Dietary soy protein maintains some indices of bone mineral density and bone formation in aged ovariectomized rats

Hormone replacement therapy (HRT) has been used to prevent osteoporosis in postmenopausal women. However, HRT may increase the incidence of some cancers and has other side effects. There is considerable interest in dietary alternatives that include the consumption of soy and isoflavones derived from soy. The purpose of this study was to determine the effects of dietary soy protein on bone density, formation and resorption in cortical and cancellous bone in aged, ovariectomized rats. Specific emphasis was placed on indices of bone formation. Rats were assigned to the following groups: baseline; sham surgery + casein diet; sham + soy protein diet; ovariectomy (Ovx) + casein diet; Ovx + soy protein diet. The diets were fed for 3 mo. The Ovx Soy group had a greater bone mineral density (BMD) than the Ovx Casein group. There was a trend (P < 0.10) for greater periosteal bone formation rates in the Sham Soy compared with the Sham Casein group. In the Ovx Soy group, indices of endocortical bone formation were greater than those of the Ovx Casein group. There were no significant differences in resorption indices or endochondral growth (bone elongation) rates with soy in either the Sham or Ovx groups. In cancellous bone, the double-labeled surface and bone formation rates were greater in the Ovx Soy group than in the Ovx Casein group. These results show that dietary soy had a beneficial effect on the preservation of BMD associated with estrogen deficiency bone loss in aged rats. These data also show that at the tissue level, soy functions in a manner different from estrogen by increasing or sustaining elevated bone formation rates after ovariectomy.