The Effect of Vitamin D Supplementation on Glycaemic Control in Women with Gestational Diabetes Mellitus: A Systematic Review and Meta-Analysis of Randomised Controlled Trials

Systematic review and meta-analysis of 5 randomized controlled trials conducted in Iran and China. The included participants were pregnant women with gestational diabetes mellitus receiving prenatal vitamin D supplementation versus placebo or control.

Vitamin D supplementation during pregnancy was evaluated in several randomized trials among women with gestational diabetes mellitus, using a range of regimens including oral vitamin D3 capsules, vitamin D3-fortified yogurt, and a single intramuscular vitamin D dose. Doses varied widely across studies, including 50,000 IU twice over 6 weeks, 500 IU daily for 16 weeks, 200 IU to 4000 IU daily in different dose arms, four 50,000 IU oral doses every 2 weeks, and a single 300,000 IU intramuscular injection.

Vitamin D supplementation during pregnancy was associated with better glycaemic control in women with gestational diabetes, with improvements seen most consistently in fasting blood glucose, glycated haemoglobin, and serum insulin. Individual trials reported significant reductions in fasting glucose, such as -17.12 ± 14.84 vs -0.96 ± 16.64 mg/dL (p < 0.001) and -9.9 ± 7.2 vs 2.9 ± 7.6 mg/dL (p = 0.04), and some trials also showed lower HbA1c, including -0.18% ± 0.48% vs 0.17% ± 0.39% (p = 0.02). However, results were not uniform across all studies or outcomes, with some insulin and glucose comparisons remaining non-significant, and the evidence base was small. Overall, the review concluded that vitamin D may promote glycaemic control in GDM, but the findings should be interpreted cautiously and further research is needed to define the optimal dose and mechanism.

The evidence base was limited to 5 randomized trials, with all studies conducted in Iran or China, which restricts generalizability. Dosing, formulation, route, and duration varied substantially across trials, and several outcomes were inconsistent or non-significant, making the optimal regimen unclear. The review also notes the need for more research to clarify mechanisms and dosing.