A prospective randomized controlled trial of the effects of vitamin D supplementation on long-term glycemic control in type 2 diabetes mellitus of Korea.

Prospective randomized controlled trial conducted at two university hospitals in Korea, including ambulatory adults with type 2 diabetes who were vitamin D deficient or insufficient, aged 30-69, stable on antidiabetic therapy, and normocalcemic at baseline. In the vitamin D group, 79 participants were randomized and 64 were analyzed/completed.

Oral cholecalciferol 1000 IU plus elemental calcium 100 mg twice daily for 24 weeks, compared with placebo tablets. The regimen totaled 2000 IU/day of vitamin D3 and 200 mg/day of elemental calcium.

High-dose vitamin D3 raised circulating 25(OH)D and lowered parathyroid hormone, but it did not improve long-term glycemic control or insulin resistance over 24 weeks in adults with type 2 diabetes and low vitamin D status. At 24 weeks, 25(OH)D was 30.2 ± 10.8 ng/mL versus 15.6 ± 7.1 ng/mL in placebo, and 43.8% versus 4.6% reached the target >32 ng/mL, both P<0.001. HbA1c was 7.40 ± 0.90% versus 7.27 ± 0.87% (P=0.415) and HOMA-IR was 2.1 (1.5-3.8) versus 2.1 (1.5-3.2) (P=0.300). Change in PTH favored vitamin D, -5.5 ± 9.8 pg/mL versus 1.4 ± 15.3 pg/mL (P=0.003), and calcium remained stable with no hypercalcemia reported.

Follow-up was only 24 weeks, so longer-term metabolic effects could not be assessed. The active arm had 64 analyzed participants after post-randomization exclusions, which limits precision. Generalizability is narrow because the trial was conducted in Korean adults with type 2 diabetes and baseline vitamin D deficiency or insufficiency.