Vitamin D supplementation has been proposed as a potential strategy to prevent type 2 diabetes. However, existing clinical trials are limited by short durations, low doses of vitamin D, variability in participants’ vitamin D deficiency status, and use of surrogate measures of body composition, insulin sensitivity, and insulin secretion.
We conducted a double-blind randomized placebo-controlled trial to investigate whether vitamin D supplementation, provided in a sufficient dose and duration to vitamin D-deficient individuals, would improve insulin sensitivity and/or secretion measured by gold-standard methods.
Sixty-five overweight or obese (BMI≥25 kg/m2), vitamin D-deficient (25-hydroxyvitamin D (25(OH)D)≤50 nmol/l) adults were randomized to a bolus oral dose of 100,000 IU followed by 4,000 IU daily of cholecalciferol or matching placebo for 16 weeks. Before and after intervention, participants had gold-standard assessment of body composition (dual X-ray absorptiometry), insulin sensitivity (hyperinsulinemic-euglycemic clamps) and insulin secretion (intravenous glucose-tolerance tests). Additional measurements included BMI, waist-to-hip ratio, blood pressure, serum lipids (ELISA), and high-sensitivity C-reactive protein (highly-sensitive ELISA). All analyses were adjusted for multiple testing using Bonferroni-correction.\
Fifty-four participants completed the study (35M/19F;age=31.9±8.5 years;BMI=30.9±4.4 kg/m2 (mean±SD)). Serum 25(OH)D increased with vitamin D supplementation compared to placebo (57.0±21.3 versus 1.9±15.1nmol/L,p=0.02). Vitamin D and placebo groups did not differ in change in insulin sensitivity (0.02±2.0 versus -0.03±2.8 mg/kg/min,p=0.9), total, first- or second-phase insulin secretion (all p>0.1), blood pressure, serum lipids, or hsCRP (all p>0.1). Results remained non-significant after adjustment for age, sex, and % body fat, and after additional adjustment for sun exposure, physical activity, and dietary vitamin D intake (p>0.1).
Vitamin D supplementation does not improve insulin sensitivity or secretion in vitamin D-deficient, overweight or obese adults, despite using sufficient doses and robust endpoint measures. It is unlikely that vitamin D supplementation would be an effective strategy for reducing diabetes risk in this population.