Researchers have found that soy isoflavone tablets are ineffective when it comes to reducing bone loss in postmenopausal women.
A previous six-month study by Iowa State University researchers had indicated that consuming modest amounts of soy protein, rich in isoflavones, lessened lumbar spine bone loss in midlife, perimenopausal women
"Our six-month preliminary study, published in 2000, indicated that soy protein, rich in isoflavones, exerted the greatest impact in slowing the loss of bone mineral density in the lumbar spine," said lead researcher D. Lee Alekel, professor of nutrition and interim associate director of the Nutrition and Wellness Research Centre (NWRC) at Iowa State.
"But we believed that we needed to replicate these results in a study with a greater sample size and longer duration, which is what we did with this three-year intervention.
"In this longer study, we had sufficient power to detect change. We monitored adverse events, had excellent compliance throughout, and accounted for potential confounding factors," she added.
During the study, researchers compared the effects of either ingesting daily 80-mg daily or 120-mg soy isoflavone tablets, compared to placebo tablets on bone mineral density (BMD) and other health outcomes.
While the 120-mg dose soy isoflavones did reveal a small protective effect on femoral neck bone BMD, researchers found no significant effect of treatment on lumbar spine, total hip, or whole-body BMD.
"This trial used isoflavones extracted from soy protein, compressed into tablet form, consumed over the course of three years, which is very different than either providing soy protein or soy foods," Alekel said.
"In our recent study, we did not demonstrate an important biological effect on BMD or bone turnover," she added.
The study appears in The American Journal of Clinical Nutrition.