Vitamin D Supplementation Improves Glycemic Control, Inflammation, and Lipid Profile in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Background: Vitamin D deficiency has been increasingly associated with the pathogenesis of type 2 diabetes mellitus (T2DM), influencing insulin resistance, β-cell function, inflammation, and cardiovascular risk. However, evidence from randomized controlled trials (RCTs) remains inconsistent. Objective: To systematically evaluate the effect of vitamin D supplementation on glycemic control, insulin resistance, inflammatory markers, and lipid profile in individuals with T2DM. Methods: A systematic review and meta-analysis of randomized controlled trials was conducted in accordance with PRISMA 2020 guidelines. PubMed, Scopus, and Embase were searched from 2010 to 2026. Eighteen RCTs involving approximately 2,000 participants were included. A random-effects model (DerSimonian–Laird) was used to calculate pooled mean differences (MD) with 95% confidence intervals (CI). Heterogeneity was assessed using the I² statistic. Subgroup analyses were performed based on geographic region, study population, and sample size. Sensitivity analysis using a leave-one-out approach and GRADE assessment were conducted to evaluate robustness and certainty of evidence. Results: Vitamin D supplementation significantly improved multiple metabolic outcomes, including HbA1c (MD = −0.42%, 95% CI: −0.32 to −0.16; I² = 27%), fasting blood glucose (MD = −9.5 mg/dL; I² = 16%), and HOMA-IR (MD = −0.98; I² = 60%). Significant reductions were also observed in C-reactive protein (MD = −0.46 mg/L; I² = 30%), low-density lipoprotein cholesterol (MD = −5.85 mg/dL; I² = 22%), and triglycerides (MD = −6.51 mg/dL; I² = 0%). Subgroup analysis demonstrated greater effects in Asian populations. Sensitivity analysis confirmed the stability of results, and GRADE assessment indicated moderate to high certainty of evidence. Conclusion: Vitamin D supplementation is associated with modest but significant improvements in glycemic control, inflammation, and lipid parameters in individuals with T2DM. These findings support its role as a potential adjunct in metabolic management, particularly among vitamin D–deficient populations.