Vitamin D deficiency is associated with insulin resistance and risk of future diabetes.
To test if supplementation with vitamin D to subjects with prediabetes will prevent progression to type 2 diabetes (T2DM).
Randomized controlled trial performed 2008 - 2015.
Clinical Research Unit at a teaching hospital.
511 subjects (mean age 62 years, 314 males) with prediabetes diagnosed with oral glucose tolerance test (OGTT) as part of the Tromsø Study 2007 - 2008 were included. 256 were randomized to vitamin D and 255 to placebo. 29 subjects in the vitamin D and 24 in the placebo group withdrew because of adverse events.
Vitamin D (cholecalciferol) 20,000 IU per week versus placebo for five years. Annual OGTTs performed.
Progression to T2DM. Secondary outcomes change in glucose levels, insulin resistance, serum lipids and blood pressure.
Mean baseline serum 25-hydroxyvitamin D (25(OH)D) level was 60 nmol/L (24 ng/ml). 103 in the vitamin D and 112 in the placebo group developed T2DM (HR 0.90; 95 % CI 0.69-1.18, Cox regression, P = 0.45, intention to treat analysis). No consistent significant effects on the other outcomes were seen. Subgroup analyses in subjects with low baseline 25(OH)D yielded similar results. No serious side effects related to the intervention were recorded.
In 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.