Vitamin D supplementation has no effect on insulin sensitivity or secretion in vitamin D-deficient, overweight or obese adults: a randomized placebo-controlled trial.

Randomized, placebo-controlled trial in otherwise healthy, non-diabetic adults aged 18 to 60 years with vitamin D deficiency and overweight or obesity (BMI at least 25 kg/m2). In the vitamin D arm, 33 participants were randomized and 28 completed and were analyzed; in the placebo arm, 32 were randomized and 26 completed and were analyzed. The study was conducted in Melbourne, Australia.

Oral cholecalciferol was given as a 100,000 IU bolus in 2 capsules, followed by 4,000 IU daily in 4 capsules for 16 weeks. The comparator was placebo capsules.

Vitamin D supplementation increased 25(OH)D but did not improve insulin sensitivity or insulin secretion versus placebo after 16 weeks. 25(OH)D rose by 57.0 6 21.3 nmol/L in the vitamin D group versus 1.9 6 15.1 nmol/L with placebo (P = 0.02). Change in insulin sensitivity was essentially null for the M value: 0.02 6 2.0 mg -> kg 21 -> min 21 with vitamin D versus -0.03 6 2.8 mg -> kg 21 -> min 21 with placebo (P = 0.9). Total insulin AUC change was also not different: -21 6 212 mU/L versus -4 6 184 mU/L (P = 0.9), and there were no between-group differences in first-phase or second-phase insulin secretion, adiposity, blood pressure, or lipids. Overall, the intervention is unlikely to reduce diabetes risk in this population.

The analyzed sample was modest after attrition, with 28 vitamin D and 26 placebo participants, limiting power for smaller effects. Follow-up was only 16 weeks, and the findings apply to a specific population of vitamin D-deficient, overweight or obese, non-diabetic adults from a single center in Melbourne. One participant withdrew after thrombophlebitis during IVGTT, and longer-term clinical outcomes were not assessed.