###### Abstract  
A number of studies have examined the role of vitamin D in reproductive processes and disorders of pregnancy such as gestational diabetes mellitus (GDM). Although observational studies have linked maternal vitamin D deficiency with a plethora of adverse pregnancy outcomes including GDM, intervention trials generally do not support the use of vitamin D supplementation for GDM prevention or management. This narrative review provides an up-to-date overview and critical appraisal of randomised controlled trials (RCTs) to describe the current state of knowledge regarding the efficacy of vitamin D supplementation for preventing and/or managing GDM. Overall, although RCT data indicates a potential benefit of vitamin D in maternal glycaemic control, results are highly disparate and the data published to date have not conclusively established the efficacy of vitamin D in GDM prevention. There are, however, several limitations within the existing literature, including some considerable challenges that are unique to vitamin D trials, which should be carefully considered in the interpretation of the evidence and design of future studies. For now, many unanswered questions remain, and there is still a need for adequately powered and well-designed trials before routine supplementation can be recommended in the context of GDM.  
Keywords:vitamin D; 25(OH)D; gestational diabetes mellitus; GDM; pregnancy; glycaemic control; randomized controlled trials; critical appraisal +
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posit that vitamin D deficiency may lead to increased inflammation and decreased insulin action, which could contribute to an increased risk of GDM. Observational studies have reported mixed results, with some finding inverse associations between maternal 25(OH)D concentrations and GDM risk factors including obesity [24], insulin resistance [25], dysglycaemia [26], dyslipidaemia [27], hypertension [28], and inflammation [29; 30]; while others report no associations [31; 32]. Similarly, some prospective studies link maternal vitamin D deficiency with an increased risk of incident GDM [33], while others do not [34; 35; 36].  
Higher levels of evidence, such as that from randomised controlled trials (RCTs) have also produced mixed results, with some [37; 38; 39; 40], but not all [41; 42; 43], supporting the use of vitamin D supplementation for preventing or managing GDM. These RCTs have been limited by small sample sizes and low statistical power, as well as low doses of vitamin D and variability in the baseline risk profiles of participants. Newly published trials [44; 45; 46] have sought to address these limitations, but the overall evidence from RCTs varies in scope, design, results and conclusions, making it difficult for decision-makers to reach definitive conclusions and recommendations for the use of vitamin D in clinical practice.  
The aim of this paper is to provide an up-to-date overview of all published RCTs to describe the current state of knowledge regarding the efficacy of vitamin D supplementation for preventing and/or managing GDM. This review is not systematic and is not intended to introduce new data or conclusions, nor does it address all studies of vitamin D in pregnancy. Rather, the aim is to provide a comprehensive overview and critical appraisal of high-level evidence related to vitamin D and GDM and to present this in an objective, accessible and compact manner to aid in decision-making and future research endeavours.