Limiting bone loss in post-menopausal women could ease the burden of osteoporosis, a disease that affects half of all women over the age of 50. According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7 bn in Europe.
Previous studies from China have linked soy isoflavones to increases in bone mineral density (BMD), while a recent large study in the Archives of Internal Medicine (2005, Vol. 165, pp. 1890-1895) reported that high soy consumption was linked with a 48 per cent decrease in fractures for women who had been menopausal for less than 10 years.
The new study, published online in the February 2006 Journal of Nutritional Biochemistry (doi:10.1016/j.jnutbio.2006.01.003), reports the effects of isoflavone supplements on the BMD and bone mineral content (BMC) of 42 women who had been menopausal for less than 13 years.
The researchers, led by Shih-Yi Huang from Taipei Medical University, Taiwan, divided the volunteers into three groups - 15 women received 100 mg isoflavones, 15 women received 200 mg isoflavones, and 12 women acted as the control group.
The participants took tablets containing 125 mg of soy protein extracts, of which 50 mg were isoflavones (35.5 mg genistein, 14.5 mg daidzein).
After one year of supplementation, the researchers measured both BMD and BMC using dual-energy X-ray absorptiometry (DEXA), and wrote: "In our study, compared to the general loss of BMD in the control group, the BMD of L1-3 [lumbar vertebrae] and the BMC of the trochanter [thigh bone] significantly increased in the 100 mg isoflavone group."
However, the researchers reported that the 200 mg supplement group did not exert a dose-dependent effect.
"No additional benefit in BMD by 200 mg isoflavone was found even when comparing all of the other percentage changes within the two treated groups," they wrote.
Indeed, use of 200 mg of isoflavones may lead to long-term adverse effects. Recent animal studies have linked genistein and/or daidzein with cancers, while in vitro studies have reported the two isoflavones can induce DNA damage. These claims have not been reproduced in actual human trials.
The mean daily intake from a Chinese diet, a diet known to be a rich in soy, is about 20 g of soy protein, which is equivalent to about 40 mg of isoflavones.
"Our study was able to identify the stable protective effect in early menopausal women produced by 100 mg isoflavones in an isolated form.
The highly active turnover phenomenon shown after one year of supplementation by related bone markers indicates the necessity of studies longer than one year for high-dosage trials," concluded the researchers.
The EU is the largest soybean and soy meal importer, with mostly consisting of Monsanto Round-Up Ready soybean, which is cultivated in the US, Brazil and Argentina.