There are likely some folks, including health professionals, who are still uncertain about whether soy feminizes men. This uncertainty is understandable given that even today, claims that soy feminizes men can still be found. These claims exist despite definitive data showing neither soy nor soy isoflavones lower testosterone levels,1 raise estrogen levels,1 affect sperm or semen,2-4 or cause gynecomastia in men.5,6 So, there may be some value in outlining the origins of this feminizing myth.
Before doing so, it is important to emphasize that when a topic is as heavily researched as soy, it becomes easy to selectively cite the scientific literature to make a point that conflicts with a conclusion based on the totality of the evidence. This is especially true in the case of diet/nutrition intervention studies. Because these studies are often small in size, there is a greater likelihood of chance findings. For this reason, when the results from a sufficient number of studies have been published, the data are typically meta-analyzed. A meta-analysis combines the data from multiple studies for the purpose of integrating the findings.
Given that soyfoods are a uniquely rich source of isoflavones, which are commonly classified as phytoestrogens, concerns about feminization are easy to conceptualize, especially if the differences between isoflavones and estrogen are not fully appreciated. Furthermore, early on, there were a few animal studies,7,8 and even a couple of human studies,9,10 showing soy lowered testosterone levels. There was also a cross-sectional study published in 2008, showing soy intake was associated with lower sperm concentration.11 But, perhaps more than anything else responsible for raising feminizing concerns, was a case report published in 2008 describing a 60-year-old man who developed gynecomastia (breast enlargement in males) attributed to his soy consumption.12
The cross-sectional study and the case report were highlighted in an article on soy in Men’s Health published in 2009 entitled “Is this the most dangerous food for men?” which included the statement “…there may be a hidden dark side to soy, one that has the power to undermine everything it means to be male.”13 However, just one year later, a meta-analysis concluded that neither soyfoods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men.14 This analysis included 15 placebo-controlled treatment groups with baseline and ending measures and 32 reports involving 36 treatment groups. In 2021, an update to this analysis also concluded that neither soy nor isoflavones affect testosterone; additionally, there was no effect on estrogen levels in men.1 This analysis included 41 clinical studies; 1,753 were involved in studies examining total testosterone levels.
So, what about the case report describing gynecomastia?12 Somewhat parenthetically, there was a second case report published in 2011, that described feminizing effects (e.g., loss of libido and low testosterone levels) in a 19-year-old vegan attributed to his soy consumption.15 However, in both cases, the men consumed 9 times more isoflavones than the typical Japanese man.16 In contrast, neither of the two intervention studies that evaluated gynecomastia showed any effects, one was 3 months in duration and intervened with 66 mg/d6 isoflavones and the other was 3 years in duration and intervened with 100 mg/d isoflavones.5 Four servings of traditional soyfoods provides about 100 mg isoflavones, which is the upper end of the dietary range.
And, what about that cross-sectional study showing soy intake was associated with lower sperm concentration?11 First of all, cross-sectional studies do not carry a lot of weight within the epidemiologic community. Cross-sectional studies are usually fast and inexpensive to conduct and are suitable for generating hypotheses, but not for reaching robust conclusions about causal relationships.17 As to the study itself, about half of the decreased sperm concentration resulted from the higher ejaculate volume in the fourth (4.1 ml) compared to the first (3.5 ml) soyfood intake quartile. Total sperm count was only reduced by about 10% when comparing extremes of soy intake, a decrease which was not statistically significant; nor was there an effect of soyfood intake on sperm motility or morphology.
Since there is no biological basis for soy increasing ejaculate volume, the observed association between soy and sperm concentration lacks credibility. More importantly, none of the 3 intervention studies showed soy affected sperm or semen parameters.2-4 Furthermore, a follow up study conducted by the research group that conducted that cross-sectional study found that soyfood intake in men was not related to clinical outcomes among couples presenting at an infertility clinic.18
Given all the data, it is hard to understand how anyone intimately familiar with scientific literature could still claim that soy feminizes men. Even Men’s Health has changed its perspective. An article published in 2015 stated that “soy contains phytoestrogens, plant-based hormones that may, in excess, lead to dips in testosterone. Up to four servings a day, however, shouldn’t hurt.” And in 2019, the magazine published an article entitled Is Tofu the New King of Protein? in which the question was asked: “What about low sperm count and man boobs? The publication notably replied: “Several news outlets (including this one—our bad) may have overblown those findings.”
- Reed KE, Camargo J, Hamilton-Reeves J, et al. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60-7.
- Mitchell JH, Cawood E, Kinniburgh D, et al. Effect of a phytoestrogen food supplement on reproductive health in normal males. Clin Sci (Lond). 2001;100:613-8.
- Beaton LK, McVeigh BL, Dillingham BL, et al. Soy protein isolates of varying isoflavone content do not adversely affect semen quality in healthy young men. Fertil Steril. 2010;94:1717-22.
- Messina M, Watanabe S, Setchell KD. Report on the 8th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment. J Nutr. 2009;139:796S-802S.
- Fleshner NE, Kapusta L, Donnelly B, et al. Progression from high-grade prostatic intraepithelial neoplasia to cancer: a randomized trial of combination vitamin-E, soy, and selenium. J Clin Oncol. 2011;29:2386-90.
- Sathyapalan T, Rigby AS, Bhasin S, et al. Effect of soy in men with type 2 diabetes mellitus and subclinical hypogonadism: A randomized controlled study. J Clin Endocrinol Metab. 2017;102:425-33.
- Weber KS, Setchell KD, Stocco DM, et al. Dietary soy-phytoestrogens decrease testosterone levels and prostate weight without altering LH, prostate 5alpha-reductase or testicular steroidogenic acute regulatory peptide levels in adult male Sprague-Dawley rats. J Endocrinol. 2001;170:591-9.
- Strauss L, Makela S, Joshi S, et al. Genistein exerts estrogen-like effects in male mouse reproductive tract. Mol Cell Endocrinol. 1998;144:83-93.
- Gardner-Thorpe D, O’Hagen C, Young I, et al. Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men. Eur J Clin Nutr. 2003;57:100-6.
- Goodin S, Shen F, Shih WJ, et al. Clinical and biological activity of soy protein powder supplementation in healthy male volunteers. Cancer Epidemiol Biomarkers Prev. 2007;16:829-33.
- Chavarro JE, Toth TL, Sadio SM, et al. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod. 2008;23:2584-90.
- Martinez J, Lewi JE. An unusual case of gynecomastia associated with soy product consumption. Endocr Pract. 2008;14:415-8.
- Thorton J. Is this the most dangerous food for men? Men’s Health (June). 2009.
- Hamilton-Reeves JM, Vazquez G, Duval SJ, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94:997-1007.
- Siepmann T, Roofeh J, Kiefer FW, et al. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition. 2011;27:859-62.
- Messina M, Nagata C, Wu AH. Estimated Asian adult soy protein and isoflavone intakes. Nutr Cancer. 2006;55:1-12.
- Wang X, Cheng Z. Cross-sectional studies: Strengths, weaknesses, and recommendations. Chest. 2020;158:S65-S71.
- Minguez-Alarcon L, Afeiche MC, Chiu YH, et al. Male soy food intake was not associated with in vitro fertilization outcomes among couples attending a fertility center. Andrology. 2015;3:702-8.
This blog sponsored by the Soy Nutrition Institute and the United Soybean Board.