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

Systematic review and meta-analysis of 46 randomized controlled trials in adults with type 2 diabetes. Across the vitamin D intervention arms, 2,164 participants were randomized and 2,149 placebo controls were included; mean age was 56.51 years and mean BMI was 29.32, with both sexes represented in most studies and many participants having baseline vitamin D deficiency. Studies were conducted across Asia, Europe, America, Africa, and Australia.

Vitamin D supplementation was evaluated across 46 randomized controlled trials using diverse oral and intramuscular regimens, including daily doses of 20 to 6,000 IU, weekly doses such as 30,000 to 60,000 IU, and intermittent bolus doses such as 50,000 to 400,000 IU. Treatment durations ranged over several weeks, with subgroup analyses suggesting larger glycemic effects at higher doses and shorter durations.

Vitamin D supplementation was associated with modest improvements in glycemic control in people with type 2 diabetes, particularly when baseline vitamin D status was deficient. Pooled effects showed reductions in fasting plasma glucose (WMD -5.02 mg/dl; 95% CI -6.75 to -3.28), HbA1c (WMD -0.20%; 95% CI -0.29 to -0.11), and HOMA-IR (WMD -0.42; 95% CI -0.76 to -0.07). The strongest effects appeared in deficient participants and in higher-dose, shorter-duration regimens. However, heterogeneity was substantial for key outcomes, with I2 = 98.2% for both fasting plasma glucose and HbA1c, so the findings should be interpreted cautiously.

Considerable between-study heterogeneity, varied dosing schedules and routes, and differences in baseline vitamin D status limit confidence in the pooled estimates. Publication bias was a concern, and adverse event reporting was not described in the extracted summary. The evidence is also indirect for some subgroups because only a subset of trials enrolled vitamin D-deficient participants.