RBP4 and type 2 diabetes mellitus: With the impact of gut microbiota it releases inflammatory adipokines, such as fatty acid binding protein 4 (FABP-4), acylation-stimulating protein (ASP), retinol-binding protein 4 (RBP4), lipocalin-2 (LCN2), and chemerin, etc., which are associated with increased inflammation, obesity, insulin resistance, and eventually T2DM (Lee et al., 2019) (Figure 2).