Soy has gone mainstream in the U.S. Toddlers snack on edamame, Chipotle sells spicy marinated tofu burritos, and plenty of people can’t start their day without a large soy-milk latte. But is soy all it’s cracked up to be—and is there any basis to concerns that the isoflavones in soy may contribute to cancer? We looked at the complicated, controversial, and ever-evolving research on soy and our health.
The good news about soy
It’s heart-healthy. Soy protein has been shown to slightly reduce levels of LDL cholesterol, the “bad” kind. A landmark 1995 study that looked at nearly 40 controlled clinical trials found that replacing 50 grams of animal protein a day with 50 grams of soy protein lowered LDL cholesterol by nearly 13 percent. But a subsequent American Heart Association update found that the reduction is actually closer to 3 percent—not to mention the fact that to get 50 grams of soy protein you’d need to eat about a pound of tofu or drink a half gallon of soy milk.
That said, replacing even a small amount of your daily animal protein with soy protein is smart for your heart and blood vessels, as it’s leaner than many animal protein sources—plus the fat it does contain is the heart-healthy type. Soy also supplies fiber, vitamins, and minerals. If a food contains at least 6.25 grams of soy protein per serving and is low in saturated fat and cholesterol, the FDA permits it to carry the claim that consuming “25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.”
It’s a stellar protein source. One cup of unshelled edamame, for example, packs 17 grams of protein, almost as much as three ounces of 70 percent lean ground beef and about the same as a hard-boiled egg. Soy is also unusual in that it’s a complete protein source, meaning it contains all of the essential amino acids rather than just some of them, as most plant sources do.
It’s high in fiber. One cup of cooked edamame contains 8 grams of fiber, as much as a dozen prunes.
It’s an alternative to cow’s milk. One cup of soy milk offers nearly as much protein as the same amount of dairy milk and, if fortified with calcium, supplies up to 30 percent of your daily calcium requirement (the exact amount varies by brand). Note that not all soy milk is fortified; check the label if you want to ensure that calcium and vitamins A and D have been added. Also check the label for sugar content, since some soy milk has a lot of added sugar. Or opt for unsweetened varieties.
The maybe good news about soy
The isoflavones in soy may strengthen your bones... Isoflavones are a type of phytoestrogen, meaning they are plant-derived compounds that weakly mimic the effect of estrogen. Because estrogen plays a role in bone health, it’s been suggested that soy isoflavones may reduce osteoporosis risk. But the evidence for this is mixed; some studies have found that women nearing menopause who consume soy protein are more likely to increase their bone density than women whose diets lack soy isoflavones, but other studies have found no impact.
…or help with hot flashes. Research on soy’s effect on hot flashes has been similarly mixed. The North American Menopause Society’s 2015 position statement on nonhormonal treatment of hot flashes says that supplements containing soy isoflavones may help, but more research is needed. And some evidence suggests that the supplements may increase breast cancer risk, as discussed below, so for now we think it's prudent to avoid them.
Safety concerns about soy
Because soy can mimic the activity of estrogen in the body, there has been some concern that it might increase breast cancer risk. The type of soy that’s potentially worrisome is supplemental soy and soy protein isolates, the kind used in nutritional bars and supplements. In fact, the phytoestrogens (estrogen-like compounds) in whole soy foods mayactually help protect against breast cancer.
The American Cancer Society says that more research is needed to understand the relationship between specific forms of soy and cancer risk. According to a statement on the organization’s website, “moderate consumption of soyfoodsappears safe for both breast cancer survivors and the general population, and may even lower breast cancer risk.” It advises avoiding soysupplements, however,until more research is done.
That view is echoed by Patty Siri-Tarino, PhD, a nutrition researcher at the Children’s Hospital of Oakland Research Institute and member of our editorial board. She points out that studies of soy isoflavone supplementation have not supported health benefits, and some studies (mostly in animals) even suggest potential harm. “Everyone should aim to get their isoflavones from real food,” she says. The epidemiological studies that suggest health benefits of soy, particularly in Asian populations, pertain to soy consumed in foods, not supplements. Plus, foods like edamame offer more than just isoflavones, including fiber, vitamins, and minerals. For other top food sources of soy, see the list below.
Hexane in processed soy foods is another concern. Hexane is a solvent used to extract oil from soybeans. If your bar or meat substitute has soy protein isolate, soy protein concentrate, or textured vegetable protein listed on its label, it likely has undergone hexane processing. Hexane is classified as a neurotoxin by the CDC, and chronic exposure in factory workers has been linked to neurological conditions. But it’s unclear whether consuming trace residues is hazardous.
If you’d like to avoid hexane-treated soy foods, look for products with the USDA organic seal, as hexane is banned in organic food production. Better yet, buy soy foods made from whole soybeans—such as tofu, tempeh, soy milk, and soy yogurt—since they don’t typically undergo hexane processing and are generally healthier for you, too. Whole soybeans (edamame) are always a hexane-free and healthy option.
Top food sources of soy
|FOOD||SERVING SIZE||SOY PROTEIN (AVG. GRAMS)
|Soy “burger”||1 patty||14|
|Edamame||1/2 cup cooked||11|
|Meatless soy crumbles||1/3 cup||10|
|Soy yogurt||1 cup||9|
|Soy milk||1 cup||7|
|Soy nut butter||2 Tbsp.||7|
Source for protein amounts: Soyfoods Association of North America.
Also see Soy: Health Food Gone Mainstream.