BackgroundThe effects of vitamin D supplementation on mild hepatic dysfunction and glycemic control are unclear in children and adolescents with either type 1 (T1D) or type 2 diabetes (T2D).HypothesisVitamin D supplementation will improve hepatic dysfunction and glycemic control.AimTo determine the effect of vitamin D supplementation on alanine transaminase (ALT), hemoglobin A1c (HbA1c), and serum 25-hydroxyvitamin D [25(OH)D] concentration.Subjects and MethodsA retrospective study of 131 subjects with either T1D (n = 88∶46 females, 42 males), or T2D (n = 43∶26 females, 17 males) of ages 3–18 years between 2007–2013. All subjects had (1) a diagnosis of diabetes for >12 mo, (2) received vitamin D supplementation for the management of vitamin D deficiency (3) had baseline and subsequent simultaneous measurements of HbA1c, ALT, and 25(OH)D. Vitamin D deficiency was defined as 25(OH)D concentration of <50 nmol/L (20 ng/mL).ResultsAt baseline, vitamin D deficiency occurred in 72.1% of patients with T2D and in 37.5% of T1D patients (p<0.001). Patients with T2D had significantly higher values for BMI SDS (p<0.001), alanine transaminase (ALT) (p = 0.001), but lower 25(OH)D (p<0.001), and no difference in HbA1c (p = 0.94), and total daily dose (TDD) of insulin per kg body weight (p = 0.48) as compared to T1D patients. After 3 months of vitamin D supplementation, there was a significant increase in 25(OH)D in both T2D (p = 0.015), and T1D patients (p<0.001); significant reduction in BMI SDS (p = 0.015) and ALT (p = 0.012) in T2D, but not in T1D. There was a clinically-significant decrease in HbA1c in T2D from 8.5±2.9% at baseline to 7.7±2.5 at 3 mo, but not in T1D, 8.5±1.2 to 8.53±1.1%.ConclusionsVitamin D supplementation in subjects with T2D was associated with statistically significant decreases in BMI SDS, ALT, and a clinically-significant decrease in HbA1c.