Is Soy Bad for You—or Is That Just a Random Health Rumor? RDNs Weigh In

If you follow a plant-based diet, soy is likely a go-to protein source for you. Many omnivores love to include soy-based foods, however, like edamame, soy sauce, soy milk, tempeh, tofu, tamari, and miso, in their snacks and meals. But there’s a cloud of confusion looming around this popular vegan option—rumors that it may contribute to conditions and diseases like cancer, infertility, hormonal imbalance, and impaired heart health. Is soy healthy—or is it actually bad for you? Time to set the record straight with evidence-based answers straight from current research and registered dietitians.

What Is Soy?

The soybean, commonly referred to simply as ‘soy’ or ‘soya’, is part of the legume family alongside other beans, peas, and lentils. This bean has a rich history, originating in Southeast Asia, where it was first domesticated in China in the 11th century. Products derived from the soybean, like miso and tofu, have been staples particularly in Asian cuisines ever since. 

However, soy and soy products’ popularity soon grew internationally, spreading to every corner of the globe. Today, you can commonly find a variety of soy and soy-based foods in most grocery stores throughout America today, from frozen soybeans (or edamame) in the pod, to various flavors of soy milk. 

Nutritional Benefits of Soy

When it comes to the health benefits and nutrient profile of soy, there’s a lot to love:

Macronutrients

• Protein: Soy is one of the few plant-based foods that is considered to be a ‘complete protein,’ meaning that it contains all nine essential amino acids, the building blocks of protein (a key component to most structures in the body). In one cup of cooked soybeans you’ll find an impressive 31 grams of protein.
• Unsaturated Fats: A big draw of plant-based proteins over animal-based ones is the heart-healthy unsaturated fats they contain. Of the 15 grams of fat found in one cup of cooked soybeans, the majority are poly- and monounsaturated fats, which help improve cholesterol levels.
• Fiber: Soy also contains complex carbohydrates in the form of fiber—10 grams per 1 cup, cooked! Fiber also helps boost heart health by lowering cholesterol levels, meanwhile supporting gut, metabolic, and immune health.

Micronutrients

• Vitamins: Soy provides a handful of vitamins including C, K, and B vitamins like thiamin, riboflavin, and folate. These combine to support immune, blood, and metabolic health.
• Minerals: You’ll also find zinc, iron, phosphorus, manganese, copper, and potassium in soy products. These minerals champion bone, blood, immune, and heart health.
• Plant Compounds: “Soy also has plenty of plant compounds that promote health, like isoflavones, which are antioxidants,” says Natalie P Carroll, MS, RDN, CDN CLC at Top Nutrition Coaching. The antioxidant quality of plant compounds reduces bodily inflammation and promotes optimal immune health.
  

Fermented Soy Products Offer Probiotics

Also, depending on the source, some soy options deliver probiotics, or healthy bacteria that fosters a thriving gut microbiome. Probiotic-rich soy options include tempeh, miso paste, and soy sauce—but tofu, edamame, and soy milk do not contain these healthy bacteria, since they're not fermented foods. A healthy, diverse gut microbiome is tied to better overall immune, gut, and brain health (though its influence reaches most body systems).

Is Soy Bad for You? Common Health Concerns, Debunked

Despite the nutrient density of soy, there’s a laundry list of potential health concerns often associated with it—and at first glance they can look understandably concerning. But are they really true, or just unfounded anxieties? Here’s what the research has to say about the most common complaints against soy.

Does Soy Affect Hormonal Health?

One fear surrounding soy is that it will negatively impact hormonal health in both men and women. This notion, thankfully, is not true. 

“Most concern here is around the phytoestrogens (soy isoflavones, found exogenously or from food) contained in soy and their ability to bind with estrogen receptors in the body,” Carroll explains. “The problem with that theory is these isoflavones are much weaker than our endogenous (or made in the body) estrogens—around 1,000 times weaker!” 

Carroll also says that phytoestrogens metabolize quickly and don’t accumulate in the body, another common misconception.

“This, however, doesn’t mean that everyone should supplement with soy protein isolate (which is found in protein powders, meat analogs, etc.), in large amounts daily—too much of a good thing is still too much,” she adds. Rather, it’s best to focus on consuming fewer highly processed foods (and drinks) and more whole food options (more on this later).

Does Consuming Soy Cause Cancer?

Possibly the most common and concerning myth surrounding soy is the claim it’s linked to cancer, particularly breast and prostate cancer. This is rooted in the concern that the phytoestrogens found in soy would contribute to these diagnoses. But there's no evidence to support this worry.

“Studies have shown that isoflavones (or phytoestrogens) do not increase circulating estrogen levels when eaten in normal and moderate amounts (one to two servings per day). It's also important to note that food sources of soy don't have large enough concentrations of isoflavones to impact breast cancer risk,” Carroll explains. 

A 2022 meta-analysis examining soy isoflavones’ relationship to breast cancer found isoflavone consumption to actually reduce the risk of breast cancer in both pre- and post-menopausal women.

