RBP4 and obesity due to melanocortin 4 receptor deficiency: In addition, although previous reviews have comprehensively reviewed RBP4 levels in individuals with obesity and metabolic dysfunctions [23–26], for insulin resistance and secretion, the two main mechanisms of T2DM, studies have focused mainly on RBP4 and insulin resistance and have paid little attention to RBP4 and pancreatic β-cell function.