Vitamin D Supplementation in Deficiency States and Combined Calcium-Vitamin D Therapy in Diabetes Prevention and Management: A Systematic Review of Clinical Evidence.

Vitamin D and calcium are necessary for maintaining metabolic and endocrine homeostasis. These micronutrients may be linked to the onset of diabetic mellitus (DM), according to recent research. To examine the impact of vitamin D and calcium supplementation on the prevention and treatment of type 1 DM (T1DM) and type 2 DM (T2DM), the body of existing research is critically assessed in this systematic review. A comprehensive and methodical literature search was conducted across multiple databases, including PubMed, Embase, Scopus, and the Cochrane Library, covering publications up to January 2025, to identify relevant studies assessing the association between vitamin D and/or calcium levels and the risk of diabetes or glycemic control. The review considered various study designs, such as randomized controlled trials (RCTs), prospective and retrospective cohort studies, as well as case-control analyses, to ensure a robust evaluation of the available evidence. From an initial pool of 3,247 publications, 68 studies qualified for final inclusion. Increased level of vitamin D concentrations was constantly linked to a lower risk of developing T2DM across multiple studies. In addition, other clinical studies demonstrated that supplementation of vitamin D resulted in significant improvements in insulin sensitivity and enhanced pancreatic β-cell function, with the most evident benefits in vitamin D-deficient subjects. While calcium alone showed inconsistent effects, combined supplementation yielded modest yet statistically significant improvements in blood glucose regulation. Vitamin D is a proven micronutrient to regulate glucose metabolism and reduce DM risk, especially in deficient populations. Calcium-vitamin D co-supplementation shows an effective agent for diabetes care; however, more in-depth studies are needed to determine how vitamin D alone, calcium alone, and both together regulate glucose metabolism, and to provide clear evidence to diabetes researchers about how these molecular mechanisms work.