Remember when tofu and soy milk were virtually the only sources of soy? No more. These days, soy is a common ingredient in a host of foods, including breakfast cereals, bread, chips, frozen dinners, margarine, meatlessburgers, desserts, water-packed canned tuna and even chocolate bars.

Is that too much of a good thing?

That's a question some nutritionists are asking as the Food and Drug Administration (FDA) considers a petition to grant soy products a qualified health claim for possible prevention of breast, colon and prostate cancer. Unlike full-fledged health claims, qualified health claims are based on emerging research that suggests, but doesn't prove, a health benefit. Nuts, olive oil and omega-3 fatty acids are some of the foods and dietary supplements that have received qualified claims in recent years.

Soy protein has already earned one full-fledged FDA health claim for heart benefits. Since 1999, low-fat, low-cholesterol products containing at least 6.25 grams of soy protein per serving -- equal to about a half-cup of tofu or a cup of soy milk -- can carry a label that says: "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease."

Since that claim went into effect, consumption of soy protein has more than doubled in the United States. If the new health claim is granted and soy consumption again doubles, industry sources project that U.S. soy intake could reach 4.5 grams per day -- or about half the daily amount consumed in Japan. That level "appears reasonable and presents no safety concerns," said the Solae Co., a soy protein research company, in petitioning the FDA for the new claim. But others are less sure.

Not only is soy a rich source of protein, it also contains complex carbohydrates that are less likely than processed products like white bread to send blood sugar soaring and boost insulin production. Soy has fiber, healthy fat and folic acid -- good for heart health and a protection against such birth defects as spina bifida. Soy also appears to help preserve bones and build muscle, and it can help lower blood pressure and blood cholesterol.

But it's the isoflavones in soy that may be of greatest benefit -- and concern. These substances have some of the same chemical properties as the female hormone estrogen.

Studies suggest isoflavones may offer some protection against breast, colon and prostate cancer in adults. But in laboratory and animal studies, isoflavones have been shown to affect immune function, the thyroid and sexual maturity. That's why in July, the Israeli Ministry of Health announced plans to recommend that young children limit soy products and advised that infants avoid them altogether. The French government recently recommended that soy products not be given to children under 3 to help minimize any risk from isoflavones.

In some quarters, these moves have been met with criticism.

"Soy really has some health benefits, and I think that it is absurd to suggest that soy should not be consumed by children," said Kenneth Setchell, professor of pediatrics at the Cincinnati Children's Hospital and a longtime researcher of soy proteins in animals and people.

A recent review of the health effects of soy infant formulas conducted by researchers at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., could find no ill effects from soy infant formula. Theoretically, soy-fed infants appear to be exposed to enough soy compounds to reach "pharmacologically active" levels, writes Aimin Chen and Walter J. Rogan in the 2004 Annual Review of Nutrition. But there is no indication that those levels have caused any problems in the 50 years the formulas have been used.

As the debate continues, here's what experts say to keep in mind when choosing soy products:

* A little goes a long way. Mark Messina, an adjunct associate professor of nutrition at Loma Linda University in California recommends that healthy people aim for about 15 to 25 grams of soy per day -- roughly the amount found in a half-cup to a full cup of tofu or a couple of cups of soy milk. "That limitation is not based on any safety concerns, but the dietetic principle of eating a varied diet," said Messina, who sometimes serves as a consultant to the soy industry.

Kaayla T. Daniel, author of "The Whole Soy Story: The Dark Side of America's Favorite Health Food" (New Trends) also believes in soy moderation. She said she sips "a little miso soup a couple of times a week and am happy to give it to my children," who are 7 and 11. But Daniel said that she is "very concerned about the amount of soy in the food supply and the numbers of people . . . who rather than eating a little soy once in a while are eating massive amounts of soy."

* Soy foods are not always low in fat or calories. An eight-ounce glass of soy milk contains four grams of fat and 100 calories -- about 20 percent more calories than a glass of skim milk. Many soy cereals come coated with added sugar. A half-cup of soy nuts has about 250 calories. So you might want to skip the chocolate-covered soy nuts and look for soy products that don't have added fat or sugar.

* Use soy to boost protein. A recent study found that adding protein to the diet of overweight and obese people prompted them to spontaneously cut their daily intake by nearly 500 calories per day. Soy is rich in protein. A half-cup of soy nuts provides about 20 grams of protein -- nearly as much as that found in half a roasted chicken breast.

* Go traditional. Roasted soy nuts, soy milk, tofu, tempeh, miso soup and edamame -- the "immature" soybeans that can be steamed or stir-fried -- are mainstays in Asian countries. Few if any studies show that such new products as soy bread, crackers and cookies have the same long-term benefits as traditional soy sources.

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