Soy products have long been a cultural staple in Asian diets, but they have only recently gained popularity in Western diets as more people search for non-dairy milks, like soy milk, and vegetarian protein sources, like tofu. However, with their increased popularity, more people have come to question the effects of consuming them.
Soy naturally has a component called phytoestrogens (specifically isoflavones) which resemble estrogen, the female sex hormone produced in the body. Though naturally produced by both females and males, estrogen is typically referred to as the female sex hormone because it is present in higher levels in females and is responsible for feminine characteristics. Since phytoestrogens and estrogen have similar structures, many nutrition scientists hypothesize that consuming high amounts may throw off the body’s natural balance of sex hormones1Patisaul, H.B. & W. Jefferson. (2010). The pros and cons of phytoestrogens. Front Neuroendocrinol. 31:400-419.https://www.ncbi.nlm.nih.gov/pubmed/20347861. It’s worth noting that other foods also contain phytoestrogens, including fruits and some grains, but soybeans have some of the highest amounts out of the foods in our diets.
So what will consuming soy do to my hormone levels? Will I get man boobs?
A meta-analysis (a study of studies) from 2010 looked at 32 published studies and concluded that consuming soy foods or isoflavone supplements did not affect testosterone levels in men2Hamilton-Reeves J.M., G. Vazquez, S.J. Duval, et al. (2010). Clinical studies show no effect of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 94:997-1007.https://www.ncbi.nlm.nih.gov/pubmed/19524224. Another meta-analysis from the same year looked at 9 studies across a total of 252 men and had similar findings. Furthermore, it found that consuming soy foods did not affect estrogen levels in men either, even when men consumed a lot of soy (the highest of any of the studies was 139 mg/day, while a typical serving of soy is about 25 mg)3Messina M. (2010). Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 93:2095-104.https://www.ncbi.nlm.nih.gov/pubmed/20378106. Therefore, no changes in either hormone levels have been shown in men due to eating soy products, and thus, you should be safe from man boobs.
So I’ll still look manly, but can soy affect fertility?
Many web articles and blog posts warn that consuming soy can lower sperm concentrations, citing a single study with data suggesting this as their source4Chavarro J.E., T.L. Toth, S.M. Sadio, and R. Hauser. (2008). Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Human Reprod. 23:2584-90. https://www.ncbi.nlm.nih.gov/pubmed/18650557. However, these articles and blog posts don’t mention that the population sampled in the study was found at a fertility center (so these are males who potentially already had fertility issues). The articles also don’t mention the fact that the association between soy consumption and lower sperm concentrations was much stronger among overweight and obese men, whose weight could contribute more to the lower sperm count than soy intake. The study noted that more research must be done to conclude definitively the relationship between fertility and soy intake, suggesting that even the researchers knew this conclusion was not the end-all and be-all.
Another study looked at semen quality (including sperm concentration) and reported that no parameters involving semen quality were affected by soy consumption in healthy young men5Beaton L.K., B.L. McVeigh, B.L. Dillingham, et al. (2010). Soy protein isolates of varying isoflavone content do not adversely affect semen quality in healthy young men. Fertil Steril. 94:1717-22. https://www.ncbi.nlm.nih.gov/pubmed/19819436. A more recent study, looking at male soy food intake and in vitro fertilization outcomes, also did not see any association between the two6Mínguez-Alarcón L., M.C. Afeiche, Y.H. Chiu, et al. (2015). Male soy food intake was not associated with in vitro fertilization outcomes among couples attending a fertility center. Andrology. 3:702-8. https://www.ncbi.nlm.nih.gov/pubmed/26097060. More studies may need to be done in different age groups and ethnicities, but overall, fertility should not be a concern while consuming soy.
Does consuming soy provide any benefits for men?
