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What Most Men Get Wrong About Soy

The science is clear: Soy is safe (and btw, it doesn't cause man boobs)

By Jessica Migala
Fact checked by Joy Ferguson
April 25, 2023

Fast Facts

Rumors persist that soy can negatively impact men’s health.
The only studies that show a negative outcome were done on people and animals who were consuming almost all of their calories from soy.
Not only is eating soy safe for your hormones, soy is good for your hear, builds muscle, and decreases your prostate cancer risk.

Tossing a hunk of tofu on the grill doesn’t have the same panache as a rib-eye, but that’s not the reason most men looking for a protein fix steer clear of soy. Though soy was initially heralded as a promising superfood, the food has been plagued by rumors that it can feminize you and lead to issues like man boobs and erectile dysfunction.

Soy’s bad publicity stemmed from two case studies that suggested excessive amounts of the protein from soybeans appeared to impact hormones.

In those reports, animals and people who were fed stupid amounts of soy developed issues like gynecomastia (man boobs) and erectile dysfunction (1). Toss in some fears that environmental estrogens contributed to men’s declining sperm count and soy was officially protein non grata.

But is soy bad for men? Should you shun soy protein powders and soy milk? Here’s what the science says.

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What is Soy?

Soybeans are a part of the legume family. Foods like tofu, tempeh, edamame, miso soup, and soy milk are made from soy, a protein isolated from soybeans.

Soy protein powder, as well as processed products like some meat substitutes and protein bars also include soy protein isolates, a dietary protein that’s been isolated from soybeans.

What Does Soy Do in the Body?

Soy contains isoflavones, which are phytoestrogens: plant-based compounds that have a similar structure to estrogen.

“Phytoestrogens bind to some of the same receptors in the body that our own estrogen does,” says Vancouver-based Matthew Nagra, ND, who specializes in plant-based and vegan nutrition.

“But,” he stresses, “They do not function in the same way as estrogen as they are 100-1000 times weaker than our own estrogens.”

After you eat soy, the isoflavones are metabolized and then may weakly have effects like estrogen in your body, according to the Centers for Disease Control and Prevention.

The key word here is weakly. Extremely weakly. Isoflavones have just a tiny fraction of the strength of estrogen, says Nagra. The puny estrogen mimics simply aren’t strong enough to meaningfully affect your hormones as long as you’re eating them in normal amounts—which can include multiple servings each day, he explains.

Is Soy Bad for Males?

Nagra stresses that just two case studies suggested that consuming excessive amounts of soy—to the point where calories came almost entirely from soy—may have impacted hormones and led to problems, such as erectile dysfunction. But before you swear off soy, it’s worth digging into the details.

In one case study (2), a 19-year-old developed ED and saw his testosterone levels and libido drop after consuming the equivalent of 360 mg of isoflavones per day in the form of soy milk, soy crisps, tofu, soy sauce, soy nuts, and soybeans. In another, a 60-year-old man who came to the doctor for breast growth reported drinking almost 100 ounces of soy milk per day (3).

That’s at least 10 times more than people eat on average, even in cultures that consume a lot of soy. For example, in Japan, older adults consume 30 to 50 mg per day of isoflavones, while in the U.S., it’s only a few milligrams per day, shows research (1).

In other words: Eating a healthy diet that includes soy won’t get you anywhere near the level of these men.

Does Soy Impact Testosterone?

There’s no indication that tossing tofu on your stir-fry or sharing a bowl of edamame will negatively affect your hormones. “There have been dozens of randomized controlled trials and meta-analyses showing that consuming soy or soy phytoestrogens does not have a clinically relevant impact on testosterone levels,” says Nagra.

The receipts: A 2022 review of 417 observational studies, clinical studies, and systematic reviews and meta-analyses in Critical Reviews in Food Science and Nutrition concluded that isoflavone intake did not impact levels of testosterone, estrogen, sperm, or semen in men (1).

An earlier 2021 meta-analyses of 38 clinical studies, published in Reproductive Toxicology (4), found that soy and its isoflavones also did not alter levels of total testosterone, free testosterone, or estrogens in men—even when eaten long-term at levels that exceed the amounts typically consumed in Asian cultures. The authors point out that their research affirmed the conclusion of a 2010 meta-analysis (5) and that “it’s unlikely that additional research will alter this conclusion.”

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Is Soy Good for Your Heart?

Not only is soy hormone-friendly, but it also provides a good source of heart-healthy unsaturated fat and fiber, as well as iron, calcium, zinc, and B vitamins, research points out (6). Soy foods also provide a lean source of protein. It may be better to consume whole sources of soy like edamame, rather than soy ultra-processed foods since these can be high in sodium.

Consuming 25 grams a day of soy (about an ounce) has been found, in some studies, to decrease total and “bad” LDL cholesterol, making it a heart-healthy food, according to a meta-analysis published in the Journal of the American Heart Association in 2019 (7).

Does Soy Impact Your Cancer Risk?

Prostate cancer is the most common cancer in men in the U.S., except for skin cancers, according to the American Cancer Society. A meta-analysis of 30 observational studies found that soy consumption is associated with a decreased likelihood of developing prostate cancer (8).

One theory why: Prostate tissues contain certain estrogen receptors, and when phytoestrogens bind to and activate these receptors, it may, in part, reduce cell division to inhibit cancer development.

Soy Builds Muscle

One of the best ways you can eat high-protein soy is as a replacement for animal proteins that can be higher in saturated fat (such as red meat)—that’s one reason why soy is considered a heart-healthy food. So, marinate and sauté tofu as your main protein or toss a handful of edamame onto a salad.

Soy can also help you build muscle. “Research shows (9) that soy protein increases muscle mass and strength just as well as whey protein, which has long been considered the top-quality protein in the bodybuilding space,” says Dr. Nagra.

How Much Soy is Too Much?

Research shows that up to 100 mg of isoflavones (the equivalent to about four servings of whole soy foods per day) is safe (1).

More than that may be as well, but when you consume a lot of one food, it leaves little room for others. Problems arise when soy (or other foods) becomes the dominant food in your diet. “At that point, there’s potential that you’ll miss out on other nutrients that you need in your diet,” says Nagra. Eat a variety of foods and protein sources.

The Bottom Line

Research clearly suggests that soy is a healthy food—and concerns can be put to rest.

Want to make soy part of your diet? Start by replacing meat with tofu or regular milk with soy milk a couple times per week. Experiment with different soy foods and their preparations to see what you like.

FAQs

Does soy cause man boobs?

No. There is no data to suggest that soy, when consumed in normal (aka not excessive amounts) will cause excess breast tissue to grow (10).

Does soy lower sperm count?

No. Large meta-analyses (4) found that soy did not affect sperm or semen in men.

Does soy cause erectile dysfunction?

No. While there are worries that soy may affect testosterone, research has found that soy consumption did not decrease reproductive hormone levels (4).

How much soy is bad for men?

Consume up to four servings a day of soy. Research shows (1) that it’s safe to consume no more than 100 mg of isoflavones per day. Soyfoods contain different amounts of isoflavones, and may vary with the brand, but in general, one serving of soy (8-oz soymilk, 3.5-oz tofu, 1-oz soy nuts) offers around 25 mg of isoflavones.