Soy and Testosterone: Harmful to Men or Just a Myth?

Soy Protein Powder and soy foods in general have  been a serious hot button amongst the health and fitness community. While research has shown it’s an all-around effective source of protein for building muscle, its effects are on men’s hormones are unclear. The term “soy boy” has even begun to be used as an insult, as some people believe a diet high in soy renders men effeminate.

So what is the verdict on soy in healthy men? I previously wrote an article discussing how phytoestrogens might negatively affect men’s health, with flax and soy being the two food items containing the highest amounts. However, emerging studies have challenged this idea, claiming that it may not be as unhealthy as we once thought. Other studies even claim that it depends on the person, making things even more confusing.

Another major issue with nutrition studies is they are all to some degree flawed, simply because it’s difficult to monitor all variables affecting human test subjects. Things like stress, lack of sleep, dishonest recording of daily meals, and additional supplements test subjects may be taking, which could interact with the study can result in skewed results. On the other hand, a researcher with an agenda or bias could publish results favorable to his/her theory, further clouding the truth. The term “fake news” in the media has illuminated what can occur when someone with an axe to grind publishes work that’s 60% fact and 40% unverified claims. The world of scientific research is no different.

In this post, I’ll discuss some of the newly emerging studies and attempt to shed more light on this issue.


Regular intake of soy foods has been believed to have feminizing effects in men due to estrogen-like molecules found in soybeans called isoflavones. Isoflavones are a class of phytoestrogens—plant derived compounds with estrogenic activity—and soy beans/soy products have been found to contain the highest amounts of them. However, claims that high soy diets can positively or negatively affect male reproductive health are often based off of animal studies, which can cause problems since the human body’s metabolic functions aren’t identical to lab mice.

Soy and Heart Disease

A 1998 study on the relationship of soy and heart disease is a perfect example of this. The study suggested that soy protein could lower blood cholesterol concentrations in animals, but when applied to human studies the results were mixed. A 2006 study showed that mice who were fed a high soy protein diet developed heart disease, and only when they were switched back to a casein protein source did their condition improve.  

Most recently, the FDA released a statement saying they would be revoking their claim that soy protein reduces the risk of heart disease.

“While some evidence continues to suggest a relationship between soy protein and a reduced risk of heart disease – including evidence reviewed by the FDA when the claim was authorized – the totality of currently available scientific evidence calls into question the certainty of this relationship. For example, some studies, published after the FDA authorized the health claim, show inconsistent findings concerning the ability of soy protein to lower heart-damaging low-density lipoprotein (LDL) cholesterol. Our review of that evidence has led us to conclude that the relationship between soy protein and heart disease does not meet the rigorous standard for an FDA-authorized health claim.”

-Dr. Mayne, Director of the FDA’s Center for Food and Safety and Applied Nutrition

Soy, Estrogen, and Testosterone

OK, so the long-standing claim of consuming soy for its heart healthy benefits is as of now off the table, but what about its effects on hormones?  Soy isoflavones are known to have weak estrogenic or hormone-like activity due to their structural similarity to 17- β-estradiol.



Estrogen, like most hormones, work by binding to specific signal receptors on the surface of cells. Estrogen receptors can be found in tissues associated with the reproductive system, liver, heart, and brain. One interesting fact is that testosterone can actually be converted to estradiol, a form of estrogen by way of an enzyme called aromatase. Managing this enzyme is key to maintaining optimal testosterone in men, as we’ll see below.

The body is always striving to maintain balance or homeostasis and it’s important to understand that having too little estrogen in the body can be just as bad as having too much. When large amounts of testosterone are present in a person say, after they’ve taken a cycle of testosterone, the body will respond by turning some of it to estrogen. The problem is once a person stops adding testosterone, the body will still be producing more estrogen than testosterone, which can result in side effects like erectile dysfunction, loss of libido, breast tissue enlargement, fatigue, and mood swings/depression. Additionally, consumption of exogenous forms of testosterone post-testosterone cycle can shut down the body’s natural production of testosterone, further leading to complications.This is why whenever a person is coming off a cycle of testosterone they are given what’s known as an “aromatase inhibitor” to avoid the excessive production of estradiol.

So how does this play into diet? Let’s say you’re a young healthy guy not taking any test-boosters or injections: what can a diet of soy do to your body? A team of researchers at Harvard University analyzed the semen of 99 men and compared it against their soy and isoflavone intake during the three previous months. What they discovered was that both isoflavone and soy intake were associated with a reduction in sperm count. Men in the highest intake category of soy foods had, on average, 41 million sperm per milliliter fewer than men who did not eat soy foods. Scary stuff.

Could soy actually be rendering the world’s men infertile  as constantly reported by major news outlets? Are we slowly becoming like Alfonso Cuarón’s film Children of Men?

End of the world?

Not so fast; As with most things the answer is not so clear cut.

A study conducted by scientists from the University of Guelph had 32 men eat low or high levels of isoflavones from soy protein for 57 days and found that it didn’t affect semen quality. These were men whose ages ranged from 22-32 years old and had normal body mass indexes (25.4 ± 3.14 kg/m(2)). One major difference between the two studies is the Guelph study used healthy men, while the Harvard study used subjects they defined as “sub fertile”, leaving you to wonder which came first, the chicken or the egg?

Gut Bacteria & Soy

One alternative explanation comes from emerging research that shows a person’s reaction to soy isoflavones and the production of equol, a metabolite of isoflavones, can vary based on the individual's gut bacteria. Produced by only 30% of the Western population, compared to 60% of Japanese, Korean, or Chinese populations, equol is believed to play an important role in converting phytoestrogens into all the heart healthy benefits that we keep hearing about. This is an important distinction, because experts will often point to these Eastern countries as examples of large populations benefiting from soy consumption. The truth may lie in the fact that Westerners are simply not equipped to properly digest/metabolize isoflavones.


Unless your body can produce equol, you’re most likely not going to benefit from a soy diet and may even be negatively affected by consuming it. A rough comparison of this situation could is lactose intolerance: A lactose intolerant person cannot become tolerant by consuming more lactose.  Similarly, consuming more soy won’t turn your body into an equol-producer.

The Verdict

It appears that the jury is still out on the benefits of soy for men. To be on the safe side, avoiding soy/soy protein is the best option in terms of keeping your testosterone levels optimal until further studies can clearly prove its efficacy. A 2002 study summarized it best:

“The clinical effectiveness of soy protein in cardiovascular, bone and menopausal health may be a function of the ability to bio transform soy isoflavones to the more potent estrogenic isoflavone, equol. The failure to distinguish those subjects who are "equol-producers" from "nonequol producers" in previous clinical studies could plausibly explain the variance in reported data on the health benefits of soy.”


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