Elevated blood apelin levels in type 2 diabetes mellitus: A systematic review and meta-analysis

•Apelin blood level in T2DM was investigated using meta-analysis.•26 case-control studies, including 1103 cases and 1079 controls, were retrieved.•The selected studies were heterogeneous and random-effects model was applied.•The pooled SMD for apelin blood levels was 2.137 (95% CI, 1.579–2.696).•The apelin blood level in T2DM is associated with disease pathogenesis. Apelin is a circulatory blood peptide acting as a ligand for the orphan G protein-coupled receptor known as APJ. Whether apelin blood levels can affect the pathogenesis of type 2 diabetes mellitus is an open question. In the present study, we aimed to assess the levels of circulatory apelin peptide in the type 2 diabetic subjects using systematic review and meta-analysis under random-effects model and standardized mean difference (SMD) as the effect size. For heterogeneity testing, Q and I2% statistic indices as well as meta-regression were applied. Using specialized biomedical online databases of Pubmed, Pubmed Central, Medline, Google scholar, Scopus and Embase databases without the beginning date restriction until July 2018, the systematic review retrieved nine studies for meta-analysis after fulfilling the inclusion and exclusion criteria. Analysis of Q and I2% statistic indices as well as meta-regression showed a high heterogeneity in the 16 selected studies (737.578 and 96.475, respectively), thus, the random-effects model was chosen. The primary analysis for the main hypothesis on a total number of 1102 cases and 1078 healthy control subjects found that the weighted pooled SMD for the impact of apelin blood concentration in type 2 diabetes mellitus was as follows: SMD = 2.136 (95% confidence interval, 1.580–2.693). The P-value for the significance of the combined SMD examined by the z-test was 0.000 and thus, it was clearly significant. This meta-analysis presents evidence that apelin circulatory levels are higher in type 2 diabetic subjects than normal controls.