You probably feel good about that, and why not? The bean has long been touted as a cancer-preventing, cholesterol-lowering superfood. Soy is nature's only plant source with all eight essential amino acids and a denser source of protein than any other bean. It's low in cholesterol and saturated fat, high in fiber, calcium, magnesium, and vitamin B6. It's a staple of healthy Asian diets, an important protein source for vegetarians, and a key ingredient in many infant formulas. Even the packaging seems virtuous: Thanks to a 1999 FDA ruling, manufacturers can state that 25 grams of soy protein a day may reduce heart disease.
But as happens with most overly optimistic health-food claims, the pendulum is swinging back on soy. New research and analysis suggest that not only have the benefits of soy been exaggerated, but too much soy may be harmful. "We've learned that soy isn't a miracle food after all," says sports nutritionist Lisa Dorfman, author of The Vegetarian Sports Nutrition Guide. "Even so, soy foods remain a healthy alternative to high fat proteins." With all this conflicting information, what's a runner to do?
The Soy Saga
The great hope and the current concern with soy centers on the same thing: its high levels of isoflavones, called phytoestrogens. Phytoestrogens are a weaker form of estrogen thought to interfere with the body's natural hormone level; soy products typically contain between five and 50 milligrams of isoflavones per serving. Back in the '90s, researchers hoped that the additional estrogen would lead to positive health effects in hormone-related conditions like menopause and breast cancer.
Those benefits, along with the claim that soy prevents heart disease, have since proven largely unfounded. In February 2006, the American Heart Association reversed its position on soy, now saying that it has little effect on lowering LDL, or "bad" cholesterol. In 2005, a panel of U.S. government experts found insufficient data supporting the claims that soy can reduce the risk of heart disease, prevent osteoporosis, and relieve menopausal symptoms. That same year, a study conducted by the National Institute of Environmental Health Sciences linked genistein (a phytoestrogen in soy) to reproductive problems and infertility in mice. What's more, the Journal of the American Dietetic Association reported in 2005 that the studies linking soy to cancer prevention were inconsistent, and acknowledged that high soy consumption might actually increase breast-cancer risk.
That's a lot of bad news. But nutritionists say that doesn't mean you have to toss out all your tofu. "There's no need to be alarmed," says David Schardt, senior nutritionist at the Center for Science in the Public Interest. While some studies suggest a negative health effect, none of the research is conclusive. "In the same way that we got overly excited about the potentials, we don't want to be overly cautious," says Schardt. His point is to view soy for what it is: food, not medicine. If we eat it as part of a healthy diet, we probably won't put ourselves at risk.
Take It Easy
In Asia, women have a lower rate of breast cancer and men have a lower rate of prostate cancer than their Western counterparts, which is part of the reason everyone got excited about soy in the first place. But a woman in Japan might have a few ounces of tofu in her soup at lunch, and a man in China a handful of soy nuts as a snack. "If we ate the same amount of soy as people in other countries, we'd be fine," says sports dietitian Nancy Clark, R.D. "Some Americans consume soy in excess through supplements, bars, and shakes." That's why Clark and other nutritionists recommend skipping soy pills altogether and eating one or two servings of soy foods a day, remembering to count the protein in your smoothie or sports bar. And if you have three servings one day, limit your intake the next day.
That said, soy foods remain an excellent meat alternative, along with eggs, nuts, and other beans. "When it comes down to it," says Clark, "it's still better to have a soy burger than a big greasy cheeseburger."