Vitamin D Supplementation in Type 2 Diabetes: A Systematic Review of Randomized Controlled Trials

Vitamin D deficiency is prevalent in type 2 diabetes mellitus (T2DM) and may influence metabolic control. The efficacy of supplementation remains contested. This systematic review aimed to evaluate the effect of vitamin D supplementation on primary glycemic outcomes (HbA1c, fasting blood glucose [FBG], HOMA-IR) and secondary metabolic parameters (lipid profile, blood pressure, anthropometrics) in adults with T2DM or prediabetes. This review was conducted according to Cochrane methodology and reported following PRISMA 2020 guidelines. Randomized controlled trials (RCTs) were identified via systematic searches of PubMed, Scopus, and Web of Science up to March 2025. Study selection, data extraction, and risk of bias assessment (using Cochrane RoB 2 tool) were performed in duplicate. A narrative synthesis was performed due to substantial heterogeneity. Thirty RCTs (n = 2627) were included. Studies varied in vitamin D dosage (1,000 to 600,000 IU), duration (8-52 wk), and participant baseline status. Overall, supplementation consistently increased serum 25(OH)D levels. For primary outcomes, significant reductions in HbA1c and FBG were reported in approximately half of the studies, particularly among participants with baseline vitamin D deficiency. Effects on HOMA-IR were mixed. Secondary outcomes (lipids, blood pressure, BMI) showed minimal or inconsistent changes. Vitamin D supplementation may improve glycemic control in individuals with T2DM who are vitamin D deficient, but appears ineffective in those with sufficient levels. It does not consistently improve other metabolic parameters. Targeted supplementation based on individual vitamin D status is recommended, rather than universal administration.