Because my husband [Jack Norris, VeganHealth.org] regularly consults for the soy industry, I’ve chosen not to write very much about soy foods and health. And, since Jack published his excellent comprehensive article on the controversies surrounding this issue, there isn’t much need for me to do so anyway.
But two issues regarding soy foods always prompt me to provide some clarification, because they seem to be universally—and consistently—misunderstood despite very clear and conclusive research findings. One pertains to the types and amounts of soy consumed in Asia which I wrote about several months ago. The other is the mistaken idea that soyfoods contain estrogen, which I want to address here.
Soyfoods are unique among commonly-consumed foods because they contain large amounts of isoflavones. And in order to understand the controversy around soyfoods, it’s necessary to understand what isoflavones are—and why they aren’t the same as estrogen. Sometimes referred to as phytoestrogens, isoflavones have a chemical structure that is very similar to the hormone estrogen, but small variations in their structure translate to important differences in physiological effects.
Both isoflavones and estrogen bind to and activate estrogen receptors (ERs) on cells. Human cells have two types of these receptors–ER-alpha and ER- beta—which have different distributions in different tissues. That doesn’t matter to estrogen, which happily binds to either type, but isoflavones are more finicky. They much prefer ER-beta. It’s an important distinction, because the two types of receptors produce different—sometimes completely opposite—effects.
As a result—while estrogen always acts like estrogen—isoflavones function differently in different tissues. They may have estrogen-like effects or anti-estrogenic effects, or no effects at all. For this reason, isoflavones are considered to be SERMs, or selective estrogen receptor modulators.(1)
Their selective nature makes some SERMs useful in medical therapy. For example, while estrogen therapy can protect bone health in postmenopausal women, it may also raise risk for breast cancer. In contrast, the osteoporosis drug raloxifene—which is a SERM—has estrogen-like effects on bones, but anti-estrogenic effects in breast tissue. So it helps protect bone without raising breast cancer risk.(2)
Because they function as SERMS, research is focused on potential health benefits of isoflavones, particularly as safe alternatives to estrogen. There is evidence that they may reduce menopausal symptoms like hot flashes,(3,4) but the findings on isoflavones and bone health are more conflicting.(5,6) And while some data suggests that isoflavone-rich soy foods could improve prognosis in women who have breast cancer,(7-10) it’s not clear that women who begin consuming soy in adulthood will actually reduce their risk of cancer.
What we do know with certainty is that isoflavones are not estrogen and, looking at how estrogen acts in the body doesn’t tell us anything about the effects of eating soy foods. In fact, if isoflavones were the same as estrogen, there wouldn’t really be any interest in studying them as alternatives to this hormone. The only way to know what isoflavones do is to study isoflavones.
- Oseni T, Patel R, Pyle J, Jordan VC. Selective estrogen receptor modulators and phytoestrogens. Planta Med 2008;74:1656-65.
- Heringa M. Review on raloxifene: profile of a selective estrogen receptor modulator. Int J Clin Pharmacol Ther 2003;41:331-45.
- Bolanos R, Del Castillo A, Francia J. Soy isoflavones versus placebo in the treatment of climacteric vasomotor symptoms: systematic review and meta-analysis. Menopause 2010;17:660-6.
- Nelson HD, Vesco KK, Haney E, et al. Nonhormonal therapies for menopausal hot flashes: systematic review and meta-analysis. Jama 2006;295:2057-71.
- Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomized controlled trials. Clin Nutr 2008;27:57-64.
- Taku K, Melby MK, Takebayashi J, et al. Effect of soy isoflavone extract supplements on bone mineral density in menopausal women: meta-analysis of randomized controlled trials. Asia Pac J Clin Nutr 2010;19:33-42.
- Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA 2009;302:2437-43.
- Guha N, Kwan ML, Quesenberry CP, Jr., Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat 2009;118:395-405.
- Caan BJ, Natarajan L, Parker B, et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev 2011;20:854-8.
- Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. Cmaj 2010;182:1857-62.