High-dose Vitamin D ₃ supplementation and glycaemic control in adult males with Type 2 diabetes: a randomised placebo-controlled trial

Background Vitamin D deficiency is common among individuals with type 2 diabetes mellitus (T2DM) and has been linked to insulin resistance and poor glycaemic control. However, findings from randomised controlled trials remain inconsistent.Objective To evaluate the effect of high-dose vitamin D₃ supplementation on insulin resistance and glycaemic parameters in adult males with T2DM.Methods This double-blind, randomised, placebo-controlled clinical trial was conducted at Taibah University, Saudi Arabia. Sixty adult males with established T2DM were randomly assigned to receive oral vitamin D₃ (cholecalciferol) 50,000 IU once weekly (n = 30) or a matching placebo (n = 30) for 24 weeks. Fasting blood samples were collected at baseline and post-intervention. The primary outcome was insulin resistance assessed using the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR). Secondary outcomes included fasting blood glucose, plasma insulin, glycated haemoglobin (HbA1c), body mass index, lipid profile, and serum 25-hydroxyvitamin D concentrations. Data were analysed using SPSS version 29 (IBM Corp., Armonk, NY, USA).Results Vitamin D₃ supplementation significantly increased serum 25-hydroxyvitamin D concentrations in the intervention group, with no meaningful change in the placebo group. Despite correction of vitamin D deficiency, there were no statistically significant between-group differences at 24 weeks in insulin resistance, fasting blood glucose, plasma insulin, HbA1c, body mass index, or lipid profile. Within-group changes in metabolic parameters were modest and not statistically significant.Conclusion In adult males with established T2DM, weekly high-dose vitamin D₃ supplementation for 24 weeks effectively corrected vitamin D deficiency but did not improve insulin resistance or glycaemic control. Vitamin D supplementation should therefore be considered primarily for correcting deficiency rather than for glycaemic management.