Researchers supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) provided convincing evidence questioning the notion that soy isoflavone tablets can help preserve bone mineral density (BMD) in women after menopause. Their study, the longest ever to evaluate the effect of soy isoflavones on BMD, appeared recently in the American Journal of Clinical Nutrition.
Isoflavones are naturally occurring compounds found in soybeans. Because they are structurally similar to estrogen, researchers have thought that they may hold promise as an alternative to estrogen therapy to protect postmenopausal women from osteoporosis. Several studies have explored the effects of soy isoflavones on bone health, but results have been mixed, ranging from a modest impact to no effect. Most of these studies had various limitations, including their short duration and small sample size, making it difficult to fully evaluate the impact of these compounds on bone health.
Building on lessons learned from earlier studies, Iowa State University’s D. Lee Alekel, Ph.D., and her colleagues at the USDA Agricultural Research Service’s Western Human Nutrition Research Center, conducted the Soy Isoflavones for Reducing Bone Loss (SIRBL) Study, enrolling more than 250 healthy postmenopausal women in this multi-center trial. The women, none of whom had osteoporosis, were randomly assigned to receive one of two doses daily of isoflavones (80 or 120 mg) or a placebo. All of the women received calcium (500 mg) and vitamin D (600 IU) supplements daily.
During three years, women in the isoflavone and placebo groups experienced similar losses in spine, total hip, and whole body BMD. Although women in the 120 mg soy isoflavone group experienced slightly less bone loss in the femoral neck region of the hip, researchers point out that this effect was very modest, particularly given that whole-body fat mass, age, and bone resorption (removal) exerted an overriding effect on bone loss.
This study is part of an emerging body of evidence that refutes a bone-sparing effect of soy isoflavone tablets. And, while researchers did not identify negative effects in soy users, the commonly held, but incorrect public view that soy isoflavones are bone-protective in healthy American women is seriously called into question.