Some people claim that consuming soy lowers testosterone in men. This seems to be repeated by a lot of bloggers and YouTubers. Is there any truth to this? Is this supported by human or animal studies?
There is some very weak evidence that soy lowers testosterone, for example, Dillingham et al (2005) and Goodin et al (2007), however the former has a very small sample size and neither study utilised a control group.
The vast majority of research shows no effect.
In general, rather than relying on individual studies which have their own idiosyncrasies and weaknesses, it is better to look for systematic reviews and meta-analyses, which use available evidence from all included studies. For the effect of soy on testosterone, Hamilton-Reeves et al (2009) reviewed 32 studies:
objective of this meta-analysis was to evaluate in men the effects of soy protein or isoflavone intake on T and other in-dicators of bioavailable T, such as SHBG, free T, and FAI as reported in clinical trials. The results indicate that neither soy protein nor isoflavone intake significantly alters any of these measures.
Die et al (2014) reviewed 8 randomised controlled trials of men with either prostate cancer or an elevated risk of prostate cancer and concluded:
a clear understanding of the impact of soy/isoflavones on PSA, total testosterone, free testosterone and SHBG levels in men with, or at identified risk of, PCa could not be derived from these data, given the limitations of sample size and study duration in individual trials
Die, M. Diana, et al. "Soy and soy isoflavones in prostate cancer: a systematic review and meta‐analysis of randomized controlled trials." BJU international 113.5b (2014). https://doi.org/10.1111/bju.12435
Dillingham, Barbara L., et al. "Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men." The Journal of nutrition 135.3 (2005): 584-591. https://doi.org/10.1093/jn/135.3.584
Goodin, Susan, et al. "Clinical and biological activity of soy protein powder supplementation in healthy male volunteers." Cancer Epidemiology and Prevention Biomarkers 16.4 (2007): 829-833. https://doi.org/10.1158/1055-9965.EPI-06-0882
Hamilton-Reeves, Jill M., et al. "Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis." Fertility and sterility 94.3 (2010): 997-1007. https://doi.org/10.1016/j.fertnstert.2009.04.038
By analyzing the the soybean chemically, we know that soy is chemically high in phytoestrogens, the organic compound that mimics estrogen. Thus, the common conceptions of it's link to hormonal imbalances. Based on the research I found, the link between soy and decreased testosterone more than a correlation.
This study follows a 19 year old man who started consuming hefty amounts of soy in his diet. After this dietary change, he observed a lost interest in sex and suffered from erectile dysfunction. His blood DHEA, testosterone, and DHT levels also plummeted significantly. One year after stopping the soy consumption, his erectile health and hormonal profile was fully regained. This study is NOT full proof, fully understand that this study follows one young man's experience, however the results are still worth noting.
This study, done with 99 infertile men, concluded that with a higher intake of soy foods comes an associated lower sperm count. Not necessarily linked to differing testosterone levels, but hormonally soy seems to have more of a cause and effect relationship in this study than a mere correlation.
This study took 35 healthy young men, and fed them various soy proteins and milk proteins (at different times). 24-hour urine samples indicated that the soy protein decreased testosterone and DHT levels and had minor effects on the other male hormones
This study took 12 men over 18 year of age, and fed them two scoops of pure soy protein powder for 28 days, and collected testosterone and other hormonal levels weekly. They concluded that soy protein powder decreases serum testosterone levels in healthy men.
This study supports the idea that soy and soy protein has no effect on testosterone levels. By having 15 placebo controlled groups consume soy (specific amounts consumed are not documented in the report), which lead them to conclude that there were no significant effects of soy protein or isoflavone intake on testosterone.
Everything is okay in moderation. Despite scientific disagreements, through various studies, regarding whether soy has a direct effect on testosterone, one can observe that a substantial daily does of soy protein is needed to demonstrate significant results. Soy may have possibly effects on testosterone and other hormones, as it introduces more hormones into the human body when consumed. Conversely, it is not detrimental to the individual unless constant consumption.
Soy does not lower testosterone. Here is a video, where the guy explains exactly why this is the case. He looks at the "evidence" suggesting that this is the case, then explains (with evidence) why this "evidence" does no hold much weight. He also shows a lot of evidence that soy does NOT lower testosterone. This video is 30 mins long, however he explains (with evidence) why this is not the case much better than I ever could. It is definitely worth a watch.
Sources that the YouTuber references (taken directly from the video description):
Analysis of 32 studies into soy protein effects on men's hormones (including testosterone) - here
Later additional analysis, including reference to the above analysis - here
The study of soy isoflavones in relation to sperm counts (which, by the way, the above article also brings up and dismisses, because scientists pay more attention to this than idiot reactionaries trying to prove what they already want to believe is true) - here
Source for WHO healthy sperm concentration - here