Once and for All, Is Soy Good or Bad for You?

soy: good or bad
Oren Aks/Thrillist

There’s a lot of misinformation circulating about soy products. Like, who started the rumor that tofu takes on the flavor of whatever sauce it’s served with? IT DOESN’T. It just tastes like tofu.

Not that tofu is a bad taste. And in any case, flavor is really the least of your concerns when it comes to soy products. Heralded as an inexpensive, nutrient-dense, plant-based source of protein, only to be dismissed by the Paleo crowd, among others, as a cancer-causing, thyroid-wrecking, baby-killing malfeasant, soy remains one of the world’s most controversial health foods. So should it be part of your diet? We wrangled doctors, dietitians, and a chef to find out.

Will soy give you breast cancer?

Soy contains phytoestrogens, or plant estrogens, which can mimic the effect of estrogen in the body. And estrogen can cause some tumors to grow. Because of that, a lot of people thought soy could actually cause cancer or accelerate its growth. Is that true?

Nope. In fact, the opposite is true. A study of more than 5,000 Chinese breast cancer survivors who ate soy protein were less likely to see their cancer return than those who did not.

“Soy, in its natural state, in healthy doses -- 25 grams or so per day-- has shown healthful benefits, reducing risks of certain cancers (colon), helping with hot flashes,” says registered dietitian Ashvini Mashru. “It has been shown to slow the progression of prostate cancer and may reduce the risk of breast cancer in Asian countries, but not Western countries.”

This discrepancy between Asian and Western countries’ breast cancer risk may be due to a variety of different lifestyle choices, including the types of soy products people eat (fermented vs. unfermented products, for example). More on that later.

Will soy make me girly if I'm a man?

Before you text your bros, “THE GF DRAGGED ME OUT WITH HER FRIENDS. WE’RE SINGING KARAOKE AND EATING EDAMAME. TOO MUCH ESTROGEN AMIRITE???” stop and think for a minute. Phytoestrogens have no effect on testosterone levels. Not only that, they also might reduce your risk of prostate cancer. The only thing you’re in danger of is getting booed off the karaoke stage, but you and your newly cancer-resistant balls can handle that, right?

Will soy wreck my thyroid?

If you have hypothyroidism and you chase your thyroid replacement with a tall glass of soy milk, yeah, that could interfere with your body’s ability to absorb the medication, since you’re supposed to take it on an empty stomach. If you have subclinical hypothyroidism and you start taking large doses of soy protein supplements, it's possible you might develop full-blown hypothyroidism.

“Only very high doses of soy phytoestrogen supplementation may induce clinical hypothyroidism in a minority of patients with subclinical hypothyroidism,” Dr. Jorge Mestman commented on a study about the effects of soy on supplements on thyroids.

So, don’t take mega-doses of soy supplements… but you shouldn't be doing that, anyway.

No supplements, got it. So what type of soy products should I eat?

The experts interviewed for this article were unanimous in their belief that organic, unprocessed, or fermented soy products are most beneficial. They all said it’s a bad idea to take soy protein and soy supplements.

“What you have to look out for is non-organic soy, which considering that 91% of soybeans are genetically engineered is hard to avoid,” says Dr. Gabrielle Francis. “Only eat brands that list organic soybeans in the ingredients list. Avoid products with soy protein isolates as well.”

“Look for organic tofu, natto, tempeh, and also edamame,” adds chef Lauren Feingold. “In the end, it’s about moderation and how unprocessed is the food.”

Essentially, their soy advice is unsurprisingly similar to one of the most common refrains tossed around in dietary circles: nothing in excess, especially if it's highly processed. But if you’re faced with a hot dog and a tofu dog at a barbecue, the choice is clear: Only one of those things is a carcinogen.

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Missy Wilkinson has soy milk, edamame, miso, and tofu in her fridge right now. Natto still squicks her out. Follow her on Twitter at @missy_wilkinson.