Effectiveness of Vitamin D Supplementation on Biochemical, Clinical, and Inflammatory Parameters in Patients with Different Types of Diabetes: A Systematic Review and Meta-Analysis

Background and Aims: Numerous clinical and observational studies have examined the role of vitamin D in glycemic control and metabolic regulation among diabetic patients, but findings remain inconsistent. This meta-analysis aimed to assess the effects of vitamin D supplementation on glycosylated hemoglobin (HbA1c%), HOMA-IR, HOMA-β, LDL cholesterol, total cholesterol, triglycerides, fasting insulin, fasting plasma glucose, C-reactive protein, and the likelihood of reversion to normoglycemia in prediabetic individuals. Methods: A comprehensive search of multiple databases was performed using keywords including “diabetes mellitus,” “type 2 diabetes,” “vitamin D supplementation,” and “VD supplementation.” Twenty studies met the inclusion criteria. Results: Vitamin D supplementation was associated with significant improvements across several parameters, including HOMA-β (SMD = 0.71; 95% CI: 0.63–0.80; p < 0.00001), HDL cholesterol (SMD = 0.07; 95% CI: 0.05–0.09; p < 0.00001), and others (SMD = −0.40; 95% CI: −0.45 to −0.34; p < 0.00001). Conclusions: Vitamin D supplementation appears to provide beneficial effects on glycemic, lipid, and inflammatory markers in patients with diabetes and prediabetes. Specifically, supplementation significantly reduced HbA1c%, HOMA-IR, LDL cholesterol, total cholesterol, triglycerides, fasting insulin, fasting glucose, and C-reactive protein while increasing the rate of normoglycemia among prediabetic individuals. Further research is needed to strengthen the evidence base regarding vitamin D’s role in diabetes management.