Low serum 25-hydroxyvitamin D (25(OH)D) has been shown to correlate with increased risk of type 2 diabetes. Small, observational studies suggest an action for vitamin D in improving insulin sensitivity and/or insulin secretion. The objective of the present study was to investigate the effect of improved vitamin D status on insulin resistance (IR), utilising randomised, controlled, double-blind intervention administering 100 μg (4000 IU) vitamin D3 (n 42) or placebo (n 39) daily for 6 months to South Asian women, aged 23–68 years, living in Auckland, New Zealand. Subjects were insulin resistant – homeostasis model assessment 1 (HOMA1)>1·93 and had serum 25(OH)D concentration < 50 nmol/l. Exclusion criteria included diabetes medication and vitamin D supplementation >25 μg (1000 IU)/d. The HOMA2 computer model was used to calculate outcomes. Median (25th, 75th percentiles) serum 25(OH)D3 increased significantly from 21 (11, 40) to 75 (55, 84) nmol/l with supplementation. Significant improvements were seen in insulin sensitivity and IR (P = 0·003 and 0·02, respectively), and fasting insulin decreased (P = 0·02) with supplementation compared with placebo. There was no change in C-peptide with supplementation. IR was most improved when endpoint serum 25(OH)D reached ≥ 80 nmol/l. Secondary outcome variables (lipid profile and high sensitivity C-reactive protein) were not affected by supplementation. In conclusion, improving vitamin D status in insulin resistant women resulted in improved IR and sensitivity, but no change in insulin secretion. Optimal vitamin D concentrations for reducing IR were shown to be 80–119 nmol/l, providing further evidence for an increase in the recommended adequate levels. Registered Trial No. ACTRN12607000642482.
(Received June 04 2009)
(Revised August 06 2009)
(Accepted August 11 2009)
(Online publication September 28 2009)
Abbreviations: FSG, fasting serum glucose; HOMA, homeostasis model assessment; IR, insulin resistance; MMP, matrix metalloproteinases; 25(OH)D, 25-hydroxyvitamin D