Objective: In observational studies, low vitamin D levels are associated with type 2 diabetes (T2D), impaired glucose metabolism, insulin sensitivity, and insulin secretion. We evaluated the efficacy of vitamin D supplementation on insulin sensitivity and insulin secretion in subjects with T2D and low vitamin D (25-hydroxyvitamin D [25(OH)D] <50 nmol/L).
Research design and methods: Sixty-two men and women with T2D and vitamin D deficiency participated in a 6-month randomized, double-blind, placebo-controlled trial. Participants received a single dose of 400,000 IU oral vitamin D3 or placebo, and the vitamin D group received an additional 200,000 IU D3 if serum 25(OH)D was <100 nmol/L after 4 weeks. Primary end points were total Rd by euglycemic clamp with assessment of endogenous glucose production and first-phase insulin secretion by intravenous glucose tolerance test.
Results: In the vitamin D group, the mean ± SD baseline serum 25(OH)D of 38.0 ± 12.6 nmol/L increased to 96.9 ± 18.3 nmol/L after 4 weeks, 73.2 ± 13.7 nmol/L after 3 months, and 53.7 ± 9.2 nmol/L after 6 months. The total exposure to 25(OH)D during 6 months (area under the curve) was 1,870 ± 192 and 1,090 ± 377 nmol/L per week in the vitamin D and placebo groups, respectively (P < 0.001). Insulin sensitivity, endogenous glucose production, and glycemic control did not differ between or within groups after treatment (P = 0.52). First-phase insulin secretion did not change significantly after treatment (P = 0.10).
Conclusions: Replenishment with a large dose of vitamin D3 to patients with T2D and vitamin D deficiency did not change insulin sensitivity or insulin secretion. These findings do not support such use of therapeutic vitamin D3 supplementation to improve glucose homeostasis in patients with T2D.
Trial registration: ClinicalTrials.gov NCT00992797.
© 2017 by the American Diabetes Association.