Soy and Manboobs??

Soy and Manboobs??

Hey friends,

This week, I want to answer a question I’ve been getting asked from some of the male readers – ‘Do soy products raise estrogen and cause manboobs?’

Ladies, feel free to keep reading if you’re curious about this topic!

What are manboobs?

As the word implies, manboobs are swollen breast tissue in males. The scientific term is ‘gynecomastia’. Manboobs form when there are significant alterations in hormones – specifically, either a decrease in testosterone or an increase in estrogen. The most likely causes are puberty, aging, various medication use, and health conditions that affect hormone levels.

What are soy products?

Soybeans have been used as a staple ingredient in Asian countries for centuries. The most common food products are tofu and miso. Soy has recently grown in popularity due to the claimed health benefits and increase in desire for plant-based diets. Now, we can regularly find things like soy protein products in America.

Soybeans have the highest isoflavone concentration of any food. Japanese adults consume an average of 30-50 mg isoflavone per day, while in America and Europe, we consume <3 mg / day. Soy protein products have minimal isoflavone content – it gets lost in production.

Isoflavones look very similar to estrogen to our bodies. They are able to bind the estrogen receptors and mimic the effects of estrogen. Hence, isoflavones are sometimes referred to as ‘phytoestrogens’.

What’s the problem?

There have been growing concerns that these phytoestrogens can feminize men by lowering sperm count, lowering testosterone, increasing estrogen, and causing gynecomastia (to name a few). One case study describes a 60-year-old man that developed gynecomastia as a (suspected) result of his soy intake. Once he quit the soy, his gynecomastia went away. It should be noted that he was drinking nearly a GALLON of soy milk per DAY.

Another case report describes a 19-year-old vegan who developed hypogonadism and erectile dysfunction as a (suspected) result of his soy intake. Again, it should be noted that he was consuming about 360 mg / day isoflavones (~9x the average amount Japanese adults are consuming).

But these individuals are anecdote and not science…

What’s the science say?

There are case reports of soy intake being associated with lower sperm counts in infertile men. There are also data showing lower testosterone post-workout when comparing soy to whey protein. Given the conflicting observations, a more rigorous look was warranted.

Several systematic reviews have been released showing no link between soy protein OR isoflavone consumption on levels of testosterone, estrogen, or anything resembling feminization. It makes sense that soy protein wouldn’t (remember, most of the isoflavones are lost during production), but it’s a bit interesting that isoflavone consumption also wasn’t associated with any of the variables looked at.

What about with large doses?

Great question.

The case reports that did report feminization in males had dosages far in excess of what a normal Japanese adult would consume.  Less than 10% of Japanese adults are consuming > 75 mg / day of isoflavones (the equivalent of about 3 servings of soy food). When looking at daily doses > 100 mg / day, we see that two studies report a marginal decrease: one reported a 5% decrease in total testosterone and the other reported a 6% decrease in free testosterone.

However, these studies were not the norm. The remainder of the studies looking at doses > 100 mg / day all showed no change in total/free testosterone levels.

One interesting study looked at doses of 900 mg / day isoflavone in prostate cancer patients. They found that only one participant had reduced levels of total testosterone, but five others had increased levels.

What about with long term consumption?

A meta-analysis found no differences in hormone levels between individuals consuming isoflavones for less than 12 weeks vs. greater than 12 weeks.

Final thoughts

There are only sparse case reports of feminization in men occurring as a result of soy or isoflavone intake (and those were in individuals consuming MEGA doses of isoflavone EVERY DAY). In contrast, scientific studies have failed to replicate those findings. The weight of the evidence suggests that soy and isoflavone intake in men, at doses in excess of what a typical adult Japanese male would consume and for longer durations, has no influence on sex hormone levels.

If you want to be cautious, feel free to have soy, just don’t eat more than 3 servings of soy in a day.

Thanks for reading this week. I know you have other asks for your time and I truly appreciate you taking a little time to read this. If you have any feedback or would just like to say ‘Hello’, feel free to shoot me an email at I read everything you send me. Seriously.

Have a great rest of your day!

P.S. Here is the reference to a meta-analysis on the topic:

Reed, K. E., Camargo, J., Hamilton-Reeves, J., Kurzer, M., & Messina, M. (2021). Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive toxicology (Elmsford, N.Y.)100, 60–67.