent breast cancer or its recurrence. Like the editorial conclusion reached by the American Journal of Clinical Nutrition in its review entitled "Phytoestrogens and Breast Cancer" (Ziegler RG. 2004. American Journal of Clinical Nutrition, Vol. 79, pages 183-184), I do not believe that the research supports a recommendation for increased intake of soy phytoestrogens by adult women for the purpose of decreased breast cancer risk.
Some of the most intriguing findings about soybean consumption involve developmental status at the time of soybean consumption. Regular consumption of soy foods during childhood or early adolescence appears to be potentially protective in a way that similar levels of consumption during adulthood are not. This age-dependent character of soy benefits may help explain some of the contradictory results when breast cancer risk in Asian women who grew up consuming soy foods is compared to breast cancer risk in U.S. women who did not.
I am concerned that some animal research has shown genistein—one of soy's premier phytoestrogen isoflavones—to potentially interfere with the activity of tamoxifen, a prescription drug widely used to help inhibit the growth of breast cancer cells. I am also concerned that some studies in postmenopausal women show that the soy isoflavones may have potentially detrimental effects when routinely consumed in deliberately increased amounts. Particularly for women of menopausal age, but also for all women considering increased soy consumption as a means of reducing breast cancer risk, I believe that decisions in this area merit the advice of a licensed healthcare practitioner. There's just too many complications here, and potential variability from individual to individual to proceed in the absence of professional advice.
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