In addition to leaving your chest and sperm unaltered, eating soy may actually be beneficial to your health! A meta-analysis of 29 studies (which included a whopping total of 17,546 men!) looking at the relationship between phytoestrogen intake and prostate cancer concluded that consumption of soy significantly reduced prostate cancer risk in Asians and Caucasians, but not Africans 7Zhang M., K. Wang, L. Chen, et al. (2016). Is phytoestrogen intake associated with decreased risk of prostate cancer? A systemic review of epidemiological studies based on 17,546 cases. Andrology. 4:745-56. https://www.ncbi.nlm.nih.gov/pubmed/27260185. This is not to say that Africans have an increased risk of prostate cancer from consuming soy, but that their risk of this cancer did not decrease in these studies. There may be additional benefits to eating soy, including improved heart health, and more information can be found in the review article linked at the end of this post.
Okay, but what if I’m a female?
Many women wonder if consuming phytoestrogens will increase their risk of breast cancer. Studies have shown that the effects of soy consumption are similar to those seen for prostate cancer in males. Consuming soy does not increase the risk of breast cancer8Fen-Jin He & Jin-Qiang Chen. (2013). Consumption of soybean, soy foods, soy isoflavones and breast cancer incidence: Differences between Chinese women and women in Western countries and possible mechanisms. Food Science and Human Wellness. 2:146-161. https://www.sciencedirect.com/science/article/pii/S2213453013000438. In fact, consuming higher amounts of soy could lower the risk of breast cancer in women on Asian diets9Chen M., Y. Rao, Y. Zheng, et al. (2014). Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies. PLoS One. 9:e89299. https://www.ncbi.nlm.nih.gov/pubmed/24586662! Surprisingly, this association between reduced breast cancer risk and soy intake was not seen in women living in Western countries; this difference is most likely due to the different average soy consumptions between the two regions. Additionally, a study conducted in North America saw a lower mortality rate in breast cancer survivors that had a higher isoflavone diet10Zhang F.F., D.E. Haslam, M.B. Terry, et al. (2017). Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry. Cancer. 123:2070-2079. https://www.ncbi.nlm.nih.gov/pubmed/28263368. This suggests that consuming soy is not linked to breast cancer and may be helpful in prevention and survival!
So, does anyone have to be concerned with consuming soy?
Soy allergies do exist, so those diagnosed with a soy allergy should obviously avoid soy products. It’s also important to note that, like any other food, there is such thing as too much soy. A 60-year-old man experienced symptoms such as erectile dysfunction and breast tenderness with high soy intake11Martinez J. & J.E. Lewi. (2008). An unusual case of gynecomastia associated with soy product consumption. Endocr Pract. 14:415-8. https://www.ncbi.nlm.nih.gov/pubmed/18558591. However, he was consuming 3 quarts of soy milk per day, meaning he was drinking up to 12 servings of soy milk every day. His symptoms disappeared after ending this habit and doctors note this was very rare and strictly due to the immensely high amount of soy consumed.
What should I know going forward?
It is worthwhile to note that most of the studies I mentioned look at people on Western diets, which contain far less soy than most Asian diets (estimated average of 0.67 mg/day versus 25-50mg/day, respectively). Thus, the soy consumption examined in the studies is most likely far too low of an amount to produce any changes in health (positive or negative)12Messina M. (2004). Western soy intake is too low to produce health effects. Am J Clin Nutr. 80:528-9. https://www.ncbi.nlm.nih.gov/pubmed/15277184. However, studies looking specifically at those who do consume an Asian diet have yet to raise concern about intake levels.
Thus, most people don’t have to be concerned with consuming too much soy. Adult population and clinical studies actually suggest benefits are seen with 2-4 servings per day, so increasing your soy consumption might be the way to go13Messina M. (2016). Soy and health update: Evaluation of the clinical and epidemiologic literature. Nutrients. 8(12). https://www.ncbi.nlm.nih.gov/pubmed/27886135. Overall, eating soy should not give you man boobs or threaten your sperm quality, and may even reduce your risk of prostate or breast cancer! So go ahead and enjoy your soy (with a hint of everything in moderation)!
For those looking at additional information not discussed above, I like this 2016 review: https://www.ncbi.nlm.nih.gov/pubmed/27886135
It covers additional topics including associations with heart health, menopause, and even cognitive function.