INS and type 2 diabetes mellitus: This meta-analysis, which enrolled 39 RCTs, demonstrated that vitamin D supplementation significantly improved glycaemic markers in T2DM patients, with notable reductions in fasting blood glucose (FBG) (−0.46 mmol/L), HbA1c (−0.30%), HOMA-IR (−0.44), and fasting insulin (−1.06 μU/mL).