Once praised as the ultimate health food, now criticized for its status as a highly genetic modified crop and common food sensitivity, many people are confused about whether or not to eat soy. One question I’ve gotten lately is whether or not soy has a feminizing effect on males.
Most studies on soy in general are done on concentrated soy protein extracts, with varying levels of soy isoflavones genistein and daidzein. Daidzein is metabolized by gut bacteria present in 30-50% of people to an isoflavone called equol. Genistein, daidzein and equol are the components of soy that have been shown to exert the most hormonal effects in tissue cultures. In vitro (outside of humans, in Petri dishes) they have been shown to inhibit enzymes required for male hormone synthesis.
In humans, studies show variable effects of soy on male hormones. In a study published in 2007 by the American Society of Nutrition1, men that ate 107mg soy isoflavones daily in the form of soy protein isolate had decreased expression of male hormone receptors in the prostate, which could be beneficial for reducing prostate cancer, but no significant effect on circulating male hormones. The authors of the study also noted the majority of prior human studies (14 out of 22) showed no significant effect of soy on male hormone levels, with the remaining studies being inconclusive. In a review published in 20022, no significant effect on semen quality or circulating hormone levels was found in 3 human studies. A 2005 study3 compared male hormone levels in Japanese men to those in men from New Zealand. Japanese men have a lower than average rate of prostate cancer thought to be due to dietary factors. In the men studied (ages 21-31) there were increased levels of isoflavones and male hormones in the Japanese men compared to the New Zealanders, leading the researchers to conclude there may be another mechanism for reduced prostate cancer rates.
In summary, I see some effect of soy protein on male hormones and receptors, but no effect on male sex characteristics such as abnormal breast development, penile or testicular shrinkage or low sperm counts. Moderate intake (i.e. 1 serving of soy up to 4 times weekly) of organic soy products – especially traditional soy foods such as natto, miso, tempeh, and tofu - seem unlikely to create feminization in men based on research thus far and are potentially protective against not only prostate cancer, but also cardiovascular disease. Consult your naturopathic physician for more information about ways to reduce prostate cancer and cardiovascular disease risk and enhance male virility.
Claire Graser, ND Candidate 2012, Bastyr University
Sources:
1. Isoflavone-rich soy protein isolate suppresses androgen receptor expression without altering estrogen receptor-beta expression or serum hormonal profiles in men at high risk of prostate cancer. Hamilton-Reeves JM, et al. J Nutr. 2007 Jul;137(7):1769-75.
2. Hormonal effects of soy in premenopausal women and men. Kurzer MS. J Nutr. 2002. Mar;132(3):570S-573S.
