Circulating retinol binding protein 4 levels in nonalcoholic fatty liver disease: a systematic review and meta-analysis

Background: Retinol binding protein 4 (RBP4) is implicated in obesity, insulin resistance and type 2 diabetes mellitus that are closely associated with nonalcoholic fatty liver disease (NAFLD). However, recent investigations regarding circulating RBP4 levels in NAFLD are conflicting. This meta-analysis is to determine whether NAFLD, non-alcoholic steatohepatitis (NASH) and simple steatosis (SS) patients have altered RBP4 levels. Methods: We performed a systematic search in PubMed, EMBASE and The Cochrane Library up until 18 March 2017, and 12 studies comprising a total of 4247 participants (2271 NAFLD patients and 1976 controls) were included in the meta-analysis. Results: There were no significant differences of circulating RBP4 levels in the following comparisons: (1) NAFLD patients vs controls (standardized mean differences [SMD]: 0.08; 95% CI: -0.21, 0.38); (2) NASH patients vs controls (SMD: -0.49; 95% CI: -1.09, 0.12); (3) SS patients vs controls (SMD: -0.72; 95% CI: -1.64, 0.20) and (4) NASH vs SS patients (SMD: -0.04; 95% CI: -0.32, 0.24). The results remained essentially unchanged in the comparisons between NAFLD patients and controls after excluding single individual study or bariatric studies (n = 2). No significant publication bias was detected. However, there was significant heterogeneity among studies and the subgroup and meta-regression analyses did not find the potential sources. Conclusions: Circulating RBP4 levels may not be associated with NAFLD. Further prospective cohort studies are required to confirm these findings.