Women who eat the most soy foods have the fewest bone fractures after menopause.

That's the word from a study of 24,403 postmenopausal Chinese women. Within 10 years of menopause, the 20 percent who ate the most soy foods reported half as many fractures as the 20 percent who ate the least soy.

Soy protected against fracture at every level of consumption over 5 grams a day. But those who ate more than 13 grams of soy a day — getting more than 60 milligrams of soy isoflavones a day — got the most benefit.

Soy protected against bone loss but did not appear to strengthen weak bones, says researcher Xiao-Ou Shu, MD, PhD, MPH, professor of medicine at Nashville's Vanderbilt-Ingram Cancer Center. Shu and her colleagues report the findings in the Sept. 12 issue of Archives of Internal Medicine.

"We saw the protective effect of soy was much better for women who just had menopause, within 10 years," Shu tells WebMD. "Afterward, soy is still protective, but it not as much as in the recently menopausal women."

Two Cups of Soy Milk — or Less

If you think that these Chinese women ate more soy than you possibly could, think again.

The highest level of consumption among Chinese women was about 13 grams of soy protein a day. A cup of soy milk contains about 6.6 grams. A half piece of tofu contains about 8 grams.

"That is definitely manageable," Shu says. "Please note that women in the middle consumption group also had a 30 percent risk reduction for bone fracture. The amount of soy food consumption in that group is about a half piece of tofu or a little more than 1 cup of soy milk per day."

The Preventive Power of Soy

Just about everyone knows that calcium is needed to build strong bones. But soy has a different bone-protecting effect, says soy isoflavone expert Kenneth D.R. Setchell, PhD, of Children's Hospital Medical Center in Cincinnati.

Setchell's research team is in the middle of a long-term experiment. The researchers are giving postmenopausal women two glasses of soy milk every day. Half the women get soy milk with a major soy component — soy isoflavones — removed.

"We found that women who consumed the soy isoflavones maintained stable bone mass," Setchell tells WebMD. "They had no bone loss in two years. And that has now been extended to four years."

The women who did not get isoflavones in their soy had significant bone loss — about 4.5 percent. Bad as that sounds, Setchell says, it's not as much bone loss as most women would see at menopause if they were not taking some kind of bone-enhancing treatment. He thinks soy protein may also play a role in bone protection.

But whatever it is about soy and bone, the key word is protection. Setchell and Shu both stress that soy is not a treatment for bone loss — its effect is to prevent bone loss.

More Health Benefits of Soy

Bone loss isn't the only reason to consume soy foods.

"It is not just for bone fracture. It is a great health benefit," Shu says. "Most studies indicate that soy is pretty safe, and the evidence is quite strong that it protects against coronary heart disease. There are also some data indicating soy may reduce the risk of breast cancer. Taking all those things together, I recommend women eat soy as much as they can."

But don't just add soy to your diet, warns Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh Medical Center.

"This is not an additive. You still have to watch your calories," Bonci tells WebMD. "If you add it into your diet, think about what you are wiling to give up. Otherwise you are going to get too big for your bones."

Bonci also warns that soy supplements aren't a replacement for soy foods. If you're going to go for soy health benefits, she says, do it with foods — not pills.

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Zhang, X. Archives of Internal Medicine, Sept. 12, 2005; vol 165: pp 1890-1895. Lydeking-Olsen, E. European Journal of Nutrition, August 2004; vol 43: pp 246-257. Xiao-Ou Shu, MD, PhD, MPH, professor of medicine, Vanderbilt-Ingram Cancer Center, Nashville, Tenn. Kenneth D.R. Setchell, PhD, professor of pediatrics, Children's Hospital Medical Center, Cincinnati. Leslie Bonci, MPH, RD, director of sports nutrition, University of Pittsburgh Medical Center.