Soy protein: Its effects on intestinal calcium transport, serum vitamin D, and insulin-like growth factor-I in ovariectomized rats

Recent reports indicate that soy protein and its isoflavones exert beneficial effects on bone in ovarian hormone deficiency. These positive effects, in part, may be due to improved intestinal calcium (Ca) absorption. We examined the role of soy protein or its isoflavones on intestinal Ca transport using ovariectomized rats. Rats were fed either a casein- or a soy protein-based diet with normal or depleted isoflavone levels. After 35 days of treatment, rats were exsanguinated and isolated cells from all intestinal segments were used to measure in vitro Ca transport. Ovariectomy significantly decreased the rates of Ca transport in duodenal and colonic cells, which were prevented by soy protein with normal isoflavone content. This enhanced Ca transport by isoflavones present in soy protein appeared to be independent of circulating insulin-like growth factor-I, 25(OH)vitamin D, or 1,25(OH)(2)vitamin D as these variables were unaffected by dietary treatments. The findings of this study imply that soy isoflavones may promote Ca absorption in a manner analogous to that of estrogen but without exerting uterotrophic effect. Further studies are needed to explore the mechanism(s) by which soy protein or its isoflavones promote intestinal Ca absorption in ovarian hormone deficiency.