Soy and Man-boobs

I have been asked so many times about soy and “man-boobs” recently that I just have to address this topic.  Because I recommend a plant-based diet to my patients at work and represent Illinois as the Vegetarian Nutrition Dietetic Practice Group State Coordinator, the topic of soy is something I discuss every day.  Soy (among many other plant foods) contains “soy estrogens” aka “phytoestrogens” which people fear mimics human estrogens and can stimulate cancer growth…..or dominate a male’s testosterone levels which would promote the spontaneous development of the dreaded man-boob (not a term I personally choose to use but one I am asked about often). 

I was also more intellectually challenged by a professor of nutrition at Illinois State University earlier this month while hosting a vegetarian exhibit booth at a nutrition seminar who asked about the effect that soy has on testosterone levels.  Good question. 

Based on an analysis of the current available literature on the effects of soy on testosterone levels, I have come to the conclusion that the answer is:  No, you will not develop man-boobs.  There are many reviews of studies done and one consensus I would like to highlight is:  “These recent studies in men consuming soyfoods or supplements containing 40--70 mg/d of soy isoflavones showed few effects on plasma hormones or semen quality. These data do not support concerns about effects on reproductive hormones and semen quality” (1). 

While soy likely does not cause hormonal disruption or imbalance, it does have some research pointing to its positive effects in regards to cancer-prevention and cardiovascular health.  As with any food, I always discuss the importance of MODERATION with my patients.  That means a soy product once every day or several times per week instead of at every meal.  Further, soy is a highly genetically modified crop in this country.  Many of my patients prefer to buy organic in the hopes of decreasing their exposure to genetically modified foods.  Finally, whole food sources are best.  I suggest consuming unprocessed/less processed soy such as edamame, tempeh, or miso instead of more processed products such as soy protein isolate or soy milk (especially with added sweeteners). 

There are some small and controversial studies that show a decrease in testosterone with high levels of concentrated soy protein isolate such as one from 2007 showing a 19% decrease in mean serum testosterone levels in 12 men over a 4-week period after consuming 56 grams daily of isolated soy protein.  For reference, on average most people require between 60-80 grams of protein per day (based on body size).  That means that this small group of study participants was eating very large quantities of a processed soy source.  My recommendation is not to do this; I would have advised against it even before I read the study. 

I’d like to point out that you likely eat phytoestrogens (plant estrogens) every day.  Soy may have the highest concentration, but a plant substance called “lignans” (basically a type of fiber) is found in many plant foods including seeds (flax, pumpkin, sunflower, poppy, sesame), whole grains (rye, oats, barley), bran (wheat, oat, rye), beans, fruits (particularly berries), and vegetables.  According to the Linus Pauling Institute, though most research on phytoestrogen-rich diets has focused on soy isoflavones, lignans are the main source of phytoestrogens in our general diets.  If you eat fruits, veggies, beans and whole grains and don’t have man-boobs yet then you’ll likely be ok eating a little soy.  Further, I have a hard time when someone is telling me they can’t eat soy (which has proven health benefits) but they’re chugging soda, alcohol, or eating sugary desserts.  You want to know what will more likely cause man-boobs?  Too much chocolate cake. 

Finally, I encourage you to take a look at my references below.  These are just my favorites right now and some of the most reliable in a sea of research on the subject.  When you need an answer regarding your health, please find a medical provider, registered dietitian or researcher who can provide you a research-based answer and not just a friend or nutrition enthusiast/health-coach who tells you to avoid soy because it gives you man-boobs…..

1. Messina M.  Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence.  Fertil Steril. 2010 May 1;93(7):2095-104.
2. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ.Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis.  Fertil Steril. 2010 Aug;94(3):997-1007
3. Kurzer, MS.  Hormonal effects of soy in premenopausal women and men.  J Nutr. 2002 Mar;132(3):570S-573S.
4. G Maskarinec, Y Morimoto, S Hebshi, S Sharma, A A Franke and F Z Stanczyk.  Serum Prostate-specific antigen but not testosterone levels decrease in a randomized soy intervention among men.  European Journal of Clinical Nutrition (2006) 60, 1423–1429.
5. Messina M, Hamilton-Reeves J, Kurzer M, Phipps W.  Effects of Soy Protein on Testosterone Levels.  Cancer Epidemiol Biomarkers Prev.  December 2007 16; 2795
6. http://lpi.oregonstate.edu/infocenter/phytochemicals/lignans/