Soy May Not Improve Bone Health in Young Women

CHAPEL HILL, N.C.--Isoflavone-rich soy protein has no effect on bone mineral content or bone mineral density in healthy young women, according to research published in the October issue of the Journal of the American College of Nutrition (21, 5:388-93, 2002) (www.jacn.org). Researchers from the University of North Carolina (UNC) compared the effects of two diets--isoflavone-enriched soy protein isolate (as Solae™, manufactured by St. Louis-based DuPont Protein Technologies) or isoflavone-depleted soy protein (an alcohol-wash product also provided by DuPont)--on the bone health of 28 women aged 21 to 25 years.

"We had seen earlier reports that isoflavones had beneficial effects on the bones of women undergoing menopause or who already were postmenopausal," said John J.B. Anderson, Ph.D., FACN, professor of nutrition at the UNC schools of public health and medicine. "We felt that if this were true for older women, it might also be true for healthy young women and could help protect their bones over their lifetimes."

In a controlled, double blind fashion, researchers allocated either a treatment diet containing approximately 90 mg/d of total isoflavones or a control diet that contained soy protein from which the isoflavone content had been removed. Bone mineral content and bone mineral density were measured at baseline and again at six and 12 months. No changes in either measure were noted at the end of the study period in either group, and researchers noted other variables also remained constant over the 12-month period, including normal menstrual patterns in both groups. Researchers concluded the isoflavone-rich preparation appeared to have little or no effect on bone in young adult women with normal ovarian function.

"We were surprised that we saw zero effect," Anderson said. "We think we found nothing because our young women were menstruating normally and therefore had normal estrogen levels. The natural estrogen levels may simply have overwhelmed any possible effect of the isoflavones."

Kathy Greaves, Ph.D., director of women's health at DuPont, noted, "[T]he circulating estrogens that are found in young women presumably would have a far greater effect on bone than the isoflavones that are found in soy protein. The study was also a very small study, which probably wouldn't have sufficient power to detect changes that might occur from eating soy protein containing isoflavones."

Earlier research conducted with Solae indicated supplementation enhanced the bone health of postmenopausal and perimenopausal women. A study out of the University of Illinois at Urbana-Champaign, which involved 66 postmenopausal women, indicated supplementing with 40 g/d of soy protein (containing 2.25 mg of isoflavones per gram of protein) for six months significantly increased bone mineral content and bone mineral density in the lumbar spine (Am J Clin Nutr, 68, 6 Suppl:1375S-79S, 1998). A subsequent study out of Iowa State University in Ames involved 69 perimenopausal women. They were randomly assigned to isoflavone-rich soy, isoflavone-poor soy or whey (control) diets. After 24 weeks, the isoflavone-rich soy protein isolate, which contained 80.4 mg/d of isoflavones, was found to have a positive effect on change in bone mineral density, measured at the lumbar spine (Am J Clin Nutr, 72, 3:844-52, 2000).