Bone-sparing effect of soy protein in ovarian hormone-deficient rats is related to its isoflavone content

Our previous studies showed that a soy-protein diet prevents ovariectomy-in duced bone loss. The purpose of this study was to determine whether isoflav ones in soy protein are responsible for this bone-protective effect. Forty- eight 95-d-old Sprague-Dawley rats were divided into 4 groups: sham-operate d fed a casein-based diet (SHAM), ovariectomized fed a casein-based diet (O VX+CASEIN), ovariectomized fed soy protein with normal isoflavone content ( OVX+SOY), and ovariectomized fed soy protein with reduced isoflavone conten t (OVX+SOY-). The OVX+SOY group had significantly greater femoral bone dens ity (in g/cm(3) bone vol) than the OVX+CASEIN group, whereas OVX+SOY- was s imilar to OVX+CASEIN ((x) over bar +/- SD; SHAM, 1.522 +/- 0.041; OVX+CASEI N, 1.449 +/- 0.044; OVX+SOY, 1.497 +/- 0.030; OVX+SOY-, 1.452 +/- 0.030). O variectomy resulted in greater bone turnover as indicated by higher serum a lkaline phosphatase activity, serum insulin-like growth factor I and insuli n-like growth factor binding protein 3 concentrations, and urinary hydroxyp roline. These increases were not affected by soy with either normal or redu ced isoflavone content. Similarly, histomorphometry revealed a greater bone formation rate with ovariectomy, and this was not altered by the soy diets . The findings of this study suggest that isoflavones in soy protein are re sponsible for its bone-sparing effects. Further studies to evaluate the mec hanism of action of isoflavones on bone are warranted.