Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials

Objective: This analysis was conducted to determine the efficacy of extracted or synthesized soybean isoflavones in the alleviation of hot flashes in perimenopausal and postmenopausal women. Methods: PubMed and The Cochrane Controlled Clinical Trials Register Database were searched for relevant articles reporting double-blinded randomized controlled trials through December 14, 2010. References within identified articles, as well as peer-reviewed articles that had come to the attention of the authors through other means, were also examined for suitability. This systematic review and meta-analysis, which evaluated the effects of isoflavones on the frequency, severity, or composite score (frequency x severity) of hot flashes compared with placebo was conducted according to Cochrane Handbook guidelines. Results: From 277 potentially relevant publications, 19 trials (reported in 20 articles) were included in the systematic review (13 included hot flash frequency; 10, severity; and 3, composite scores), and 17 trials were selected for meta-analyses to clarify the effect of soybean isoflavones on hot flash frequency (13 trials) and severity (9 trials). Meta-analysis revealed that ingestion of soy isoflavones (median, 54 mg; aglycone equivalents) for 6 weeks to 12 months significantly reduced the frequency (combined fixed-effect and random effects model) of hot flashes by 20.6% (95% CI, -28.38 to -12.86; P < 0.00001) compared with placebo (heterogeneity P = 0.0003, I-2 = 67%; random effects model). Meta-analysis also revealed that isoflavones significantly reduced hot flash severity by 26.2% (95% CI: -42.23 to -10.15, P = 0.001) compared with placebo (heterogeneity, P < 0.00001, I-2 = 86%; random effects model). Isoflavone supplements providing more than 18.8 mg of genistein (the median for all studies) were more than twice as potent at reducing hot flash frequency than lower genistein supplements. Conclusions: Soy isoflavone supplements, derived by extraction or chemical synthesis, are significantly more effective than placebo in reducing the frequency and severity of hot flashes. Additional studies are needed to further address the complex array of factors that may affect efficacy, such as dose, isoflavone form, baseline hot flash frequency, and treatment duration.