Treating patients with vitamin D supplements is unlikely to prevent prediabetes from developing into type 2 diabetes, new research finds.
Vitamin D is generally known to reduce the risk of insulin resistance, which is often a precursor to type 2 diabetes. People who are deficient in vitamin D therefore have a higher risk of type 2 diabetes.
Researchers at the Tromsø Endocrine Research Group, The Arctic University of Norway, evaluated data from 511 subjects with prediabetes between 2007 and 2008. The mean age of the participants was 62 years, and 314 were men.
256 participants were randomised to vitamin D supplements for five years (20,000 IU per week) and 255 to placebo. Oral glucose tolerance tests were performed annually. 116 participants in the vitamin D group completed the study, while this figure was 111 in the placebo group.
The primary aim of the researchers was to assess the progression to type 2 diabetes. Secondary outcomes included glucose levels, insulin resistance, serum lipids and blood pressure.
Levels of vitamin D nearly doubled from baseline after five years in the vitamin D group, but remained largely stable in the placebo group.
However, type 2 diabetes developed in 40.2 per cent of the vitamin D group and 43.9 per cent of the placebo group. No significant differences in secondary outcomes were observed between the groups, with normal blood glucose levels found in 55 vitamin D group participants and 41 participants in the placebo group.
“Our study does not support giving vitamin D for the prevention of [type 2 diabetes] or for improvement of insulin resistance or hyperglycemia,” the authors wrote.
“If there is a positive effect of vitamin D in this regard, the effect must be small. Very large studies with inclusion of vitamin D deficient subjects will be needed to show such a putative effect.”
The study was published in The Journal of Clinical Endocrinology and Metabolism.

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