The effects of melatonin supplementation on inflammatory markers among patients with metabolic syndrome or related disorders: a systematic review and meta-analysis of randomized controlled trials

Objective: This systematic review and meta-analysis of randomized controlled trials (RCTs) was carried out to determine the effect of melatonin supplementation on the inflammatory markers among individuals with metabolic syndrome (MetS) and related disorders. Methods: We searched the following databases up to March 2018: PubMed, MEDLINE, EMBASE, Web of Science, and Cochrane Central Register of Controlled Trials. Three reviewers independently assessed study eligibility, extracted data, and evaluated risk of bias of included primary studies. Statistical heterogeneity was assessed using Cochran’s Q test and I-square (I²) statistic. Data were pooled using the random effect model and standardized mean difference (SMD) was considered as the summary effect size. Results: Six trials of 317 potential reports were identified to be suitable for our meta-analysis. The pooled results using random effects model indicated that melatonin supplementation significantly reduced C-reactive protein (CRP) (SMD = − 1.80; 95% CI − 3.27, − 0.32; P = 0.01; I²: 95.2) and interleukin 6 (IL-6) concentrations (SMD = − 2.02; 95% CI − 3.57, − 0.47; P = 0.01; I²: 91.2) among patients with MetS and related disorders; however, it did not affect tumor necrosis factor-α (TNF-α) concentrations (SMD = − 1.87; 95% CI − 3.81, 0.07; P = 0.05; I²: 94.4). Conclusions: In summary, the current meta-analysis showed the promising effect of melatonin administration on reducing CRP and IL-6, but not TNF-α levels among patients with MetS and related disorders. Additional prospective studies are recommended using higher supplementation doses and longer intervention period.