The results revealed that lower 25(OH)D levels increased the risk of T2DM (Pooled ESRR = 1.34; 95%CI: 1.16, 1.53), Vitamin D supplementation ameliorated FBG (ES = −0.56; 95%CI: −1.00, −0.11), HbA1c (ES = −0.11; 95%CI: −0.20, −0.02), insulin (ES = −0.38; 95%CI: −0.59, −0.18) and HOMA-IR (ES = −0.37; 95%CI: −0.57, −0.16) in T2DM patients, especially in those with Vitamin D deficiency (FBG = −0.98; HbA1c = −0.27; HOMA-IR = −0.52). The gene discussed is INS; the disease is type 2 diabetes mellitus.