Vitamin D supplementation reduces insulin requirements in children and adolescents with type 1 diabetes: a systematic review and meta-analysis of randomized trials

Background Vitamin D has been proposed to support immune regulation and improve insulin sensitivity in type 1 diabetes (T1D), but the extent of its added clinical benefit remains unclear. This review summarizes randomized evidence regarding its influence on insulin use, metabolic outcomes, and β-cell function. Methods Searches across five major databases through October 14, 2025, identified randomized controlled trials evaluating vitamin D supplementation in T1D. Total daily insulin dose (TDD) served as the primary endpoint, while glycated hemoglobin (HbA1c) and C-peptide measurements were assessed as secondary outcomes. Pooled mean differences (MD) with 95% confidence intervals were generated using Review Manager (RevMan) version 5.3 and STATA MP version 17.0, with risk of bias and certainty assessed through established tools including GRADE. Results Nine trials encompassing 370 children and adolescents qualified for analysis. Vitamin D supplementation produced a statistically significant reduction in total daily insulin dose (MD = − 0.13 U/kg/day; 95% CI − 0.22 to − 0.04). No meaningful differences were observed in HbA1c, stimulated C-peptide (SCP) levels measured after standardized stimulation tests, or fasting C-peptide (FCP) levels. Supplementary analyses of inflammatory and immunologic indicators likewise showed no significant differences between the vitamin D and control groups. Conclusion Vitamin D supplementation may reduce daily insulin requirements in young individuals with T1D; however, its overall effect on glycemic control and preservation of β-cell function remains uncertain. Future well-designed randomized controlled trials, particularly in newly diagnosed patients with preserved residual β-cell function, are warranted to determine whether earlier intervention provides greater clinical benefits. Clinical trial registration Not applicable.