Depending on what you eat everyday, soy-based foods like tofu, soy milk, miso, tempeh, and edamame may sound like classic "health" foods. But for vegetarians, vegans, and other dieters who have come to rely on this common meat alternative in their diets, grocery store items rich in soy have developed scary reputations for a purported "disease risk." Some previously published research can be downright scary, with claims that increased soy can mess with your hormones, the thyroid, and possibly cause cancer.
So which side of this debate is actually right — does soy deserve that health halo, or should you swear the stuff off of your shopping list for good?
As is often the case when it comes to nutrition, the answers aren’t black and white. But for the most part, "Soy-based foods are some of the best foods you can eat on the planet," says Jaclyn London, MS, RD, CDN. "Soybeans provide a plant-based protein source; a slew of vitamins and minerals crucial for reducing risk of chronic disease; and fiber that helps you fill up and feel satisfied."
While some small, poorly designed studies have led to inflammatory headlines over the years, it’s important to think about all foods in context. Eating plant-based foods in their closest-to-nature (a.k.a. least processed) form? Super nutritious. But taking supplements made with the compounds in soybean? Not so much.
"That’s where we’ve seen health risks," London explains. "In fact, it’s not uncommon to see research reflecting consuming compounds in supplement form rather than eating the foods themselves." Those supplements are linked to increased disease risk, while real, whole foods are linked to decreased disease risk.
Why Soy Is Controversial
Let’s take a trip back to the 1990s, when soy foods first started really hitting it big. At the time, many experts believed that soy had the power to fight problems like obesity, heart disease, and even cancer. After all, people in Asia eat a ton of soy. And studies showed that these populations had significantly lower rates of obesity, heart disease, and breast cancer compared to people in the U.S. Clearly, soy was the miracle food, right?
Not necessarily. Those studies only looked at associations, not causation. Just because people who consume a lot of soy also happen to be healthier than people who don’t eat soy doesn’t automatically mean that soy is the key to their superior state. Countless other factors — from genetics, to lifestyle, to the rest of their diet — could also play a role.
When researchers began taking a closer look to find out what made soy so healthy, they ran into some surprises. Soy, it turned out, contains estrogen-like compounds called isoflavones. And some findings suggested that these compounds could promote the growth of some cancer cells, impair female fertility, and mess with thyroid function. Some health experts also trash-talk soy because of its potential to be an endocrine disrupter — meaning it can mimic estrogen in the body, which may lead to a hormone overload.
At the same time, other studies were still showing that soy consumption could cure high cholesterol and help women cope with the symptoms of menopause. And Dawn Jackson Blatner, RDN, a Chicago-based dietitian, says that while whole soy does contain natural plant estrogens, they're much weaker than actual human hormones, and shouldn't case you worry. Add it all up, and you can see how this little green bean became a source of mass dietary confusion.
What We Know Today
As with all foods, experts still don’t know everything there is to know about soy. But research in recent years suggests that moderate consumption of minimally processed soy foods (more on what those are later) not only isn’t bad for you, it probably has some benefits. Here’s what we can say about soy today:
Soy may decrease your risk of certain cancers, among other chronic diseases.
How did soy even get linked to cancer risk in the first place? Stephanie Clarke, RDN, a dietitian based in Washington, D.C., says it has to do with processed grocery products. Soy protein isolates, a highly processed form of soy used in cereals, protein bars, and snacks (among other foods), may contain more soy isoflavones, which are organic compounds that can also be considered endocrine disruptors in high amounts. Elevated levels of this kind of soy may lead to unbalanced hormone levels, which can play a factor in cancer risk.
The majority of recent, high-quality studies, however, have found that unprocessed soy doesn’t increase breast cancer risk, and very high consumption could even offer some protection.
Eating soy could help protect against other types of cancer, too. Findings show that soy consumption may slightly lower the risk for gastrointestinal cancers and have a protective effect in prostate cancer survivors. Eating a high-fiber diet is also tied to lower colon cancer rates, and soy foods like edamame and tempeh both have plenty of roughage.
The only instance in which you may wish to limit soy consumption? If you've previously been diagnosed with estrogen receptor-positive breast cancer, Clarke says. Your doctor may advise that it's best to skip soy altogether if estrogen is at play in this case.
Soy might improve fertility and help with hot flashes.
Soy appears to be beneficial for fertility, as long as you don’t eat too much. Women undergoing in vitro fertilization who have environmental exposure to BPA are more likely to get pregnant if they also ate soy. That’s likely because soy’s isoflavones help neutralize the BPA’s endocrine-disrupting effects, researchers say.
Just don’t go overboard. Consuming over 100mg of soy isoflavones (the equivalent of 6-ounces uncooked tempeh or 16 cups soy milk) daily was linked to reduced ovarian function, found a Journal of Nutrition review. But moderate soy consumption didn’t pose a problem.
As for soy solving hot flash problems? It might help, but not for everyone. Among women whose bodies produce the soy metabolite equol, those who ate the most soy experienced significantly fewer hot flashes and night sweats compared to those who ate the least, found one Menopause study. (Between 20% and 50% of North American and European women produce equol. Some research centers can test for it in a urine sample, but there’s an easier option: Try adding soy to your diet for four to six weeks and see what happens. If it helps, you produce equol. If it doesn’t, you probably don’t, the study authors say.)
Eating soy in place of meat will probably protect your heart.
Early research suggested that soy could help lower levels of bad cholesterol. But more recent findings have shown that might not be the case, and in 2008, the American Heart Association said that there wasn’t enough evidence to say for sure that soy lowered the risk of heart disease.
Still, it’s safe to assume that soy has some benefits going for it. In general, replacing animal foods with plant foods like soy lowers saturated fat intake and ups fiber intake, both of which are help your heart. In other words, swapping that steak out for tofu or tempeh is a heart-smart move. But having steak followed by a bowl of soy ice cream for dessert probably won’t be as helpful.
You should pay more attention to your soy intake if you have thyroid issues.
Soy foods don’t affect thyroid function in people with healthy thyroids, found a Loma Linda University review of 14 studies. But if you have an underactive thyroid, you might want to watch how much soy you eat. Soy foods have been shown to interfere with the body’s absorption of thyroid medication — but only if you overdo it, suggests a 2016 Nutrients review. The evidence is still far from conclusive, but experts still advise to wait at least four hours after consuming soy to take your thyroid medicine.
The Best (and Worst) Types of Soy to Eat
All of soy’s potential benefits come with an important caveat: To reap them, you need to pick minimally processed forms of soy — think tempeh, tofu, miso, and edamame, all three experts say.
These foods serve up soy’s entire nutritional package without added sugar, unhealthy fats, sodium, or preservatives that you usually find in highly processed foods.
Soy frankenfoods like meat analogs, soy bars, soy yogurts, or protein powders usually only contain soy protein isolates, rather than nutrition from the whole soybean. "Just as other processed foods are lower in nutrient density, removing the protein from the other enzymes and bacteria needed for digestion affects the nutritional quality," says Dr. Taz Bhatia, MD, integrative health expert and author of What Doctors Eat.
As for how often you should eat soy? As with all foods, moderation is the way to go. Generally, three to five servings of minimally processed soy foods per week are perfectly fine, Bhatia says. If you’re unsure, or you have an underlying health condition (like hypothyroidism), bring it up with your doctor the next time you discuss your diet.
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