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It’s often said that there are two topics you don’t bring up at family dinners: religion and politics. But if you’re a nutrition-minded person, there’s another thing you just don’t talk about: soy. The little bean has stirred many a heated debate since it began being cultivated widely as a food crop in the United States in the 1940s. And with its long, meandering history in and out of health halos and GMO threats, it’s no wonder we’re all so riled up.
Medical studies in the early 1960s determined that soy was an excellent source of protein and could make a good meat replacement for vegetarians and omnivores alike. By the 1990s, research showed soy might be the answer to all our health problems—a potential cure for obesity, heart disease, and , among other things. The logic seemed obvious: Asian populations are less obese, have lower rates of heart disease, and report than people in the United States. And what do Asian populations eat more than just about anyone in the US? Soy, of course.
But when scientists took to their labs to corroborate the theory, their studies could not clearly establish soy as the health boon we all thought it was. In fact, they started to worry soy might actually harm our health, that the high concentrations of estrogen-like compounds genistein and daidzin—collectively known as isoflavones—found in soy could actually rev cancer cells and cause them to spread faster.
Researchers turned up reason to believe those plant-based estrogens, or phytoestrogens, caused other problems as well, including impaired fertility in women. Yet other studies at the time continued to extol the benefits of soy, suggesting that the little bean could counteract menopause symptoms, improve asthma, and lower cholesterol. Hence the simultaneous barrage of warnings and hype we’ve gotten about soy consumption. (Beat menopausal weight gain naturally with !)
The best we can do, of course, is look to recent research for answers to these conundrums. Here’s what the science says now about the 5 most common soy confusions:
False: Soy Causes Breast Cancer To Spread
Although scientists generally agree that soy won’t cause cancer where it doesn’t exist, some studies have wondered if the phytoestrogens in soy energize pre-existing cancer cells and cause them to spread faster. One of the initial studies to spark this fear was a 1987 claiming a diet of 20% soy protein “enhanced” prostate cancer. Another study reported that “moderate” soy consumption activated certain genes that could potentially cause cancer cells to spread in some women with breast cancer.
Bottom Line: “Research is constantly evolving and the idea that soy could be harmful for cancer came from animal studies and lab studies that tested estrogen-like compounds in soy,” says Marjorie L. McCullough, strategic director of nutritional epidemiology for the . “Studies in women have suggested soy could possibly be beneficial.” Even if you’re worried about cancer, you can feel fine about a couple servings a day of soy foods like tofu, edamame, and miso.
False: Soy Makes Men Grow Breasts
In 2008, a retired Army vet walked into a doctor’s office with enlarged breasts, low sexual desire, and erectile dysfunction and after extensive testing, was given an order to drink less soymilk. After doctors discovered the man had estrogen levels eight times higher than normal, a result of a recent switch to soymilk because of lactose intolerance.
But it wasn’t just a splash of soymilk added to that caused his breasts to puff up: The man admitted to drinking three quarts of soymilk every day. That’s 12 servings of soy—way more than the American Institute of Cancer Research’s recommended 1 to 2 daily servings.
So the problem for the Army vet may not have been that he was drinking soymilk, but that he was eating too much of one particular thing. And later studies into soy’s effect on men confirm this.
Bottom Line: “A healthy diet has a variety of healthy foods in it,” says , founder of Yale University’s Prevention Research Center. And soy can absolutely be one of those foods. “We have soymilk in the Katz family refrigerator.”
True: Soy Worsens Underactive Thyroid
In the early 2000s, two FDA officials wrote a letter in protest of the FDA’s promotion of the health benefits of soy. One of their arguments? Isoflavones in soy inhibit thyroid activity, potentially causing goiters and other thyroid abnormalities. Further studies have claimed soy can make an already dysfunctional thyroid even worse.
One in babies with hypothyroid disease showed that soy-based formula interfered with their medication and caused their thyroid to lose even more function. Another in adults with pre-established thyroid problems showed that a diet typical of vegetarians (30 grams of soy per day) caused “a 3-fold increased risk of developing overt hypothyroidism.”
Bottom Line: Soy can in fact worsen problems with thyroid function in anyone who already has hypothyroidism. Anyone with the condition should wait four hours after taking medication to eat anything with soy—and keep it to moderate amounts.
True: Processed Soy May Be Bad In Large Quantities
Everyone, regardless of thyroid function, should try to avoid highly concentrated sources of soy like supplements, pills, powders, even soy-based protein bars, according to Katherine McManus, director of nutrition at in Boston.
In addition to the unpronounceable ingredients that typically make up the bulk of those products, (supplements, in particular, aren’t regulated by the FDA so you never know exactly how much of a pill is soy and how much is something else) taking supplements or eating high concentrations of soy could boost your soy intake way past the daily recommendation.
Other than the wicked high estrogen levels and swollen breasts seen in the man who overdid it on soymilk, ultra-high levels of soy have also been linked with dysfunctional ovaries.
In a examining the effects of soy on ovaries, researchers found that consuming more than 100 milligrams of soy isoflavones a day can lead to reduced ovarian function. Yet, moderate soy consumption had no effect on ovaries.
“We were never intended to eat three soy-based energy bars a day and a soy smoothie and a soy veggie patty … and, and, and,” says , RD, author of The SuperFoodsRx Diet and Prevention’s nutrition advisor. “I think we took the message that soy is good for your health too far.”
That message, along with how cheap it is to manufacture soy, led to soy derivatives like soy protein isolate, soybean oil, and soy lecithin being used in a majority of packaged foods. Because of its prevalence in convenience foods, most Americans aren’t even aware of how much soy they actually eat.
Bottom Line: McManus and other nutritionists agree that the best way to eat soy is in its least processed forms: miso, edamame, tofu (check out these ), tempeh, or unsweetened soymilk. They say it’s best to be wary of other forms of soy—such as textured soy veggie patties and soy protein powders—and only reach for those with the . In some cases, other ingredients matter more than whether or not a product contains soy. “We in the west tend to hyper process everything,” says Katz. “So if you make a soy ice cream with 50 ingredients on the list, sure soy is one ingredient, but what’s more problematic is the other 49 ingredients which could include a lot of added sugar.”
True, Kind Of: Soy Cuts Heart Disease Risk
File this under “confusing:” even while soy was demonized for spreading cancer, feminizing men, and hurting your thyroid, it was praised for healing your heart. Early claimed the bean lowered cholesterol, blood pressure, and overall congenital heart disease.
Yet later says eating soy may actually have has little direct impact on your heart. But, and it’s a big but: Although the direct heart-related benefits of soy are likely small, many nutritionists believe the benefit is more about what soy replaces than eating soy itself.
“Sometimes the heart’s health is not about directly influencing heart health but the effect of shifting in something that’s better for your heart than what you usually eat,” says Bazilian. In most cases, she says, someone is replacing red meat with tofu, tempeh, or edamame, which are low in saturated fat and high in fiber.
Bottom Line: Soy isn’t going to significantly reduce your heart disease risk on its own, but can be a good substitution for saturated-fat-loaded foods.