The Role of Vitamin D Supplementation in Type 1, Type 2, and Gestational Diabetes: A Comprehensive Updated Narrative Review.

Vitamin D has emerged as a modulatory factor in the pathogenesis and management of diabetes mellitus due to its influence on pancreatic β-cell function, immune regulation, and inflammatory pathways. This narrative review critically examines mechanistic and clinical evidence linking vitamin D status with type 1 diabetes (T1DM), type 2 diabetes (T2DM), and gestational diabetes (GDM). In T1DM, vitamin D's immunomodulatory effects are thought to protect β-cells from autoimmune destruction; epidemiological studies associate vitamin D sufficiency with lower T1DM incidence and improved glycemic control, although causality remains under investigation. In T2DM, vitamin D deficiency is associated with worsened metabolic control and may contribute to disease development in at-risk individuals; however, it does not influence the initial onset of T2DM in patients who are already diagnosed. Intervention trials indicate that correcting the deficiency can modestly improve insulin sensitivity, β-cell function, and metabolic parameters. GDM has similarly been linked to hypovitaminosis D, with low maternal vitamin D levels associated with higher GDM risk and adverse perinatal outcomes; mechanistic insights suggest that adequate vitamin D supports glucose homeostasis in pregnancy, and emerging trials demonstrate improved insulin resistance with maternal vitamin D supplementation. Across these diabetes subtypes, maintaining sufficient vitamin D levels appears to confer metabolic benefits and may serve as an adjunct to current preventive and therapeutic strategies. However, definitive evidence from large-scale trials is required to establish optimal vitamin D supplementation protocols and confirm its efficacy in diabetes care.