Role of soy protein with normal or reduced isoflavone content in reversingbone loss induced by ovarian hormone deficiency in rats

Soy protein, a rich source of isoflavones, fed immediately after an ovariec tomy prevents bone loss in rats. Reports of the effectiveness of natural an d synthetic isoflavones in preventing or treating osteoporosis led us to ex amine the effect of soy protein in reversing established bone loss. Seventy -two 95-d-old female Sprague-Dawley rats were assigned to 6 groups. The rat s were either sham operated (SHAM; 2 groups) or ovariectomized (OVX; 4 grou ps) and then fed a casein-based, semipurified diet. Thirty-five days after surgery, 1 SHAM and 1 OVX group were killed to examine the occurrence of bo ne loss. Thereafter, the other SHAM and 1 OVX groups continued to receive t he casein-based diet. Whereas the remaining 2 OVX groups received diets in which casein was replaced by soy protein with normal (OVX+SOY) or reduced ( OVX+SOY-) isoflavone content for 65 days. The OVX control group had signifi cantly lower femoral and fourth lumbar Vertebral bone densities than the SH AM group. Femoral density of rats fed SOY or SOY- diets were not significan tly different from SHAM or OVX controls. This suggests a slight reversal of cortical bone loss that may be partially due to higher femoral insulin-lik e growth factor I mRNA transcripts resulting from both the SOY and SOY- die ts. The ovariectomy-induced increases in indexes of bone turnover were not ameliorated by either of the soy diets, suggesting that any positive effect of soy was achieved through enhanced bone formation rather than slowed bon e resorption. Long-term consumption of soy or its isoflavones may be needed to produce small but continued increments in bone mass.