In terms of prostate cancer, “soy intake has actually been found to decrease prostate specific antigen (PSA) levels, which is a marker for prostate tumor growth,” Carroll says. Many studies have corroborated this finding, including a 2018 meta analysis, 2019 review, and additional review from 2019.

Does Soy Affect Thyroid Health?

Current research does not support the claim that soy consumption negatively impacts thyroid health. A 2019 systematic review and meta analysis found that soy supplementation had no significant impact on thyroid hormones, with high consumption showing only modest increases in thyroid stimulating hormone (TSH). While further research is warranted, this impact on TSH levels was suspected by the research group to have minimal clinical significance.

Does Soy Affect Fertility?

Another concern is that soy may negatively impact both male and female fertility and reproductive health. This is also not true.

As a dietitian whose client base is primarily people looking to get pregnant, are pregnant or recently had a baby, Carroll is particularly well-versed in this topic. “We have studies that show soy intake being beneficial for those undergoing intrauterine insemination (IUI) and in vitro fertilization (IVF) treatment—indicating that dietary soy intake was positively related to the probability of having a live birth during infertility treatment with these assisted reproductive technologies,” she explains. “Plus, soy has little to no impact on our circulating hormones, and no significant impact on our menstrual cycle length—all of which would impact fertility.”

Men don’t need to fear soy either. “A meta analysis of 41 different studies indicated that there were no significant effects from soy protein or isoflavones on male reproductive hormone levels or production,” Carroll says.

Even further, soy may help to remediate any negative reproductive health harm (in the way of impaired hormone production and decreased fertility) done by the formerly popular plastic ingredient, bisphenol A (BPA). In fact, one study found that soy foods may actually offer protection against the adverse reproductive effects of BPA. “This, of course, should continue to be studied, but it is promising that normal intake of soy protein and isoflavones are a part of a healthy fertility diet,” she adds.

Is Soy Bad for Heart Health?

This is another false claim about soy. The unsaturated fats, fiber, and micronutrient content of soy all promote optimal heart health through encouraging healthy cholesterol, blood pressure, and heart rhythms. Research backs up these findings, with studies from both 2019 and 2020 finding inverse relationships between soy intake and heart disease.

Downsides to Soy Consumption to Consider

Allergies and Gastrointestinal (GI) Complaints

Two very legitimate concerns surrounding soy are related to allergies and GI troubles. Soybeans are one of the top eight major allergens as identified by the Food Allergen Labeling and Consumer Protection Act of 2004 (FALCPA). If you or a loved one has a soy allergy, it’s important to avoid it. Similarly, legumes of all kinds, including soy, can cause GI discomfort like gas, bloating, stomach pain, diarrhea, or constipation in some people.

Sustainability and Environmental Impact

When it comes to sustainability of soy, the discussion is a bit more nuanced. Mass production of commodity crops like soy and wheat have certainly contributed to deforestation globally, which produces large amounts of greenhouse gas emissions (GHGs) and biodiversity loss. Soy is pretty water intensive to grow, as well. However, 77 percent of the soy produced globally is used to feed livestock. And ultimately, research shows that eating 1 kilogram of tofu produces far fewer GHGs cumulatively than an equal amount of meat.

That said, some of the best ways you can purchase soy with a smaller carbon footprint is to opt for organically or regeneratively grown products and (again) to embrace moderation.

Healthiest Ways to Enjoy Soy

The best, most nutritious way to enjoy soy is to choose for soy foods in as close to their natural form as possible. “I prefer minimally processed soy products like tofu, tempeh, unsweetened soy milk, tamari, miso paste, soy sauce, and edamame over foods that are made with soy protein isolate and lots of added fats, sodium, and sugars,” Carroll says. “In fact, many of the meat analog products out there aren’t ‘healthier’ than their meat counterparts when you begin to look at the label and see the saturated fat, sugar and sodium contents.”

Tofu, tempeh, miso paste, tamari, and soy sauce are excellent in stir fries, soups, stews, marinades, noodle dishes, and more. Meanwhile, edamame is delicious sprinkled with salt and enjoyed right out of the shell, or in salads, salsas, grain bowls, and soups. Unsweetened soy milk can be used in virtually any recipe calling for cow’s milk, including baked goods, sauces, mac and cheese, biscuits, and more.

Enjoying soy in moderation can range from a few servings per week up to one to two servings per day, depending on your preferences.

The Bottom Line

Despite soy’s position at the center of several hot health debates, there is actually an overwhelming amount of evidence debunking the majority of these claims. Realistically, soy is incredibly nutrient dense , offering a wide array of macronutrients, micronutrients, and even probiotics in some cases. When enjoyed regularly in a balanced diet, these vital nutrients and compounds influence our health in so many important ways. As long as you’re not sensitive or allergic to soy, and you genuinely enjoy it, moderate consumption of soy-rich foods and drinks is an excellent addition to a healthy lifestyle.