Vitamin D Supplementation and Glycaemic Control in Type 2 Diabetes: A Narrative Review of Randomised Trials

Abstract

Low circulating vitamin D is common in people with type 2 diabetes, and it has long been suspected that correcting it might improve blood glucose. Observational studies, laboratory work on the pancreatic beta cell and a handful of small trials have kept the idea alive. This narrative review asks what randomised trials published up to the end of 2013 can actually say about vitamin D supplementation and glycaemic control in type 2 diabetes. It draws on the biology of vitamin D, on epidemiological associations, on reported supplementation trials and on the methodological literature that governs how such trials should be read, and it reports no new data. The trials point in different directions. Benefit, where it has been reported, tends to appear for insulin resistance rather than for glycated haemoglobin, in people who were deficient or glucose intolerant at the start, and in studies that were small and short. Trials in people with established diabetes and adequate vitamin D status have generally been unrewarding. Differences in dose, preparation, baseline status, duration and outcome measurement make the studies difficult to pool, and an association seen in cohorts is not evidence that supplementation changes glycaemia. The review concludes that supplementation is reasonable where deficiency is documented but is not established as a glucose-lowering treatment, and it sets out what a decisive trial would need to contain.