Many people are confused about soy. It continues to be a controversial food in our culture. Because I recommend a plant-based diet to my patients at my place of 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” or “phytoestrogens” which people fear mimics human estrogens and could stimulate cancer growth or dominate a male’s testosterone levels.
Based on an analysis of the current available literature on the effects of soy and testosterone levels, the consensus points to safety in moderation: “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 reported a 19% decrease in mean serum testosterone levels among 12 men over a 4-week period in response to the daily consumption of 56 grams 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 and instead choose a wide variety of protein sources in the daily diet.
It is important to remember 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, it is lignans that contribute the majority of phytoestrogens in our diets. If you eat fruits, veggies, beans and whole grains, then you consume plant estrogens every day in a healthy way. Research continues to find that the natural complexes found in plants are protective of our health. Men and women alike may include soy foods safely in the diet in moderation, along with other whole food products.
- 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.
- Hamiilton-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
- Kurzer, MS. Hormonal effects of soy in premenopausal women and men. J Nutr. 2002 Mar; 132(3):570S-573S.
- 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. Euro J Clin Nutr (2006) 60, 1423–1429.
- Messina M, Hamilton-Reeves J, Kurzer M, Phipps W. Effects of Soy Protein on Testosterone Levels. Cancer Epidemiol Biomarkers Prev. December 2007 16; 2795