"Isoflavones derived from red clover and soy have weak estrogenic and antiestrogenic activities and are able to bind to estrogen receptors, potentially mediating hot flashes triggered by estrogen deficiency," explained lead author Heidi Nelson from the Oregon Health and Science University.
The new study, published in the latest issue of the Journal of the American Medical Association (Vol. 295, pp. 2057-2071), performed a meta-analysis of six red clover isoflavone trials and 11 soy isoflavone trials. Only published, English-language, randomized, double-blind, placebo-controlled trials were considered.
Isoflavones are well known phytoestrogens - active substances derived from plants that have a weak estrogen-like action.
Isoflavones from soy have been shown to provide a number of health benefits, including the promotion of heart health and the maintenance of bone health in post-menopausal women.
They have also been studied for their role in cancer prevention and slowing down the ageing process in peri-menopausal women, and have proved to be a popular alternative to hormone replacement therapy for those wishing to control menopause symptoms without resorting to drugs.
The researchers found that, for the red clover isoflavone studies, only one trial reported a significant improvement in the frequency of hot flushes during the 12-weeks of intervention.
Three trials using soy isoflavone supplements, with daily doses ranging from 50 to 150 milligrams and containing genistein, daidzein and genistein, or daidzein, genistein and their glucoconjugates, reported significant improvements in the frequency of hot flashes.
When all 11 soy isoflavone trials were combined with respect to the length of the trial, all combined results favored the soy extracts, although it is not clear if this benefit is statistically significant. The authors do not discuss these combined results.
"Hot flash frequency was not reduced when all the trials of red clover isoflavone extracts were combined, and results for soy isoflavone extracts were contradictory even among the largest and highest quality trials," concluded Nelson.
There are several important limitations concerning this meta-analysis, including the small number of trials used. Of the 4249 abstracts identified, only 43 trials met the inclusion criteria, with 17 concerning isoflavones. Highly selective small sample populations, short follow-up times and the use of self-reporting of adverse effects are also limitations.
Placebo effects for hot flushes are also said to be large, while dropout rates in the placebo groups are high due to a lack of effect. All of these factors undermine to some extent the conclusions of this meta-analysis, and, it could be argued, that the authors do not answer the questions they asked.
The authors rightfully call for "larger, more rigorous, and more standardized trials, which include head-to-head as well as placebo comparisons… to accurately determine relative benefits and adverse effects."
It is unclear if these results will affect the postmenopausal women isoflavone market, with significant research focusing of the benefits of soy isoflavones and bone health already in the public domain.
Indeed, 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.
Demand for soy proteins and other products has been growing rapidly, driven largely by the research showing its health benefits. Market analysts The Freedonia Group predict that by 2007 US demand alone for soy products will rise by nearly five per cent each year to $8.23 (€6.7) bn.