DIETARY SOYBEAN PROTEIN PREVENTS BONE LOSS IN AN OVARIECTOMIZED RAT MODEL OF OSTEOPOROSIS

The purpose of this study was to examine whether soybean protein isola te prevents bone loss induced by ovarian hormone deficiency. Thirty-tw o 95-d-old Sprague-Dawley rats were randomly assigned to four treatmen t groups [sham-operated (sham); ovariectomized (ovx); ovx + soybean; o vx + 17 beta-estradiol (E(2))] and killed after 30 d. Rats in the sham , ovx and ovx + 17 beta-estradiol groups were fed a casein-based diet, and the soybean group was fed soybean protein isolate instead of case in; the diets were otherwise comparable. Rats in the ovx group had sig nificantly lower densities of the right femur (P < 0.001) and the four th lumbar vertebra (P < 0.05) than rats in the sham group. These lower bone densities were not observed in animals receiving 17 beta-estradi ol or fed soybean. The ovx group also had significantly (P < 0.01) gre ater serum concentrations of 1,25-dihydroxycholecalciferol than the ot her three groups. Our findings suggest that dietary soybean protein is effective in preventing bone loss due to ovarian hormone deficiency. Because serum activities of both alkaline phosphatase and tartrate-res istant acid phosphatase were significantly greater in the ovx group an d in the ovx + soybean group but not in the group receiving 17 beta-es tradiol, compared with sham animals, this confirms that ovariectomy en hances and 17 beta-estradiol suppresses the rate of bone turnover. Des pite the higher rate of bone turnover in the soybean-fed animals, the vertebral and femoral bone densities of these rats were significantly greater than those of rats in the ovx group, suggesting that formation exceeded resorption. Further studies are needed to clarify whether th is protective effect on bone is due to the protein itself or to the pr esence of isoflavones in soybean protein.