Is soy bad for you? Experts weigh in

Despite the ubiquity of soy, there are many rumors swirling about whether or not it’s good for you. To get answers, Fortune spoke with experts—here’s what you need to know.

What is soy?

Soy products are derived from the soybean plant, which is a member of the legume family—you might recognize its seeds as the popular food edamame. The U.S. is the largest grower of soybeans in the world, according to the Department of Agriculture. 

It is arguably one of the most important crops, providing protein for animals and humans, in addition to being processed into biofuel, vegetable oil, and other food products.

Some common soy food products include: tofu, edamame, tempeh, miso, soy milk, soy sauce, soy protein isolate, soybean oil, and textured vegetable protein.

Is soy bad for you? 

The concerns about soy primarily stem from two studies published in 1987 and 1998, says registered dietitian Lauren McNeill, who specializes in plant-based nutrition. Researchers were investigating how raw soybeans consumed by mice affected their hormone levels and health outcomes. They found that soy increased the mice’s cancer risk and stimulated breast growth.

Soy contains phytoestrogens, meaning it has a similar chemical structure to estrogen and can potentially mimic, and increase, estrogen activity in the body. That left people concerned that phytoestrogens, like those found in soy, would put you at increased risk of breast cancer, as estrogen activity can act as a catalyst for cancer growth.

Other concerns about soy included how it could potentially inhibit thyroid function based on animal and test-tube studies.

But those studies are fundamentally flawed, McNeill says. 

“Soy is not inherently what mice eat in general…it’s not a good study to be doing to translate into humans,” McNeill tells Fortune. There’s actually evidence pointing to soy’s overall benefits for human health.

@tastingtothrive_rd Reply to @chubaga is soy bad for you? #vegandietitian #soymyths #vegansoymyths ♬ original sound – Plant-Based Dietitian | Lauren

A number of more recent studies and reviews of research have debunked concerns about soy being harmful to your health. Research has found that soy has no significant impact on thyroid function—thought that relationship is still being investigated. A 2010 study also put to the test what kind of impact soy had on male reproductive hormones. They found there was no impact from soy on testosterone or other sex hormones in men.

“We now have very clear human data showing there’s no association between soy consumption and increase of cancer risk and increase of cancer recurrence,” says Dr. Neil Iyengar, a breast medical oncologist at Memorial Sloan Kettering Cancer Center in New York.

Potential health benefits of soy

“There are a lot of advantages to including soy in [your] diet,” McNeill says. 

In fact, several studies have indicated that eating soy not only helps lower your cancer risk, but can also broadly improve health outcomes.

Soy foods like tofu, edamame, and tempeh are packed with fiber, protein, vitamins, and minerals, with the added bonus of being plant-based, which Iyengar sees as beneficial for his patients because of the overall health and longevity benefits that come from a plant-forward diet.

“The good thing about soy is that it’s such a rich source of nutrients … I certainly would not tell people to avoid soy,” he says. “In fact I would tell people to incorporate soy as part of a healthy cancer-protective diet.”

A 2009 study of 5,042 female breast cancer survivors in China—women aged 20 to 75 years with diagnoses between March 2002 and April 2006—found that those with diets higher in soy had a significantly lower risk of death and cancer recurrence. Meanwhile for men, soy consumption appears to help lower their risk of prostate cancer as well, according to a 2018 study.

Meanwhile, a 2019 review explains that phytoestrogens—found in soy—have been associated with a number of benefits, including cardiovascular and cognitive function, skin health, aiding weight-loss, and minimizing the symptoms of menopause.

Other benefits of soy include:

• Lower total cholesterol
• Reduced hypertension
• Lower fasting blood sugar levels
• Appetite suppression
• Minimizing skin aging

The best way to consume soy

Some forms of soy are more processed than others, such as soy protein isolate or textured vegetable protein—common in protein powders and vegan meat alternatives—and soybean oil, found in many processed food products and used as a cooking oil in many restaurants. It’s best to stick to minimally processed soy foods to maximize the benefits from protein, fiber, and other nutrients found in soy. 

Here are some soy foods to incorporate into your diet:

• Tofu
• Edamame (including dried crunchy edamame snacks)
• Tempeh
• Soy milk

McNeill’s favorite way to prepare tofu is tossing it in olive oil, garlic powder, salt, paprika, and nutritional yeast, then pop it into the air fryer or oven to add to stir fry, salads, and wraps. She’s also a fan of steaming edamame and seasoning it with salt.

“The less processing the better,” Iyengar says.

Iyengar recommends one to two servings of soy per day to see the health benefits observed across numerous studies. That recommendation isn’t to say it’s bad to consume more than that, though. He says because foods like tofu and edamame are so high in protein and fiber, you’ll likely fill up pretty easily from soy—and eating more than one to two servings could mean you won’t have room for other nutrient-dense foods.

“You can eat soy daily if you want to,” Iyengar says.

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