A meta-analysis of five case-control and two cohort studies examined the effects of soy intake on endometrial and ovarian cancer. 169,051 women and, 3516 with endometrial or ovarian cancer in the U.S., China, Italy and Japan with an average age of 54 were evaluated for their soy intake based on soy containing foods or soy isoflavone intake.[i]
In each of the studies, women who consumed the highest dietary intake of soy had a lower risk for endometrial and ovarian cancers compared with the women who had the lowest intake.
Commentary: It is not surprising to see this report as we have seen previous observational studies with similar results, showing lack of endometrial proliferation, endometrial safety and/or reduced risk of endometrial cancer. Only one previous study that I’m aware of, did demonstrate that after 5 years, but not after one year or 3 years, who were given 150 mg per day of soy isoflavone tablets had an increased occurrence of endometrial hyperplasia (but no cases of atypical hyperplasia or endometrial cancer).[ii]
The mechanisms whereby soy appears to have an influence on lowering the risk of hormonal cancers, including breast, appear to be multiple. These include: through its ability to bind to certain estrogen receptors and actually have an estrogen blocking effect, raising sex hormone-binding globulin which decreases circulating estrogens, affecting selected enzyme pathways which result in anti-carcinogenic effects, direct tumor growth inhibition, and having antioxidant effects.
My advice: for most women, and for those who are not allergic to soy or have indigestion with soy products, I recommend 1-2 servings per day of the following soy foods: cooked soy beans, roasted soy nuts, soy milk, tofu, tempeh, edamame, tofu pate (my favorite).
[i] Myung S- K et al. Soy intake and risk of endocrine-related gynaecological cancer: A meta-analysis. BJOG 2009 Dec; 116:1697
[ii] Unfer V, et al. Endometrial effects of long-term treatment with phytoestrogens: a randomized, double-blind, placebo-controlled study. Fertility and Sterility 2004;82:145-148). 150 mg of soy isoflavones per day is above the average intake in an Asian diet (ranging from about 40-90 mg per day