Vitamin D supplementation reduces insulin resistance in Japanese adults: a secondary analysis of a double-blind, randomized, placebo-controlled trial

Nutr Res. 2016 Oct;36(10):1121-1129. doi: 10.1016/j.nutres.2016.07.006. Epub 2016 Jul 27.


Higher circulating 25-hydroxyvitamin D (25[OH]D) concentration has been linked to a lower prevalence of insulin resistance and type 2 diabetes mellitus. However, randomized controlled trials have not clarified the effect of vitamin D supplementation on insulin resistance in healthy adults. The objective of this study was to assess the effect of vitamin D supplementation for 1 year on insulin resistance; the study was a secondary analysis of a clinical trial. We hypothesized that increased 25(OH)D concentration after vitamin D supplementation for 1 year would significantly improve insulin resistance. Ninety-six healthy adults participated in this study, of whom 81 completed the study. The participants randomly received daily either 420 IU vitamin D3 or placebo in a double-blind manner for 1 year. The levels of fasting insulin, glucose, and other parameters were assessed at baseline and after 1 year of intervention. Homeostasis model assessment of insulin resistance index was calculated from insulin and glucose levels. Visceral fat area and physical activity were also investigated. Serum 25(OH)D and 1,25-dihydroxyvitamin D concentrations were significantly increased by approximately 29.5 nmol/L and 7.0 pg/mL, respectively, after 1-year vitamin D supplementation. After vitamin D supplementation, fasting glucose levels and values of homeostasis model assessment of insulin resistance index significantly decreased from 88.3 to 85.3 mg/dL (P < .01) and 1.17 to 0.84 (P < .01), respectively, and the results were independent of physical activity and visceral fat accumulation. In conclusion, the present study showed that vitamin D supplementation for 1 year effectively improves fasting glucose level and insulin resistance in healthy Japanese adults.

Keywords: 25(OH)D; Insulin resistance; Percent body fat; Physical activity; Visceral fat area.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Blood Glucose / metabolism*
  • Cholecalciferol / blood
  • Cholecalciferol / pharmacology*
  • Cholecalciferol / therapeutic use
  • Dietary Supplements*
  • Double-Blind Method
  • Fasting
  • Female
  • Humans
  • Insulin / blood*
  • Insulin Resistance*
  • Japan
  • Male
  • Middle Aged
  • Reference Values
  • Vitamin D / analogs & derivatives*
  • Vitamin D / blood


  • Blood Glucose
  • Insulin
  • Vitamin D
  • Cholecalciferol
  • 1,25-dihydroxyvitamin D
  • 25-hydroxyvitamin D