Soy affects trabecular microarchitecture and favorably alters select bone-specific gene expressions in a male rat model of osteoporosis

We have recently reported that soy isoflavones particularly when provided in the context of soy protein are capable of preventing loss of bone mineral density due to orchidectomy in F344 rats. We hypothesize, that soy isoflavones also exert beneficial effects on bone microstructural properties, in part, by enhancing bone formation. Therefore, in the present study, we examined the dose-dependent effects of soy isoflavones on femoral bone microarchitectural properties and select bone-specific gene expressions in the same rat model. Seventy-two, 13-month old rats were either orchidectomized (ORX; 5 groups) or sham-operated (Sham; 1 group) and immediately placed on dietary treatments for 180 days. Four of the ORX groups were fed either casein- or soy protein-based diets each with one of two doses of isoflavones either 600 or 1200 mg/kg diet. Rats in the remaining ORX control and Sham groups were fed a control casein-based diet. Soy protein at the high isoflavone dose, and to a lesser extent with the lower dose, reduced the magnitude of the ORX-induced decreases in trabecular bone volume (BV/TV) and trabecular number (Th.N) and increase in trabecular separation (Tb.Sp) at the femoral neck site. These modulations of trabecular microstructural properties by isoflavones may be due to increased mRNA levels of alkaline phosphatase (ALP), collagen type I (COL), and osteocalcin (OC), which are associated with enhanced bone formation. These findings confirm our earlier observations that the modest bone protective effects of soy isoflavones are due to increased rate of bone formation.