The results of two meta-analyses conducted by a team from Peking University in Beijing, China and the University of Yamanashi in Japan have determined that not only does supplementing with soy help inhibit bone resorption and increase bone formation, it also improves spinal bone mineral density.
For the first review, published in the February, 2008 issue of the European Journal of Clinical Nutrition, Pei-Yu Wang and colleagues selected 9 randomized, controlled trials which tested high isoflavone isolated soy protein or isoflavone tablets in a total of 432 perimenopausal or postmenopausal women. Urinary deoxypyridinoline, a marker of bone resorption, and serum bone-specific alkaline phosphatase (BAP), a bone formation marker, were measured before and after the treatment periods, which ranged from 4 to 48 weeks. The second meta-analysis, published in the February, 2008 issue of the journal Clinical Nutrition, included 10 trials involving 608 peri- or postmenopausal subjects who ingested soy products or isoflavones for 3 months to 4 years. Spine bone mineral density was measured by dual x-ray absorptiometry upon enrollment and at the end of the studies. Bone mineral content was determined in 6 of the trials.
The first meta-analysis found an average decrease in urinary deoxypyridinoline of 2.08 nanomoles per millimole, and an average increase of 1.48 micrograms per liter of BAP in women who received isoflavones compared with those who received a placebo. "Isoflavone intervention significantly inhibits bone resorption and stimulates bone formation," the authors stated. "These favorable effects occur even if less than 90 milligrams per day of isoflavones are consumed or the intervention lasts less than 12 weeks."
The second review also found a bone-building benefit for soy. In the clinical trials examined, women who received soy experienced a significant increase of 20.6 milligrams per cubic centimeter in spine bone mineral density compared with those who received a placebo. Bone mineral content also increased, but to a lesser extent. Benefits were greater when more than 90 milligrams per day isoflavones were consumed, or when the trial lasted at least six months. "The results clearly suggested that isoflavones contributed significantly to the increase of spinal bone mineral density, especially in postmenopausal women," Dr Wang and his coauthors conclude.
In their introduction to one of the reviews, the authors remark that with increases in life expectancy, osteoporosis has become common, and although hormone replacement therapy is "the first choice of treatment," it carries with it the risk of breast cancer and, in some studies, failed to demonstrate significant hip fracture reduction or bone protection. "Thus, new bone protection options are needed," they write. The lower incidence of osteoporosis-related fracture among Asian women who consume 10 to 20 times more soy than Western women may point to a bone-protective effect of soy isoflavones. The authors recommend large randomized clinical trials using graded dosages of isoflavones to measure their long term effects on bone mass as well as fracture risk.
Related Health Concern: Osteoporosis
Considering the health risks associated with conventional hormone replacement therapy (HRT), many women are reluctant to consider estrogen replacement therapy. Fortunately, phytoestrogens from soy, including genistein and daidzin, provide a possible alternative. We now know that genistein and daidzin bind loosely with estrogen receptors and that diets high in soy may protect against estrogen-induced cancers. Soy may also have an impact on bone health.
A six-month study to investigate bone density and bone mineral content in response to soy therapy was conducted. In this study, women received daily either phytoestrogens derived from soy protein or milk-derived protein (which contained no phytoestrogens). The results showed significant increases in bone density and bone mineral content for the lumbar spine in the women receiving the phytoestrogens derived from soy protein diets. Increases in other skeletal areas also were noted in the women on the soy diets. Researchers concluded that soy isoflavones show real potential for maintaining bone health (Potter SM et al 1998).
Another study found that soy foods reduced the risk of fracture in postmenopausal women, particularly among women who just finished menopause (Zhang X et al 2005). In this study, Chinese officials studied soy consumption among approximately 24,400 postmenopausal women and discovered that women with the highest soy intake were less like to suffer from fractures.