Soy: Too Much of a Good Thing?

How much soy should you be eating? The antioxidant-rich superfood may be falling out of favor.

By Elizabeth Devita-Raeburn
Photograph: Photo by: iStockphoto

Soy-based foods rocketed to popularity about a decade ago, largely because of a spate of studies that found midlife Asian women were less susceptible to heart disease, hot flashes, and osteoporosis than their American counterparts. Since Asian women are regular consumers of soy, researchers guessed that isoflavones — antioxidants in soy protein that have an effect similar to estrogen — might be responsible for these benefits. Most of us didn’t wait for science to confirm this hunch; we just loaded up on soy lattes, tofu stir-fry, and soy protein shakes.
Falling Star
Ten years and many studies later, nutritional experts are suggesting we slow down. Here’s the turnaround: In 2000, the American Heart Association officially recommended that people eat more protein from soy products to reduce their risk of heart attacks and stroke. But in 2008, in testimony before a Food and Drug Administration panel, the organization asked the FDA to no longer allow claims that soy products help prevent coronary heart disease. The AHA had changed its position after reviewing 22 studies and concluding that consuming soy products had no significant effect on the risk factors for heart disease.
Other health claims about this bean have turned out, so far, to be unsubstantiated. "There are a lot of studies that say eating soy probably doesn’t stop hot flashes, and a couple that say maybe," says endocrinologist Cynthia Stuenkel, MD, of the University of California, San Diego. Similarly, a review in a recent issue of Menopause produced conflicting results on whether soy improves bone density.
At the same time, questions have been raised about the impact of consuming large amounts of the estrogen-like isoflavones. Of special concern are supplements or additives that concentrate micronutrients extracted from soy foods; these give you a much higher dose than you would get from eating whole soy. In fact, according to Cynthia Sass, a registered dietitian based in New York City, a couple of studies suggest that supplements have risks; in one, high levels of isolated isoflavones spurred the growth of breast tumors in mice.
What to Eat — and Not Eat
Soybeans are the only vegetables to contain a complete set of amino acids, which makes them the protein equivalent of meat, eggs, and milk. "If you eat soybeans as a substitute for meat, you’ll increase your intake of fiber and lower your consumption of cholesterol," Sass says. You get the biggest nutritional bang if you stick with whole soy foods like tofu, soy milk, and edamame. And check the labels of processed foods: Your best bets, Sass says, are those that list whole soybeans on the label; she gives Soy Joy bars and Dr. Praeger’s burgers as good examples.
And while no one knows how much soy is too much, the current government guidelines say that if soy is your main source of protein (as it is for many vegetarians), you should stop at 25 grams, or three to four servings, a day. (See "What’s a Serving Size?" to see how that translates.) That advice applies to most midlife women, but those at high risk for breast cancer should talk to their doctors, says oncologist Marisa Weiss, MD, founder of She adds that women with a history of hormone positive breast cancer might be told by their physicians to minimize consumption of soy foods and to avoid consuming any concentrated soy products — because the isoflavones in soy can have weak estrogen-like effects. But for the rest of us, "Soy is a healthy addition to your diet," Sass says, "if consumed in moderation."
What’s a Serving Size?
1 cup edamame
6 ounces tofu
6 ounces
1 ounce (about 1/4 cup) soy nuts
1 cup soy milk
Originally published in MORE magazine, March 2009.

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