Soy isoflavones plus soy protein effects on serum concentration of leptin in adults: A systematic review and meta-analysis of randomized clinical trials

Background: Some studies conducted on soy isoflavones and soy protein reflected that leptin can be reduced by soy isoflavones or soy protein. The aim of present study is to assess the effect of soy isoflavones plus soy protein on serum concentration of leptin among adults. Methods: To find randomized clinical trails (RCTs) assessing the effect of soy isoflavones containing soy protein on serum concentration of leptin in participants with age ≥18 years old, databases including PubMed, Scopus, ISI Web of Science, Cochrane and ClinicalTrials.gov were searched up to March 2021. We calculated the effect size by the mean change from baseline of leptin concentrations and its standard deviation for intervention and comparison groups. DerSimonian and Laird random effects model was used to estimate the overall summary effect and the heterogeneity. Risk of bias was conducted by Cochrane Collaboration's tool. Study protocol was registered in PROSPERO (No. CRD42021228366). Results: Eight articles with 421 participants and six articles with 336 participants were known to be eligible for systematic review and meta-analysis, respectively. A pooled analysis revealed that the combination of soy isoflavones and soy protein had a borderline non-significant on serum concentration of leptin (weighted mean difference (WMD): −1.03 ng/ml, 95% confidence interval (CI): −2.11, 0.05 ng/ml). This combination reduced serum leptin levels in studies with baseline levels of leptin ≤22 ng/ml, in short-term trials (≤56 days), studies that used soy isoflavones with dose ≤96 mg/day and among subjects with health risk factors or diseases. Conclusion: Soy isoflavones plus soy protein had a non-significant decreasing effect on the serum concentration of leptin. However, making firm conclusion needs more studies on participants with different health condition, and different soy isoflavones doses and intervention duration.