Retinol binding protein (RBP) 4 is the only specific transport protein for vitamin A in the circulation whose function is to deliver vitamin to target tissues. In obesity and type 2 diabetes, expression of Glut4 is significantly impaired in adipocytes. Glucose transport via Glut4 is the rate-limiting step for glucose use by muscle and adipose tissue.

Yang et al. noted that adipocytespecific deletion of Gluts led to notable elevation of RBP4 causing systemic insulin resistance, and that reduction of RBP4 improved insulin resistance. This identified a novel role of RBP4 in regulating insulin action and RBP4 is recorded as an adipocytederived hormone. Thus, measurement of serum or plasma RBP4 is a useful means for understanding of metabolic disorders.