Effect of soy protein containing isoflavones on endothelial and vascular function in postmenopausal women: a systematic review and meta-analysis of randomized controlled trials

Importance: The beneficial role of soy protein in cardiovascular health has been well documented in observational studies. However, evidence from clinical trials on effects of soy protein on endothelial function in postmenopausal women has been conflicting. Objective: We aimed to systematically review and conduct a meta-analysis of randomized controlled trials (RCTs) investigating the impact of soy protein supplement containing isoflavones on endothelial function in postmenopausal women. Evidence review: We searched PubMed-Medline, SCOPUS, Embase, and Google Scholar until March 2020 to find RCTs evaluating the impact of soy protein supplementation on endothelial function parameters. Random effects model (using DerSimonian-Laird method) was applied to synthesize quantitative data. We performed the leave-one-out method for sensitivity analysis. To quantitatively assess heterogeneity, the I-2 index was applied. Findings: From a total of 267 studies identified from the initial search 15 and 5 studies were considered appropriate for inclusion into the systematic review and meta-analysis, respectively. In the meta-analysis, an insignificant enhancement in flow-mediated dilation (FMD) after soy protein supplementation (0.882%; 95% CI: -1.059 to 2.822; P = 0.373) was found. However, subgroup analysis showed that supplementation of isolated soy protein had significant effect on FMD (3.39%; 95% CI: 0.733-6.01; P = 0.01). Conclusions and Relevance: Our findings suggest that soy protein supplementation does not lead to meaningful improvement in FMD in postmenopausal women. However, this finding is based on a limited number of studies. Additional high-quality large-scale RCTs are warranted.