Background: Type 1 diabetes mellitus (T1DM) presents significant glycemic management challenges in pediatric populations. Therapeutic role in glucose homeostasis through immunomodulatory and direct metabolic pathways. This study explored whether vitamin D supplementation enhances glycemic regulation in children with T1DM and vitamin D deficiency.

Methods: Participants received either vitamin D3 2000 IU daily or matching placebo. Primary outcomes included changes in HbA1c and continuous glucose monitoring-derived time-in-range (TIR). Secondary endpoints assessed fasting glucose, glycemic variability, C-peptide preservation, inflammatory biomarkers, and safety parameters.

Results: Vitamin D supplementation significantly improved glycemic regulation compared to placebo. HbA1c decreased by 0.8% (95% CI: -1.2 to -0.4%, p<0.001), while TIR increased by 12.3 percentage points versus 2.1% in controls (p<0.001). Time-above-range decreased by 11.6 percentage points, and glycemic variability improved substantially. C-peptide preservation was enhanced (p<0.001), and inflammatory markers (IL-6, TNF-α) decreased significantly in the vitamin D group. The intervention demonstrated excellent safety with minimal adverse events.