Mythbuster: Does Soy Decrease Testosterone?
Let’s talk about the relationship between soy consumption and male androgen levels. This debate is decades old and there are strong advocates for and against soy decreasing testosterone, so I wanted to address this from an unbiased, evidence-based perspective. A quick google search of this reveals a lot of ‘experts’, some of who have likely done their due diligence and are unbiased, and others who just want to be experts or who have an ulterior motive. I have no dog in this fight, so I’ll do my best to just present the facts and the conclusions that I draw from them.
The scientific or theoretical bases for this concern is as follows. Soy and soy-derived products contain isoflavones, a biological molecule that mimics estrogen and is often termed phytoestrogens. These so-called phytoestrogens may exert adverse effects on male androgens levels. Regardless of whether this occurs biologically, the question is does soy have any clinical or measurable impact on serum testosterone, free testosterone, sex hormone binding globuline (SHBG) or free androgen index (FAI)?
One thing I want to address before I get started is that I am not going to address other potential benefits of soy. It may have many other nutritional benefits, but in an effort to remain focused, I will only discuss soy and its relationship with testosterone. Also, there are dozens of articles and publications on the subject, so I will try to focus on just a few that I find most relevant.
Getting started with the evidence that soy does lower Testosterone
A 2000 study looked at the effects of replacing meat with soybeans and the effects on sex hormone concentrations in men
- 42 adult men did a cross-over trial where they consumed either lean meat or tofu for 4 weeks, then had a wash-out period and were switched over to the other diet
- Conclusion: Replacing meat with soy protein has a minor effect on biologically active sex hormones
A 2000 study looked at soy intake and serum androgens (and estrogens)
- They took 69 Japanese men and assessed soy consumption based on food frequency questionnaire
- Conclusion: Total and free testosterone concentrations were inversely correlated with soy product intake after controlling for the covariates, but these correlations were of borderline statistical significance
A 2007 study looked to assess the impact of soy protein powder supplementation
- They included 18 male subjects who consumed 2 scoops of soy protein powder daily for 28 days
- Conclusion: Soy protein powder decreases serum testosterone levels in healthy men
A 2013 publication looked at the effect of whey and soy protein on hormone responses to resistance exercise
- The took 10 men who completed a 14 day cross over trial exercising and then consuming whey or soy protein, and then switching to the other protein.
- Conclusion: The investigators observed lower testosterone responses following supplementation with soy protein
There you have it right? 4 studies, and there are more, that clearly demonstrate soy lowers testosterone. Well, it’s not that simple. There are other studies that draw the opposite conclusion, so let’s talk about the flip side, evidence that states soy does not lower testosterone
A 2006 study looked at soy intake on antioxidant capacity, testosterone levels
- They fed healthy young men a high soy diet for 4 weeks
- Conclusion: no effect on plasma testosterone values
A 2007 study looked at protein source, resistance training and sex hormones
- 20 subjects supplemented with 50g/day of protein from different sources (Soy concentrate; Soy isolate; Soy isolate and whey blend, and Whey blend only) in combination with a resistance-training program
- Conclusion: 12 week supplementation with soy protein does not decrease serum testosterone or inhibit lean body mass change
A 2010 meta-analysis looked at whether soy protein or isoflavones reduced testosterone in men
- They included 15 placebo-controlled studies and 32 other reports involving 36 treatment groups that were assessed separately.
- Conclusion: No significant effects of soy protein or isoflavone intake on Testosterone, SHBG, free Testosterone, or FAI (free androgen index) were detected regardless of statistical model.
I just want to point out that this last study is a meta-analysis. This is the gold standard for combining multiple studies to draw conclusions and it’s the best publication I could find that assessed the relationship between soy and testosterone.
Ultimately, I think the jury is still out.. There just isnt enough good data in large, randomized clinical trials. Most of the studies are small scale and/or easily criticized for methodological flaws. A lot of these studies that show testosterone lowering effects have individuals consuming excessive amounts of protein per day. For example, one of my references they were consuming 56 g/day. This is a lot of soy for the average person, but it may not be for an athlete or weight lifter.
The best study is the 2010 meta-analysis which found no significant effect of soy or isoflavones on Testosterone, SHBG, free Testosterone, or FAI (free androgen index).
Like most things, soy in moderation probably does not have a significant effect on your serum testosterone levels. Excessive soy intake may have a suppressive effect to some degree, but this is unlikely to apply to most individuals. If you are an athlete or weight lifter who consumes high amounts of protein per day, it may be reasonable to minimize the amount of protein from soy you consume per day. Soy has a lot of other benefits: high in protein, protective against heart disease & high cholesterol, antioxidant effects
Habito, R., Montaito, J., & Leslie, E. (2000). Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr, 84(4), 557-563.
Nagata, C., Inaba, S., & Kawakami, N. et al. (2000). Inverse Association of Soy Product Intake With Serum Androgen and Estrogen Concentrations in Japanese Men.Nutrition and Cancer, 36(1), 14-18.
Goodin, S., Shen, F., & Shih, W. J. et al.(2007). Clinical and Biological Activity of Soy Protein Powder Supplementation in Healthy Male Volunteers. Cancer Epidemiology Biomarkers & Prevention, 16(4), 829-833.
Kraemer, W. J., Solomon-Hill, G., Volk, B. M., & Kupchak, B. R et al. (2013). The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Responses to Resistance Exercise in Men. Journal of the American College of Nutrition, 32(1), 66-74.
Disilvestro, R. A., Mattern, C., & Wood, N. et al. (2006). Soy protein intake by active young adult men raises plasma antioxidant capacity without altering plasma testosterone. Nutrition Research, 26(2), 92-95.
Kalman, D., Feldman, S., & Martinez, M. et al. (2007). Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr Journal of the International Society of Sports Nutrition, 4(1), 4.