The role of vitamin D in glycaemic control: a review of randomised controlled trials

Abstract

Vitamin D deficiency and diabetes mellitus are both global health concerns with increasing prevalence. Recent research suggests a potential link between vitamin D status and the development and progression of various types of diabetes. This review aims to analyse the clinical evidence on the relationship between vitamin D supplementation and glycaemic control across different populations: individuals with prediabetes, type 1 diabetes (T1D), type 2 diabetes (T2D), and gestational diabetes mellitus (GDM). A systematic search of randomised clinical trials (RCTs) published from 2014 to 2024 was conducted in PubMed, Scopus, and Web of Science. Sixteen studies met the inclusion criteria. The findings were mixed. In prediabetes, vitamin D improved serum 25(OH)D levels, but effects on glycaemic parameters were inconsistent. In T1D, supplementation was associated with reduced insulin requirements and improved C-peptide levels, especially in children with vitamin D deficiency. T2D results were variable: some trials reported improvements in HbA1c and HOMA-IR, while others found no significant changes. In GDM, supplementation improved fasting insulin response and reduced adverse pregnancy outcomes. Overall, vitamin D supplementation shows potential benefits in certain contexts, particularly in T1D and GDM. However, heterogeneity in dosage, duration, and baseline characteristics limits the generalisability of findings. Further high-quality RCTs are needed to define optimal regimens and identify subpopulations most likely to benefit.