• Rolf Jorde, Stina T Sollid, Johan Svartberg, Henrik Schirmer, Ragnar M Joakimsen, Inger Njølstad, Ole M Fuskevåg, Yngve Figenschau, and Moira Y S Hutchinson.
• Tromsø Endocrine Research Group (R.J., S.T.S., J.S., R.M.J., Y.F.), Department of Clinical Medicine, Department of Clinical Medicine (H.S.), Epidemiology of Chronic Diseases Research Group (I.N.), Department of Community Medicine, Department of Medical Biology (Y.F.), UiT The Arctic University of Norway, 9037 Tromsø, Norway; Division of Internal Medicine (R.J., S.T.S., J.S., R.M.J.), Division of Diagnostic Services (O.M.F., Y.F.), University Hospital of North Norway, 9038 Tromsø, Norway; and Division of Head and Motion (M.Y.S.H.), Department of Rheumatology, Nordland Hospital, 8092 Bodø, Norway.
• J. Clin. Endocrinol. Metab. 2016 Apr 1; 101 (4): 1647-55.

ContextVitamin D deficiency is associated with insulin resistance and risk of future diabetes.ObjectiveThe objective of the study was to test whether supplementation with vitamin D to subjects with prediabetes will prevent progression to type 2 diabetes mellitus (T2DM).DesignThis was a randomized controlled trial performed in 2008 through 2015.SettingThe study was conducted at the clinical research unit at a teaching hospital.PatientsFive hundred eleven subjects (mean age 62 y, 314 males) with prediabetes diagnosed with an oral glucose tolerance test as part of the Tromsø Study 2007–2008 were included. A total of 256 were randomized to vitamin D and 255 to placebo. Twenty-nine subjects in the vitamin D and 24 in the placebo group withdrew because of adverse events.InterventionsInterventions included vitamin D (cholecalciferol) 20 000 IU/wk vs placebo for 5 years. Annual oral glucose tolerance tests were performed.Main Outcome MeasureProgression to T2DM was the main outcome measure. Secondary outcomes were change in glucose levels, insulin resistance, serum lipids, and blood pressure.ResultsThe mean baseline serum 25-hydroxyvitamin D level was 60 nmol/L (24 ng/mL). One hundred three in the vitamin D and 112 in the placebo group developed T2DM (hazard risk 0.90; 95% confidence interval 0.69–1.18, Cox regression, P = .45, intention to treat analysis). No consistent significant effects on the other outcomes were seen. Subgroup analyses in subjects with low baseline 25-hydroxyvitamin D yielded similar results. No serious side effects related to the intervention were recorded.ConclusionsIn subjects without vitamin D deficiency, vitamin D supplementation is unlikely to prevent progression from prediabetes to diabetes. Very large studies with inclusion of vitamin D-deficient subjects will probably be needed to show such a putative effect. This study tested if supplementation with vitamin D to subjects with prediabetes will prevent progression to type 2 diabetes (T2DM).

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