What is the impact of vitamin D supplementation on glycemic control in people with type-2 diabetes: a systematic review and meta-analysis of randomized controlled trails

Randomized controlled trials in adults with type 2 diabetes. Included 46 RCTs (n=4313; 2164 intervention, 2149 control). Mean age 56.5 years; mean BMI 29.3 kg/m2; both genders (one study included only women). Interventions included oral vitamin D supplementation (predominant) and intramuscular vitamin D in a minority; one trial used calcitriol. Study durations ranged 8–48 weeks. Most trials were placebo-controlled; blinding varied (double-blind, single-blind, or open-label). Trials conducted across Asia (31 studies), Europe (9), the Americas (3), Africa (1), and Australia (1).

Vitamin D supplementation modestly improves glycemic control in adults with type 2 diabetes and vitamin D deficiency, reducing fasting plasma glucose by about 5.02 mg/dL, HbA1c by 0.20 percentage points, and HOMA-IR by 0.42. Higher doses and shorter treatment durations, and deficient baseline vitamin D status, were associated with larger effects. Results show substantial heterogeneity and potential publication bias; use of vitamin D status assessment and correcting deficiency could support diabetes management as an adjunct therapy, though further high-quality trials are needed.

Substantial heterogeneity across trials; potential publication bias; relatively short intervention durations and variable dosing; risk of bias in several trials (allocation concealment, blinding); lack of control for sun exposure, diet, and antidiabetic medications in some studies; incomplete methodological reporting in some trials.