Soy protein with or without isoflavones failed to preserve bone density in gonadal hormone–deficient male rat model of osteoporosis

Soy with its isoflavones has been shown to positively influence bone mineral density in female ovariectomized rats; hence, we hypothesized a similar effect in orchidectomized (ORX) male rats. Forty male Sprague-Dawley rats, aged 95 days, were divided into 4 groups and were either sham operated (Sham) or ORX. The ORX groups were fed a soy protein–based diet (SOY), an isoflavone-depleted soy protein diet (SOY−), or a casein based diet for 65 days after surgery. Orchidectomy increased the rate of bone turnover, resulting in reduced bone mineral density and bone mineral content by 3.5% and 14%, respectively, and compromised biomechanical properties. The mean femoral length of ORX animals was also significantly shorter than Sham animals, but ORX rats that were fed SOY diet did not experience this reduction in bone length, implicating a role for soy protein in bone growth (4.02 ± 0.02, 3.93 ± 0.01, 3.99 ± 0.02, 3.91 ± 0.01 for Sham, ORX, SOY, SOY−, respectively). The SOY and SOY− positively influenced the biomechanical properties of bone such as yield and ultimate force, the measures of bone elasticity, and plasticity. In terms of bone histomorphometry, the data indicate that SOY− tends to reduce ORX-induced increase in bone turnover as evidenced by suppressed bone formation rate/mineralized surface by about 9%. Overall, our results indicated that soy protein, regardless of its isoflavone content, was unable to prevent the ORX-induced femoral decrease in bone density and mineral content. However, soy may enhance the quality of bone as indicated by increased yield force.