INS and type 2 diabetes mellitus: To fill this gap and promote the development of intervention strategies for the prevention and treatment of diabetes, the aim of this study was to assess the correlations between serum 25(OH)D levels and TIR/GV parameters, in T2DM populations, while simultaneously investigating the associations of 25(OH)D levels with pancreatic β-cell function (including glucose-stimulated insulin secretion function) and insulin resistance.