Phytoestrogen-based medications are commonly used by menopausal women, and especially by obese postmenopausal women, to relieve menopausal symptoms. Substitution of animal with soy protein is often used in weight loss regimens, yet the effect of phytoestrogens, the main constituent of soy foods, on body composition is not completely understood. We conducted a systematic review and meta-analysis to investigate the associations between phytoestrogen supplementation and body weight and the main parameters of body composition in postmenopausal women. A literature search was done using 5 electronic databases from inception to April 2018. Randomized controlled trials (RCTs) with postmenopausal women comparing phytoestrogen supplementation followed by usual diet and placebo were included in the present meta-analysis. From 5932 references, we identified 23 RCTs that met our inclusion criteria, with a total of 1880 postmenopausal women. No association was observed between phytoestrogen supplementation and body weight, body mass index, waist and hip circumference, total fat mass or percentage of body fat. However, the use of phytoestrogens supplementation was associated with a slight decrease in waist-hip ratio; the pooled mean difference was -0.01 cm (95%CI: -0.01 to -0.006). In subgroup analysis, we found a modest decrease in body weight with phytoestrogens supplementation compared with placebo in healthy postmenopausal women [pooled mean difference of changes -0.28 kg (95%CI: -0.52 to -0.04)] and in RCTs with a median number of participants of 66 or less [pooled mean difference of changes -0.49 kg (95%CI: -0.87 to -0.11)]. In contrast, phytoestrogen supplementation was associated with increased body weight in postmenopausal women with preexisting metabolic disorders (prediabetes, type 2 diabetes, prehypertension and hyperlipidemia) [pooled mean difference of changes: 0.78 kg (95%CI: 0.53-1.03)]. In addition, there were some indications that some types of phytoestrogens, such as daidzein, but not soy products or isoflavone mix, could lead to modest adverse changes in body composition in menopausal women. Therefore, future studies should investigate the potential adverse effects of phytoestrogen supplementation on body composition among postmenopausal women.

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