Vitamin D Deficiency and the Risk of Gestational Diabetes Mellitus: A Systematic Review and Meta-analysis of Observational Studies

Vitamin D has been implicated in glucose metabolism and insulin sensitivity, but evidence regarding its association with gestational diabetes mellitus (GDM) remains inconsistent. This study aimed to evaluate the association between maternal vitamin D deficiency and the risk of GDM. A systematic search was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines. Studies assessing vitamin D status at or beyond 24 weeks of gestation were included in this study. Vitamin D deficiency was defined as serum 25-hydroxyvitamin D [25(OH)D] levels <20 ng/mL (<50 nmol/L). Pooled odds ratios (ORs) and mean differences (MDs) were calculated using fixed- or random-effects models. Risk of bias was assessed using the Newcastle–Ottawa Scale. Seventeen observational studies were included in the qualitative synthesis, of which 12 were eligible for meta-analysis. Vitamin D deficiency was significantly associated with an increased risk of GDM (pooled OR = 1.42; 95% confidence interval [CI]: 1.24–1.64; P <0.00001; I 2 = 12%). Serum vitamin D concentrations demonstrated significantly lower vitamin D levels in women with GDM (pooled MD = −1.33 ng/mL; 95% CI: −2.36 to −0.31; P = 0.01; I 2 = 68%). Maternal vitamin D deficiency assessed at or after 24 weeks of gestation is significantly associated with an increased risk of GDM. These findings support vitamin D deficiency as a potential risk modifier for GDM, although causality cannot be inferred due to the observational nature of the evidence. Well-designed randomized controlled trials are needed to determine whether vitamin D supplementation during pregnancy can reduce the incidence of GDM.