Is Soy Bad For You?

When replacing meat and dairy products with soy alternatives, can there be too much of a good thing? We looked at the science behind soy.

Soy has become more widely adopted into some vegan and vegetarian diets. But, you also might’ve heard it can cause a plethora of problems including fatigue, digestive ills, weight gain, hormonal imbalances, and even cancer. So, is soy actually bad for you?

When you dig through the science on soy, it becomes clear that it isn’t (or shouldn’t be) public enemy number one and need not be avoided like cyanide. Instead, there are reasons why you want to welcome soy foods like tempeh and miso into your kitchen more often.

Soy in Our Diets

Be it tofu or textured vegetable protein, these foods hail from the soybean. Soy is consumed in a wide range of forms, such as tofu, tempeh, soy sauce, miso, and natto. As a major commodity crop, today you’ll find soy is in all kinds of foods that go well beyond tofu and soy milk. You’ll spot guises of soy including soy lecithin and soy protein isolate in energy bars, cereals, vegetable oils, faux cheese, ice cream, yogurt, hot dogs, and a myriad of other imitation animal products. The ubiquitousness of soy makes it hard to avoid eating it in one form or another.

The Benefits of Soy

When you dig into the nutrition numbers of lesser processed forms of soy foods, it’s pretty darn impressive. “From protein to fibre to vitamins, soy has a little bit of everything to help meet the nutritional needs of runners, says Lauren Antonucci, a board-certified sports dietitian based in New York. “In all the hoopla surrounding soy, it’s important to remember that it’s a legume, which is one of nature’s healthiest types of foods.”

Soy is one of greatest food sources of phytoestrogens (a.k.a. isoflavones) including genistein and diadzein, and it’s these compounds that make eating it good or bad for health depending on who you ask. But what is not up for debate is the antioxidant prowess of these compounds. One study showed that runners who consumed soy-derived isoflavones experienced an increase in their antioxidant defenses against the rigors of exercise. “The more you exercise, the greater the chance for a higher amount of oxidative damage to occur in your body, making antioxidants like isoflavones potentially useful to help combat the damage,” explains Antonucci. She adds that it’s always ideal to get your antioxidants from whole foods like soy as opposed to that from a pill.

Plus, a new report in the Journal of Nutrition found that women with higher intakes of soy foods as a whole and also soy isoflavones appear to be at a lower risk of developing type 2 diabetes. But for unknown reasons, this benefit was not found in male subjects. What’s more, there is some evidence to show that greater intakes of isoflavones from soy can help keep our bones strong and healthy, mainly by reducing bone resorption. But this impact seems to be greatest in menopausal women. It’s worth noting that the average daily intake of soy-derived phytoestrogens is considerably higher in Japan and China than on this side of the Pacific.

For vegetarians, soybeans are one of the few plant-based proteins that are considered complete, which means they have all the essential amino acids needed for bodily processes such as making muscle, explains Antonucci. “Runners have higher protein needs than the general public, so they will benefit from consuming more food sources of complete protein,” she adds. While it’s best to focus on eating less processed forms of soy, research shows that adding a scoop of soy protein powder to your post-run smoothie is a viable vegetarian option for bolstering muscle repair and growth. And doing so does not appear to nosedive testosterone levels.

And it’s worth noting that research is piling up to show that people who eat more plant-based protein like tofu at the expense of animal-based protein may live longer. Case in point: An investigation in the European Journal of Nutrition found that after three months, people who replaced 30 grams of animal protein daily with the same amount of protein from whole soy foods experienced a drop in body weight and blood triglyceride numbers, both of which can be protective against cardiovascular disease. Further, a Dutch study found that women who swapped out some of the animal protein in their diets with soy protein saw their cholesterol and insulin sensitivity improve. “When you eat more plant proteins you also get some nutritional benefits like an increased intake of fiber and certain micronutrients not found in animal protein,” notes Antonucci.

Noshing on soy can also directly do the heart some good. A meta-analysis of 17 previous studies conducted by researchers in China concluded that eating more soy foods is associated with a significantly lower risk of cardiovascular disease, stroke, and coronary heart disease (although, the impact of soy on heart health was stronger among individuals in Asian countries than those residing in Western countries). This discrepancy could be owed to the simple fact that, overall, people in countries like China typically have a much higher soy intake than those living in North America and Europe. Another report found that individuals who ate soy foods three or more times a week had a lower risk for all-cause mortality, including that from heart disease, than those who consumed less. There is also some evidence that a higher intake of soy isoflavones via soy protein can modestly reduce blood pressure numbers, particularly in people who already have hypertension.

