Soybean isoflavones in bone health

Soybean isoflavones are structurally similar to estrogen, bind to estrogen receptors, and exhibit weak estrogenic activity. It has been reported that isoflavones play an important role in the prevention of hormone-dependent diseases, including osteoporosis, cardiovascular diseases, cancer, and postmenopausal syndrome. There are many researches indicating isoflavones prevent bone loss caused by estrogen deficiency in animal models. Furthermore, it has been demonstrated that a combination of isoflavone treatment and exercise cooperatively prevented bone loss in the estrogen-deficient status. Epidemiological studies demonstrated the relationship between the lower incidence of osteoporosis in Asian women and a diet rich in soy foods. Although a number of observational studies confirm the findings from the animal studies, the results from intervention studies are still controversial. One of the potential reasons for these inconsistencies could be individual differences in the isoflavone metabolism. Recently, it has been suggested that the clinical effectiveness of isoflavones might partly depend on the ability to produce equol, a gut bacterial metabolite of daidzein showing stronger estrogenic activity than the predominant isoflavones. Several candidate bacteria responsible for equol production have been suggested, for example Lactococcus 20-92 strain. From these findings, food factors enhancing equol production have received great deal of attention recently. On the other hand, safety assessment of isoflavones has been conducted by the Japanese Food Safety Commission. Further studies are required to address the numerous questions on the potential benefits, mechanisms of action, and safety of isoflavones.