I get this question all the time – but doesn’t soy contain estrogen? Doesn’t soy lower your testosterone? Doesn’t soy give you man boobs …
You think I am joking – I literally get this a few times a week!
Soy is a staple for many cultures and has played an important role in the diets of East Asian countries for centuries. In each gram of soy, you will find roughly 3.5mg of Isoflavones – diphenolic compounds and a class of phytoestrogens that are selective estrogen receptor modulators.
Before you jump to conclusions you should know: there are different isoforms of estrogen receptors, with opposite functions, and there are different phytoestrogens that each modulate different estrogen receptor isoforms to varying degrees. (yeah, it is more complex than even I thought)
Now, the two primary isoflavones in soybeans are genistein and daidzein. For a good review on their roles on other tissues in the body, I recommend reading the entire review in the Nutrients Journal. In the review, Scientist Mark Messina, who has dedicated his life and reputation to studying soy, put together a 42 page article on how soy may reduce the risk of coronary heart disease, breast cancer and prostate cancer, why soy alleviates hot flashes and may beneficially affect renal function, alleviate depressive symptoms, and improve skin health. He also reviews the studies showing that isoflavones do not adversely affect the breast, thyroid, or uterus of post menopausal women. (Note: Soy, just like anti-acids and calcium supplements, may interview with thyroid hormone replacement therapies, but it does not affect or cause hypothyroidism).
Now, the question at hand:
Is Soy lowering your Testosterone. There have been a lot of poor posts and headlines of this in the past. Typically written by unqualified writers making unsubstantiated claims! I DO NOT claim to know everything, and I admit I only read about 20 articles out of hundreds posted on this topic! But I made sure those 20 articles were some of the most recent articles published in peer-review scientific journals. With science, anything can change with new findings, but with the evidence that is out there, I want to clear the air …
SOY does not Lower your testosterone.
A recent meta analysis from 2010, where 15 placebo-controlled treatment groups were looked at and 32 reports involving 36 treatment groups were assessed – There was no significant effect of soy protein or isoflavone intake on Testosterone, sex hormone-binding globulin, free testosterone, or the free androgen index.
For my non-science folks, a meta analysis is where they compile results from a bunch of individual studies to see what the results are on a larger scale. In Science, it is hard for any one study to hold weight on a topic, BUT if they compile the results of several studies, they can better understand the question at hand and it also will account for other confounding factors in each individual study.
Another cool study from 2000, showed that when they replaced meat protein with soybean, tofu, after adjusting for weight they were able to see a 8.8% increase in sex hormone-binding globulin and no change in serum testosterone and its metabolites, consistent with previous studies.
Now for men who are worried about their testosterone, their semen, or their fertility – don’t worry, it has been tested! There was a study published in Clinical Science were they gave healthy male volunteers a supplement of the same phytoestrogens found in soy and measured their semen quality, serum sex steroid and gonadotrophin levels, ejaculate volume, sperm concentration, total sperm count, motility, and morphology. The supplements were shown to increase serum genistein and daidzein levels but there was no effect on the endocrine measurements, testicular volume or any of the semen parameters!
As you can imagine, there are a few studies that do not fit this model! There are a host of papers that show that genistein and lower testosterone and others that show they synergistically raise testosterone levels! All of the ones I could find that show strong contrast to the studies above were done on mice. The most recent study to be published claims that genistein and daidzein together increase the sperm number, increase sperm motility, increase testosterone, and increase cholesterol levels. I am sticking with the meta-analysis that shows no effect. We have to remember that Soy intake and genistein have been shown to protect against prostate cancer, which for men, is a huge plus! I will make sure to do an additional post on this one day.
As always I love answering your questions! Below are a few of the articles I used to make this post.
Disclaimer: All opinions are my own. I do not provide medical advice. If you have questions please ask your doctor & do your own research. I am still a medical student and a PhD candidate. I have 5 degrees including 2 masters in nutrition & metabolism. I am a certified personal trainer & have researched these topics for 9 years. The information I post is research based & is information not always taught in school.