While yes, soybeans contain a stew of beneficial compounds such as amino acids, fibre, isoflavones and lecithins that may work synergistically to improve metrics like cholesterol numbers associated with heart health, keep in mind that highly-processed forms of soy aren’t likely to have the same impact. For example, soy protein isolate—a protein that has been isolated from soybeans using chemical engineering and added to everything from veggie burgers to boxed cereal—won’t improve your heart health in the same way fresh edamame would.

To that point, a research review in the Journal of Clinical Lipidology found no evidence that the consumption of isolated soy isoflavones impacts blood levels of lipoprotein, a substance that appears to be an independent risk factor for cardiovascular disease. Citing “inconsistent findings” since the claim was authorized in 1999, the Food and Drug Administration has proposed to revoke food companies’ ability to use a label claim stating that there is a relationship between soy protein intake (a dose of 25 grams a day) and reduced risk of coronary heart disease—mainly via a reduction in cholesterol numbers.

Now, this does not suggest that soy protein can’t play a role in a heart-healthy diet, as research shows it indeed can, but just that on its own, it might not be as powerful as was once thought. “The overall nutritional profile of soy is still heart-healthy, which is why I will continue to recommend it as part of a diet designed to improve heart functioning,” states Antonucci. “Even though a bag of apples can’t make a heart health claim, nobody should question that they aren’t good for your heart.”

Additionally, whole forms of soy such as tempeh deliver oligosaccharides, a special type of carbohydrate that acts as a prebiotic to nurture the beneficial bacteria in your gut which may bring about positive improvements in immunity and digestive health. But these can come with gassy side-effects to the uninitiated, so if you’ve never typically eaten too much soy, it’s best to ease into this food group.

The Misunderstood Aspects of Soy

The phytoestrogens in soy have a structural similarity to estradiol, the main estrogen in both men and premenopausal women, so it’s not uncommon to come across advice that men should steer clear of soy foods or risk feminization.

The theory goes that soy’s phytoestrogens can inhibit the activity of enzymes involved in testosterone production, and thereby lower its levels while simultaneously increasing estrogen levels. But men should fret not; serving up a tofu stir-fry won’t deflate your muscles. Research has shown that men who consume soy in a variety of forms—foods, protein powders, isoflavone supplements—do not have a clinically significant impact on testosterone levels. Soy also doesn’t appear to negatively impact the testosterone-to-estrogen ratio in men or reduce fertility via lower sperm counts or erectile dysfunction. And contrary the preaching’s of some anti-soy bloggers, including soy foods in your diet appears to help—not hurt—in the battle against prostate cancer.

This isn’t to say that excessively high intakes of soy isoflavones won’t negatively impact hormone levels in males, but unless you eat tofu with a shovel and drink a tanker of soy milk, the amounts of soy consumed in a typical diet, say a serving or two a day, appear to be of little concern. As they say, it’s the dose that makes the poison.

Since phytoestrogens are chemically similar to estrogen and therefore can attach themselves to estrogen receptors and activate them, there is a perception that eating soy may play a role in hormone-influenced conditions in women such as breast cancer because estrogen may encourage the growth of certain breast cancer tumours. Yet, research suggests that consuming soy and its phytoestrogens won’t throw your hormones out of whack, only has a modest impact on estrogen and other hormone levels in women, and doesn’t have a worrisome impact on breast cancer risk.

It’s likely that soy phytoestrogens can’t activate estrogen receptors to the same degree as does real estrogen. In fact, previous research found eating soy food appears to lessen the risk for developing breast cancer, but only in Asian populations and not among women living in Western nations, which hints at the idea that for a benefit to occur, soy also needs to be consumed regularly during childhood. “Human studies linking soy intake and breast cancer occurrence just haven’t panned out, but this association continues to be blown out of proportion,” Antonucci says. Still, women deemed to have a higher risk of developing breast cancer like having a strong family history of the disease are best to eat whole soy foods and not use high amounts of pure isoflavone supplements.

