Estrogen is known to increase bone mass density and to improve bone turnover markers, but is associated with a higher risk of hormone related cancer. Therefore, many women seek alternatives to estrogen to improve their bone health. Studies show that, in spite of their significantly weaker estrogen action, isoflavones have bone building effects. This is the reason why in Asian countries osteoporosis is very rare after menopause, despite their low consumption of dairy products. Whereas genistein is mainly responsible for the reduction of menopausal symptoms, daidzein is the most protective isoflavone against osteoporosis. In the human body there is a constant process of breaking down and remaking of bones. The osteoclasts are cells which break down the bone, whereas the osteoblasts build up bone. If the activity of the osteoclasts is higher than that of the osteoblasts, there will be a gradual reduction of bone strength, leading to osteoporosis. Isoflavones inhibit the activity of osteoclasts, while stimulating the activity of osteoblasts. This moderate stimulus is sufficient enough to stimulate a continuous formation of bones.
ChenYu-Ming demonstrated that soy isoflavones have a beneficial effect on bone mineral content, especially in women who were over 4 years in menopause, had a lower body weight or a lower calcium intake . A study by Tsuang using quite large quantities of isoflavones (25mg/day) in ovariectomized rats, showed that isoflavones reduced bone loss and actually increased the bone mineral densities of long bones by up to 60% . A recent meta-analysis published in Maturitas and co-workers reviewed 5 studies about the effects of isoflavones on bone mineral density and bone turnover makers . They concluded that soy isoflavones significantly increased the bone mineral density of the lumbar spine and moderately decreased urine deoxypyridinoline, a bone resorption marker. Soy isoflavones may prevent osteoporosis and improve bone strength in postmenopausal women.
 Chen et al. Beneficial effect of soy isoflavones on bone mineral content was modified by years since menopause, body weight, and calcium intake: a double-blind, randomized, controlled trial. Menopause. 2004 May-Jun;11(3):246-54.
 Tsuang et al. Isoflavones prevent bone loss following ovariectomy in young adult rats. Journal of Orthopaedic Surgery and Research 2008, 3:12.
 Taku et al. Soy isoflavones for osteoporosis: An evidence-based approach. Maturitas. 2011 Sep 27