(HealthDay News) -- Genistein, a natural phytoestrogen derived from soy products, worked better than placebo pills at building bone mineral density in a two-year study of older, postmenopausal Italian women.

After two years, those taking the soy product had better bone mineral density than those taking placebo. "[Even] after one year, there was a clear difference in the women who got the genistein," said Steven Wilson, a biostatistician at National Jewish Medical and Research Center, Denver, and a co-author of the study, published June 19 in the Annals of Internal Medicine.

The research was funded by the Italian Ministry of Education and the University of Messina, Italy.

Genistein, an isoflavone phytoestrogen, is found abundantly in soybean products. Experts who study it hope it can build bone without the adverse side effects -- such as increased risk of heart disease and certain cancers -- associated with hormone replacement therapy.

In the study, a team led by Dr. Francesco Squadrito of the Azienda Ospedaliera Universitaria Policlinico, Messina, randomly assigned 389 women who were past menopause (ages 49 to 67) to take either 54 milligrams of genistein daily for 24 months or a placebo pill.

Both the genistein and the placebo pills also contained calcium and vitamin D, known to build bone. The doses per day were 500 milligrams of calcium and 400 international units (IUs) of vitamin D.

The women had osteopenia, defined as bone mineral density lower than normal but not low enough to be classified as osteoporosis, a condition of lowered density that renders bones vulnerable to dangerous fractures.

The researchers focused on evaluating the bone mineral density at the lumbar spine and the hip, conducting bone mineral density measurements at the study start, and again at 12 and 24 months.

After just one year, "the bone had not only stopped deteriorating but actually began to reform and become more dense," Wilson said. "The placebo group, still receiving a normal dose of calcium and vitamin D, continued unfortunately their decline," he said, referring to the age-related decline in bone density that occurs after menopause, as estrogen levels decline.

In the study, those who took genistein did not have increased endometrial thickness, which can be a problem with some hormone treatments for decreased bone density, according to the researchers. The genistein group did have more gastrointestinal side effects than the group that did not take it.

Soy has potential for bone-building, but more research is still needed, added Dr. Frank Sacks, professor of medicine at Brigham and Women's Hospital and the Harvard School of Public Health in Boston. "There are safety issues with soy phytoestrogens" related to breast and uterine cancer, he stressed.

"This study would not change the overall view [among many experts] that soy phytoestrogen supplements should not be taken," Sacks said.