Prediabetes is a condition characterised by intermediate hyperglycemia, which elevates the risk of developing type 2 diabetes mellitus (T2DM), cardiovascular disease, and other metabolic disorders. Prediabetes, with an estimated global incidence of 480 million persons, has become a significant public health concern. Lifestyle adjustment is fundamental to prediabetes therapy; however, supplementary therapies aimed at enhancing insulin sensitivity and mitigating the progression to T2DM are now under investigation [1,2].

Vitamin D, a fat-soluble secosteroid hormone mostly produced in the skin by ultraviolet B radiation exposure, is crucial for calcium homeostasis and bone metabolism. Recent studies have enhanced the comprehension of vitamin D's function in glucose regulation, immunological modulation, and inflammation. The existence of vitamin D receptors (VDR) in pancreatic beta-cells and peripheral tissues associated with insulin action has initiated research into their function in insulin sensitivity and secretion [3].

Epidemiological studies have repeatedly indicated a significant incidence of vitamin D insufficiency in persons with prediabetes and type 2 diabetes mellitus (T2DM). Moreover, observational studies have demonstrated inverse correlations between blood 25-hydroxyvitamin D [25(OH)D] levels and indicators of insulin resistance, fasting hyperglycemia, and inflammatory cytokines. Randomised controlled trials (RCTs) assessing the metabolic effects of vitamin D supplementation have shown inconsistent results, sometimes constrained by small sample numbers, brief durations, or insufficient stratification of baseline vitamin D status [4].

Vitamin D insufficiency is prevalent in Bangladesh across all age demographics, attributed to low sun exposure, increased melanin levels, and inadequate food intake. The prevalence of prediabetes is escalating swiftly, necessitating the implementation of effective and cost-efficient preventative interventions.

This research aims to evaluate the impact of vitamin D supplementation on insulin resistance, blood sugar parameters, and overall inflammation in a large cohort of vitamin D-deficient individuals with prediabetes in Bangladesh. By focusing on a well-defined high-risk group and employing a rigorous randomized controlled trial design, the study intends to explore the therapeutic potential of vitamin D in the prevention of metabolic disorders.

Study Design and Participants This randomized, double-blind, placebo-controlled experiment was executed from January 2018 to December 2019 at 12 tertiary care centres in Bangladesh. The study protocol received approval from the appropriate institutional review boards and was executed in compliance with the Declaration of Helsinki. Inclusion Criteria • Adults aged 30 to 60 years • Diagnosed with prediabetes (FPG 100–125 mg/dL and/or HbA1c 5.7%–6.4%) • Serum 25(OH)D <20 ng/mL Exclusion Criteria • Diagnosis of diabetes mellitus • Use of vitamin D or calcium supplements in the past 6 months • Chronic kidney disease, liver disease, or malabsorption disorders • Pregnancy or lactation Intervention Participants were randomized to receive either: • Vitamin D3: 50,000 IU orally once weekly for 12 weeks • Placebo: Identical capsule once weekly for 12 weeks Data Collection Baseline and follow-up assessments included: • Fasting plasma glucose (FPG) • HbA1c • Fasting insulin • HOMA-IR • Serum 25(OH)D • High-sensitivity C-reactive protein (hs-CRP) • BMI and physical activity level Statistical Analysis The data were analyzed using SPSS version 27. Continuous variables were presented as mean ± standard deviation. Within-group comparisons were performed using paired t-tests. ANCOVA models were applied to account for covariates. A p-value of less than 0.05 was regarded as statistically significant.