For years, soy seemed to be on every list of so-called superfoods. With good reason: It’s packed with protein and fiber, and it contains isoflavones—estrogenlike compounds thought to help strengthen bones, dampen hot flashes, lower cancer risk, and more.

But lately, soy’s healthy reputation has taken a hit, in part because what “soy” means isn’t always clear. Some soy products are whole foods such as tofu, soy milk, and edamame. Other are processed, such as textured vegetable protein, soy protein isolates (used in packaged foods such as veggie burgers and energy bars), protein powders, and isoflavone supplements. We took a hard look at the research. 

Heart of the Matter

In 1999, the Food and Drug Administration allowed foods with at least 6.25 grams of soy protein per serving to carry a label saying that low-fat, low-cholesterol diets containing 25 grams of soy protein per day may reduce the risk of heart disease. In part, that was based on a review of studies that showed that 47 daily grams of soy protein—equivalent to 2½ cups of tofu or 7 cups of soy milk—lowered LDL cholesterol by almost 13 percent. Later research put the LDL drop at closer to 3 percent, so the FDA is reconsidering the label claim. According to the American Heart Association, soy foods can help lower cholesterol because they’re low in saturated fat and high in fiber, and can replace less healthy foods in the diet. “But there’s nothing unique about soy compared to other beans,” says Alice Lichtenstein, D.Sc., Gershoff professor of nutrition science and policy at Tufts University in Massachusetts.  

Breast-Health Effects

Studies of Asian women have found that those who eat soy, a staple of Asian diets, regularly have a lower breast-cancer risk. But some animal studies suggest that the isoflavone genistein may promote the growth of estrogen-positive tumors and mute the effects of breast-cancer drugs such as tamoxifen.

And whether soy is protective may depend on when in life a woman starts eating it, says William Helferich, Ph.D., professor of nutrition at the University of Illinois at Urbana-Champaign. The American Cancer Society says that there’s no reason for women, including breast-cancer survivors, to avoid soy foods but that until further research is done, it’s prudent to skip supplements, which contain much higher levels of isoflavones.

Other Benefits?

Isoflavones have been studied for many conditions, but there are no firm conclusions. Limited evidence suggests, for example, that soy foods may lower the risk of prostate cancer. And genistein supplements may reduce hot flashes, though they need further study. (And as with all supplements, you can’t be sure what you’re getting.)

The bottom line: Unless you’re allergic, three daily servings (one serving is 1 cup of soy milk, ½ cup of edamame, or 3 ounces of tofu) is unlikely to have negative effects. Whole forms are best; that’s what has been used in the studies that show benefits, says Anna Wu, Ph.D., professor of preventive medicine at the University of Southern California’s Keck School of Medicine. It’s not known whether protein powders, supplements, or processed soy foods have the same effect, and the latter may contain ingredients you don’t want to load up on, such as sodium and sugar.