RBP4 and type 2 diabetes mellitus: NAFLD, non-alcoholic fatty liver disease; MASLD, metabolic dysfunction-associated steatotic liver disease; T2DM, type 2 diabetes mellitus; FGF21, fibroblast growth factor 21; ANGPTL8, angiopoietin-like protein 8; RBP4, retinol-binding protein 4; LECT2, leukocyte cell-derived chemotaxin 2; SeP, selenoprotein P; USG, ultrasonography; AUC, area under the curve; ELISA, enzyme-linked immunosorbent assay; BMI, body mass index; SD, standard deviation; PTEN, phosphatase and Tensin homolog; IL-6, interleukin-6