(USA TODAY) -- Soy foods may be safe, and possibly even beneficial, for breast cancer survivors, a new study says.

Many breast cancer doctors have been cautious about recommending soy products -- such as soy milk, tofu, edamame or miso soup -- because they contain plant estrogens. Most breast cancers are fueled by estrogen, which can make it risky to take additional hormones, such as for menopausal symptoms.

Studies of soy also have produced conflicting results, says Xiao Ou Shu of the Vanderbilt-Ingram Cancer Center in Nashville.

On the plus side, soy is a good source of protein with little fat and no cholesterol, which makes it popular with vegetarians, dieters and people who can't handle milk. Women in regions with high soy consumption, such as Asia, tend to have lower rates of breast cancer, Shu says. But Asian women differ in many ways from Americans, especially in their weight, so scientists say soy can't get all the credit.

Studying soy in America has been difficult, Shu says, because women here eat little of it. About 28% of Americans ate soy at least once a week in 2003, her study says.

That's why Shu decided to study soy in more than 5,000 breast cancer survivors in China, where soy is a staple.

She divided women into four groups, based on how much soy they ate. Women with low soy levels consumed an average of about half a cup of soy milk a day, while the high-soy group had about three cups a day, Shu says.

After four years, 7.4% of those who ate the most soy had died, compared with 10.3% of those who ate the least soy, according to her study. It's published today in The Journal of the American Medical Association.

Those results suggest that it's probably safe for breast cancer survivors and other women to have one or two servings of soy foods a day, says Walter Willett, chairman of the nutrition department at the Harvard School of Public Health. But he cautions against women taking large doses of genistein pills or other soy supplements, which are unproven.

Previous studies have shown that genistein, an estrogen-like compound in soy, promotes the growth of breast tumor cells in lab dishes and animals, Shu says.

The latest study suggests that women benefited from eating whole soy, possibly because other nutrients from the plant act together in healthy ways, Shu says.

Willett says scientists are still trying to understand all of soy's hormonal effects. It's possible, he says, that soy acts like the breast cancer drug tamoxifen, which blocks the effects of estrogen.

But American patients may respond to soy very differently from those in China, says Claudine Isaacs of Georgetown's Lombardi Comprehensive Cancer Center. Research suggests prenatal and childhood exposure to hormones and other chemicals can program how the body responds to them, Shu says. And other research suggests that soy is most effective at preventing cancer in women who are exposed to it during and after adolescence.

So it's possible that a middle-aged American woman who adds soy to her diet for the first time may not reap the same benefits as Chinese women who grow up with it, Isaacs says.

"It makes sense not to go to extremes," Willett says. "Having some soy in the diet is probably a good thing, especially if it replaces red meat," which is linked to an increased risk of colon cancer, he says. "But there's still good reason not to go overboard."

Copyright 2009 USA TODAY, a division of Gannett Co. Inc.