Effects of vitamin D(2) or D(3) supplementation on glycaemic control and cardiometabolic risk among people at risk of type 2 diabetes: results of a randomized double‐blind placebo‐controlled trial

AIMS: To investigate the effect of short‐term vitamin D supplementation on cardiometabolic outcomes among individuals with an elevated risk of diabetes. METHODS: In a double‐blind placebo‐controlled randomized trial, 340 adults who had an elevated risk of type 2 diabetes (non‐diabetic hyperglycaemia or positive diabetes risk score) were randomized to either placebo, 100 000 IU vitamin D(2) (ergocalciferol) or 100 000 IU vitamin D(3) (cholecalciferol), orally administered monthly for 4 months. The primary outcome was change in glycated haemoglobin (HbA1c) between baseline and 4 months, adjusted for baseline. Secondary outcomes included: blood pressure; lipid levels; apolipoprotein levels; C‐reactive protein levels; pulse wave velocity (PWV); anthropometric measures; and safety of the supplementation. RESULTS: The mean [standard deviation (s.d.)] 25‐hydroxyvitamin D [25(OH)D](2) concentration increased from 5.2 (4.1) to 53.9 (18.5) nmol/l in the D(2) group, and the mean (s.d.) 25(OH)D(3) concentration increased from 45.8 (22.6) to 83.8 (22.7) nmol/l in the D(3) group. There was no effect of vitamin D supplementation on HbA1c: D(2) versus placebo: −0.05% [95% confidence interval (CI) −0.11, 0.02] or −0.51 mmol/mol (95% CI −1.16, 0.14; p = 0.13); D(3) versus placebo: 0.02% (95% CI −0.04, 0.08) or 0.19 mmol/mol (95% CI −0.46, 0.83; p = 0.57). There were no clinically meaningful effects on secondary outcomes, except PWV [D(2) versus placebo: −0.68 m/s (95% CI −1.31, −0.05); D(3) versus placebo −0.73 m/s (95% CI −1.42, −0.03)]. No important safety issues were identified. CONCLUSIONS: Short‐term supplementation with vitamin D(2) or D(3) had no effect on HbA1c. The modest reduction in PWV with both D(2) and D(3) relative to placebo suggests that vitamin D supplementation has a beneficial effect on arterial stiffness.