The study by Chinese researchers, published in the May/June issue of Menopause (vol 11, no 3, pp246-254), shows that postmenopausal women taking 80 mg of isoflavones daily saw positive benefits on bone mineral content, particularly if they were more than four years into menopause, had lower body weight or had a lower calcium intake.
"Many studies have shown that soy isoflavones have an effect in preventing oestrogen-related bone loss, but no data reported whether such an effect could be influenced by other important factors affecting bone loss," noted the researchers.
There have also been studies finding no effect on bone health, such as that published last week in JAMA.
But the current research, funded by soy isoflavone maker Acatris, shows that certain factors may be so important to bone health that they overpower any potential benefit seen from isoflavones.
The team from the Chinese University of Hong Kong enrolled 203 women, aged 48 to 62 years old, to receive either placebo with 500 mg calcium and 125 IU of vitamin D, a mid-dose (40 mg isoflavones with 500 mg calcium and 125 IU of vitamin D), or a high-dose (80 mg isoflavones with 500 mg calcium and 125 IU of vitamin D) supplement everyday for one year.
The researchers found significant benefit of high dose isoflavone supplementation on bone mineral content at the total hip and trochanter compared to those women on low-dose or placebo. There was no significant improvement in bone mineral density.
In addition, there was a significant benefit to women had been postmenopausal for more than four years, as well as those with a lower body weight.
Women with a lower level of calcium intake, less than 1095 mg on average per day, also saw such an effect although in those with a high calcium intake, soy isoflavones seemed to make no difference to their bone health.
The researchers write that the results were not surprising, as "the body weight is a much stronger predictor of bone mass than many other factors, including menopause status".
They also note that "soy isoflavones at current doses may not have any additional benefits to bone mass among women with a high body weight."
"Our findings suggest that the beneficial effects of isoflavone supplementation could be potentiated in women with lower body weight or in women with low dietary calcium intake," they conclde.