Impressive data from the many studies on cultured bone cells and rat models of
postmenopausal osteoporosis support a significant bone-sparing effect of the soy
isoflavones genistein and
daidzein. Translating this research to the clinic has been more challenging, and thus far only a few clinical studies have attempted to tease out the influence of
phytoestrogens on bone from the many other components of the diet. Human studies have shown promising although variable results. Studies have been mostly of short duration and with relatively small sample sizes, making it difficult to observe significant and accurate changes in bone. Levels of intake of the
soy protein and
isoflavones are varied, and the optimal
isoflavone intake for bone-sparing effects remains to be determined. Clinical studies thus far performed can be broadly divided into those that have assessed biochemical evidence of reduced bone turnover from measurement of
surrogate markers of osteoblast and osteoclast activity, and those that have examined changes in bone mineral density. There are no studies examining effects on fracture rate. This review focuses specifically on the potential influence of
phytoestrogens on bone by examining the evidence from 17 in vitro studies of cultured bone cells, 24 in vivo studies of animal models for
postmenopausal osteoporosis, 15 human observational/epidemiologic studies, and 17 dietary intervention studies. On balance, the collective data suggest that diets rich in
phytoestrogens have bone-sparing effects in the long term, although the magnitude of the effect and the exact mechanism(s) of action are presently elusive or speculative.