• Robust correction of vitamin D deficiency: Serum 25(OH)D increased from 19.2 to 30.9 ng/mL (large effect size, Glass’s Δ = 1.2)• Minimal effect on glycemic control: nonsignificant change in HbA1c (small effect size, Glass’s Δ = 0.1)• Negative predictors: higher insulin dose and BMI predicted lower postsupplementation vitamin D levels• Lifestyle factor: sedentary behavior was associated with higher HbA1c. This evidence concerns the gene INS and vitamin D deficiency.