Soy isoflavones in the treatment of menopausal symptoms

Due to their structural similarity to the human 17beta-estradiol, isoflavones are classified as phytoestrogens. The main dietary source is soy which mainly supplies genistein and daidzein. In the intestine, daidzein is metabolized to equol, but due to a high variability in the gut microflora, only 30-50 % of the adult population produce significant amounts of equol. Isoflavones are capable of binding to the estrogen receptors ER-alpha and ER-beta, with a higher affinity to ER-beta. Isoflavones have estrogenic or antiestrogenic effects depending on the concentration of endogenous estrogen and the amount and type of estrogen receptors. In Western nations the prevalence of vasomotor symptoms is in the range of 60-85 %, whereas only 25 % of Japanese women suffer from hot flashes. The significantly lower incidence of menopausal symptoms in Asian countries is assumed to be due to the estrogenic effect of a diet rich in isoflavones resulting from the high consumption of soy products. Soy isoflavones, therefore, may also lead to an improvement of menopausal symptoms, especially of hot flashes. However, of 17 randomised, controlled clinical trials, 12 showed no statistically significant improvement of hot flashes after a soy or isoflavone treatment compared to placebo. Five studies have found a significantly greater improvement of vasomotor symptoms in the soy compared to the placebo group. There are several possible explanations for the inconsistent results such as varying amounts of isoflavones, different severity of symptoms in the study samples at baseline or differences in the ability to convert daidzein to equol. To date, a final judgement on the efficacy of soy isoflavones in the treatment of hot flashes is not possible. Due to toxicological concerns, women at high risk for breast or endometrial cancer should not use isoflavone supplements without medical consultation.