Background: The evidence indicates that vitamin D [25(OH)D] improves glycaemic outcomes in type 2 Diabetes mellitus patients. The outcome measures used to determine the accuracy of this hypothesis are: glycosylated hemoglobin (HbA1c), fasting plasma glucose (FPG) and homeostasis model assessment-insulin resistance (HOMA-IR). Methods: We performed a systematic review and meta-analysis which included all previous randomised controlled trial (RCT) studies that assessed the effects of vitamin D on glucose metabolism. We carried out an extensive electronic database search of published and unpublished RCTs, evaluating the association between vitamin D and glycaemic outcomes in type 2 diabetes mellitus patients. We searched Cochrane Library, PubMed, EMBASE, CINAHL Plus with Full Text, MEDLINE, BioMed Central, Turning Research Into Practice (TRIP), Health Technology Assessment (HTA), and Latin American and Caribbean Health Sciences (LILIACS) between the years 2005 and 2016. The full texts of relevant studies were retrieved and a snowballing technique was used to discover further studies missed from the initial database search. This was done by hand-searching for references within the retrieved articles. Results: A total of 17 studies were included in the review. The pooled effect of 15 studies that measured HbA1c showed an insignificant effect of vitamin D on HbA1c (Mean difference (MD) = −0.06 mmol/l; 95% CI = −0.26 to 0.14; I2 = 76%). A pooled analysis of seven studies that measured the effect of vitamin D on blood glucose also found no significant effect of vitamin D on T2DM (MD = −0.03 mmol/l; 95% CI = −0.69 to 0.63; I2 = 76%). Three studies that analysed the effect of vitamin D on insulin sensitivity also observed no significant effect (MD = −1.51 mmol/l; 95% CI = −3.61 to 0.60; I2 = 67%). Conclusion: In conclusion, although vitamin D has been extensively studied in relation to some glycaemic outcomes and some indications that increased plasma vitamin D concentrations might be linked to prevention of T2DM, firm universal conclusions about its benefits cannot be drawn. Further studies with better designed trials and larger sample sizes are needed to draw firmer conclusions
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