Does eating soy help prevent breast cancer?

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.

References

Cooke GM. A review of the animal models used to investigate the health benefits of soy isoflavones. J AOAC Int 2006 Jul-2006 Aug 31;89(4):1215-27.

Duffy C, Perez K, Partridge A. Implications of phytoestrogen intake for breast cancer. CA Cancer J Clin 2007 Sep-2007 Oct 31;57(5):260-77.

Linos E, Willett WC. Diet and breast cancer risk reduction. J Natl Compr Canc Netw 2007 Sep;5(8):711-8.

Messina M, McCaskill-Stevens W, Lampe JW. Addressing the soy and breast cancer relationship: review, commentary, and workshop proceedings. J Natl Cancer Inst 2006 Sep 20;98(18):1275-84.

Michels KB, Mohllajee AP, Roset-Bahmanyar E, et al. Diet and breast cancer: a review of the prospective observational studies. Cancer 2007 Jun 15;109(12 Suppl):2712-49.

Rice S, Whitehead SA. Phytoestrogens and breast cancer--promoters or protectors? Endocr Relat Cancer 2006 Dec;13(4):995-1015.

Tempfer CB, Bentz EK, Leodolter S, Tscherne G, Reuss F, Cross HS, Huber JC. Phytoestrogens in clinical practice: a review of the literature. Fertil Steril 2007 Jun;87(6):1243-9.

Wu AH, Yu MC, Tseng CC, et al. Epidemiology of soy exposures and breast cancer risk. Br J Cancer 2008 Jan 15;98(1):9-14.

Wuttke W, Jarry H, Seidlova-Wuttke D. Isoflavones--safe food additives or dangerous drugs? Ageing Res Rev 2007 Aug;6(2):150-88.

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