Research from the University of Messina in Italy points to genistein, and isoflavone from soy (Glycine max), as a possible treatment for poor bone density and osteopenia in post-menopausal women. While the scientists didn't track fractures or other negative outcomes of poor bone density, they did document an increase in BMD in the treatment arm of the study (the whole study included 389 women). The general conclusions indicate that soy contains phytoestrogenic isoflavones that reverse the trend of bone loss that occurs in post-menopausal women.
The researchers caution that, genistein being a phytoestrogen, the use of soy is dangerous for those with a history of endometrial or breast cancer (considered to often be estrogen-dependent). In response, I offer two considerations: first, no research has even indicated conclusively that phytoestrogens stimulate cancer in vivo, and additional research points to the possible cancer-preventive role of these chemicals. Second, ethnic and geographic groups that traditionally use an abundance of soy in their diet generally show lower rates of breast cancer.
As with many plant chemicals taken in their unadulterated states (and especially as crude, whole preparations), I suspect soy isoflavones play an amphoteric role in the human physiology, increasing what is deficient and reducing the effects of that which is excessive -- in this case, helping to modulate the effects of estradiol.