A total of 608 subjects provided data for the analysis, which reported the favourable effects were significant with consumption of more than 90 mg of isoflavones per day. The results are published on-line in the journal Clinical Nutrition. Previous studies have reported conflicting results concerning soy isoflavones (40 to 99 mg/d doses) and bone health for postmenopausal women. But the new meta-analysis adds to the debate by reporting that such doses of soy isoflavones (less than 90 mg/d) may improve bone density. 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 intake of isoflavones varied greatly amongst the trials used for the new meta-analysis, ranging from 4.4 to 150 mg/day, while trial duration also ranged from three to 24 months, state the researchers from Peking University (China), University of Yamanashi (Japan), Soochow University (China). Lead author De-Fu Ma reports that isoflavone consumption was associated with a 20.6 mg per sq. cm increase in spine bone mineral density (SBMD), compared to subjects consuming placebo. Moreover, isoflavone intake of more than 90 mg/day and lasting at least six months was associated with a 28.5 and 27 mg per sq. cm increased in SBMD, respectively. In addition, the spine bone mineral content (SBMC) was 0.93 grams higher on average in subjects taking isoflavones, compared to placebo.
"The mechanism of the SBMD-increasing effect of isoflavones is not well understood," wrote the researchers. "Isoflavones may combine with the estrogen receptor, albeit with lower affinity than 17-beta-estradiol, and stimulate estrogen activity, thus having an estrogenic effect on bone. "On the other hand, soy diets were shown to increase calcium absorption in rats. "In addition, there are also some human studies reporting that isoflavones in soy protein isolate can significantly increase bone formation by stimulating serum bone-specific alkaline phosphatase synthesis or significantly decrease bone resorption by inhibiting urinary deoxypyridinoline incretion." "Future randomised controlled trials should include a larger sample size, use of graded isoflavone dosages and an examination of the long-term effects of isoflavone supplementation on bone mass and fracture risk," they concluded.
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. Source: Clinical Nutrition Published on-line ahead of print, doi:10.1016/j.clnu.2007.10.012 "Soy isoflavone intake increases bone mineral density in the spine of menopausal women: Meta-analysis of randomized controlled trials"
Authors: D.-F. Ma, L.-Q. Qin, P.-Y. Wang, R. Katoh