11, 12 Since the previously published reviews on this topic, 8, 9 we found 11 additional eligible studies by updating the search and using a more comprehensive set of search terms compared to the prior reviews, we found six new RCTs completed since the previous review and five older studies not included in previous systematic reviews that met the inclusion criteria in our review. Indeed, previous studies have reported glycemic benefits and improvements in beta cell function among vitamin D deficient adults upon vitamin D supplementation. Further, neither review systematically examined the role of differing lengths and doses of vitamin D supplementation in the reported treatment effects and whether baseline vitamin D deficiency may impact glycemic outcomes after vitamin D repletion. They also included observational studies, which are limited in ability to give insight on causality. included healthy as well as subjects with impaired fasting glucose of type 2 diabetes. 8– 10 These reviews, however, were limited by clinical and methodological heterogeneity in the included studies two recent reviews included studies using various different forms of vitamin D such as intramuscular delivery of vitamin D or active vitamin (i.e., calcitriol) the meta-analysis by Nigil Haroon et al. In contrast to observational studies supporting the beneficial role of vitamin D in glucose metabolism, three most recent systematic reviews examining the efficacy of vitamin D treatment for improving glycemic control in patients with type 2 diabetes showed no benefits. In past years, multiple randomized controlled trials (RCTs) have been conducted to evaluate whether supplementation with vitamin D can improve glycemic control in adults with type 2 diabetes mellitus.

Several mechanisms have been proposed to explain the potential role of vitamin D in glucose metabolism: (1) direct stimulation of insulin secretion through the vitamin D receptor on pancreatic beta cells, (2) lowering of systemic inflammation and subsequent improvement in insulin resistance and (3) improving peripheral insulin resistance via vitamin D receptors in muscles and liver. 1– 4 However, whether vitamin D treatment improves glycemic control has not been convincingly demonstrated. Observational association between vitamin D deficiency and diabetes has been well described.

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