Based on some animal and test tube studies, there have been rumors that making soy foods a dietary staple can negatively impact thyroid health by encouraging hypothyroidism. But the available research on humans does not seem to support that eating soy adversely affects thyroid functioning in people with healthy thyroids. For instance, a randomized controlled study, which followed 403 menopausal women for two years, reported that daily supplementation with 80 and 120 mg of soy isoflavones had no significant effect on thyroid hormone functioning. But with that said, there is a need for studies that are designed solely for investigating the effects of high amounts of soy food consumption on thyroid functioning among various demographics. “If you have a thyroid condition such as hypothyroidism or taking thyroid medication, it’s best to discuss your soy consumption with a physician or dietitian to determine if you need to moderate your intake,” advises Antonucci.

Soybeans also contain “anti-nutrients” such as phytates and tannins which are compounds that can impair the digestion and absorption of protein, vitamins, and minerals. But before you freak out and toss your veggie burger in the trash, you need to know that processing methods such as soaking and boiling soybeans, commonly used when making items like soy milk, greatly reduces levels of these anti-nutrients. Fermenting soy such as during the production of tempeh and miso also further removes some of these troubling compounds.

The Dangers of Soy

Not all soy is created equal. Antonucci cautions against eating too many highly processed forms of soy including soy protein bars and soy-infused veggie burgers with a laundry list of ingredients. “Sugar-sweetened soy yogurt is not a health food,” she says. A diet that includes high amounts of products made with processed forms of soy is likely an unbalanced diet that won’t help runners get all the nutrients they need.

And remember that soy sauce won’t give you much, if any, of the nutritional benefits of soy, but will almost certainly eat into your daily sodium allotment. People often report feeling better after cutting out soy from their diet because in doing so, they are eating fewer ultra-processed foods and not because they have taken a pass on edamame when going out for a sushi night.

Because it’s so cheap, soybean oil, made by extracting the fat from soybeans, is the grease of choice, either on its own or as part of a vegetable oil blend in restaurant kitchens and packaged processed foods. Soybean oil is especially high in omega-6 fat, which can be concerning. It’s not that omega-6 fat is unhealthy—in fact one type called linoleic acid is essential, which means you have to get some in your diet—it’s just that most Americans consume much higher levels of omega-6 fats in comparison to omega-3 fats. “This problem arises when people eat too much greasy fast-food and processed foods and not enough omega-3 rich foods like fatty fish and walnuts,” says Antonucci. A ratio skewed heavily towards omega-6 fats may promote unwanted weight gain and drive up inflammation in the body, which can place you at a greater risk of ailments like heart disease and may even reduce recovery from hard runs. So it’s okay to include some soybean oil in your diet (it has a fairly high smoke point making it a good option for searing steak), but only as long as you balance things out by also eating plenty of omega-3 heavyweights.

In terms of concern about eating soy foods because they might be made from genetically modified soybeans, look for certified organic products, as they must be made from non-GMO beans.

The Bottom Line:

Yes, you can go ahead and eat soy daily and feel good about it. Just be sure that you’re consuming an appropriate amount—about three servings—of lesser processed soy foods. Some forms of soy like these below are more nutritious than others, so here’s a quick rundown.

Soy Nuts

These are dried and toasted soybeans and have even more protein and fiber than typical nuts like almonds. They’re also a leading source of isoflavones. Snack on them by the handful or toss onto salads for some satisfying crunch.


Made from soybeans that are soaked, cooked, slightly fermented and then formed into a firm patty, meaty tempeh is like tofu on nutritional steroids. Try it grilled like steak or crumble and use in pasta sauce, chili, tacos or stuffed potatoes.


Packing an umami-punch, pasty miso is made by combining cooked soybeans with salt, koji (a starter enzyme that breaks down proteins), and rice or barley. It’s home to gut-friendly bugs. Whisk it into salad dressings, a glaze for fish, mashed potatoes, and dips.


Made from curdling soy milk, tofu is a versatile meat protein alternative. Brands that list calcium sulfate as an ingredient contain more of this bone-friendly mineral. Use firm versions in stir-fries and kebabs and blend silky soft tofu into smoothies, dips, and salad dressings.


These nutrient-dense green immature soybeans are high in plant protein, fiber, vitamins and minerals. Boil up a bunch of frozen shelled edamame for an ultra-healthy snack or add them to soups and salads.

Soy Milk

Some of the nutrients are lost during processing, but most brands are fortified with vitamin B12 and vitamin D. Plus, it’s one of the very few plant-based “milks” to have protein numbers that approach what is found in the moo variety. To side-step added sugars, choose cartons labeled “unsweetened.”

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