Effects of soy isoflavone and/or estrogen treatments on bone metabolism in ovariectomized rats

This study investigated whether soy isoflavone intake, with or without estrogen treatment, can reduce postmenopausal bone loss, and whether soy isoflavones can be an alternative for estrogen replacement therapy using a postmenopausal osteoporotic rat model in which ovariectomized female rats were fed a low calcium, high fat diet. Nine-week-old female Sprague-Dawley rats were ovariectomized and then fed low (0.1 %) calcium diets with or without soy isoflavone supplementation (80 or 160 ppm) for 6 weeks. Some ovariectomized rats were fed the same diets but also injected with estrogen (10 mu g/kg of body weight) subcutaneously. Serum calcium and phosphate levels were normal in all rats. Serum alkaline phosphatase activities were not affected by the treatments. Serum tartrate-resistant acid phosphatase activities and urinary hydroxyproline levels were not different between experimental groups. Bone mineral (calcium and phosphorus) contents were increased in the rats supplemented with 80 ppm soy isoflavone or the rats treated with only estrogen without soy isoflavone. Therefore, the effect of 80 ppm soy isoflavone supplementation was the same as estrogen injection, but there was no beneficial effect from combining soy isoflavones and estrogen injections. When 160 ppm soy isoflavone was used, the benefits were lessened or disappeared altogether. These results suggest that appropriate soy isoflavone supplementation prevents postmenopausal bone loss without estrogen injection and may have efficacy as an alternative to estrogen therapy.