How Much Soy Can You Really Eat Before It Affects Your Testosterone?

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Soy is a great plant-based protein since it contains all of the essential amino acids, and there's been enough progress in the fitness world that you can find soy protein everywhere from smoothie shops to post-workout bars. Hell, even the FDA states you should eat some soy for potential heart health benefits. But soy also catches flack for what’s perceived as being a man boob–giving, estrogen-bestowing testosterone-killer. So what's the deal?

The soy and testosterone relationship

Studies suggesting that soy affects testosterone have fueled the frenzy around this topic, however, many of these studies are flawed — from lacking control groups to focusing on a small number of test subjects to neglecting to collect crucial data.

According to Jason Kovac, MD, PhD, of the Men’s Center in Indianapolis, Indiana, “Soy proteins contain significant amounts of the phytoestrogen isoflavonoids that change to estrogenic substances with potential hormonal properties.” But eating plant estrogens doesn’t mean you’re going to grow man boobs.

Kovac highlights the 2010 meta-analysis published in Fertility and Sterility as the best literature to consider, where researchers reviewed 15 placebo-controlled treatments and 32 reports. The results suggested that soy protein and isoflavones do not affect testosterone levels in men — regardless of age.

Soy may not affect everyone in the same way

Kovac asks his patients with abnormal hormone levels about how much soy they eat to make sure they’re not eating excessive amounts, but not for the reason you might think. “Since estradiol is converted from testosterone in body fat," Kovac says, "there is an explanation for higher levels of estradiol in men with higher body fat. If you are prone to high estrogen levels, eating a moderate amount of soy should decrease your overall estrogen levels, while too much can increase it."

The amount of soy per day that has been studied

We only know as much as what researchers studied and for how long. The amount of soy scrutinized in the Fertility and Sterility meta-analysis, up to 71 grams soy protein and 20 to 900 milligrams isoflavones per day for 10 weeks,was much higher than what the average Japanese adults eats, which averages 6 to 11 grams of soy protein and 25 to 50 milligrams of soy isoflavones per day.

The studies each used a specific type of soy, such as soymilk, tofu, soy grits, isolated soy protein (up to 56 grams), supplemental isoflavones, or a mix of traditional and processed soy.

How much soy you can eat

Since a varied diet is usually better for getting a balanced range of nutrients, not all of your protein should come from soy. Here are a few soy scenarios that fall within the ranges studied that could be included daily with other protein sources.

24 grams soy protein

  • 1 cup soymilk
  • 3 ounces tempeh

26 grams soy protein

  • 1 cup sweetened soy yogurt
  • 1/4 block extra-firm tofu
  • 1/2 cup shelled edamame

58 grams soy protein

  • 1 scoop soy protein powder
  • 1 soy protein bar

You can see that two to three servings per day keeps you within the range of soy studied. But if you’re downing soy 24/7, including highly processed foods that often contain hidden soy, you may be getting too much, according to some experts.

“Guys occasionally chomping on edamame can have at it, since it’s high in fiber and nutrient-rich," says Wendy Jo Peterson, MS, RDN, author of Mediterranean Diet Cookbook for Dummies and a nutritionist who coaches military men. While Peterson is a fan of whole soy, she is leery of isolated soy protein since it is so concentrated. “If I’m finding processed soy in a lot of their staple foods like cereals, breads, and salad dressings, I take note.”

Peterson refers to one small study without a control group that included young healthy males. The study found that two scoops of pure soy protein powder (56 grams) decreased blood testosterone levels by 19 percent after four weeks. T levels increased within two weeks of skipping the protein powder.

Peterson questions whether men under extreme stress are represented in soy studies. “There’s no reason to add more stress to an already stressful situation,” she says, but for guys that have low testosterone, “Adding the stress of whether or not soy is increasing testosterone is not worth it.” In cases of duress and low testosterone, she steers guys towards other proteins, while allowing for some whole soy.

The bottom line on soy and testosterone

“Men shouldn’t worry about consuming a regular diet of soy,” advises Kovac. Just get plant proteins from a variety of beans (including soybeans), lentils, quinoa, nuts, and seeds — even if you keep an active lifestyle.

“There is no data showing that soy negatively effects muscle growth in men," says Marie Spano, MS, RD, a board-certified sports dietitian for the Atlanta Falcons. Spano refers to a recent clinical trial that found 22 grams per day of a soy-dairy protein supplement for three months did not alter testosterone in young men doing resistance training. Muscle strength and thickness increased similarly to the whey protein and placebo groups.

All that said, if you're still worried about low testosterone, the cause may not be your diet. See a urologist or endocrinologist who specializes in men’s health and get your levels tested.

Michelle Dudash, RDN is a registered dietitian nutritionist, Cordon Bleu-certified chef, and cookbook author.