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

Two-center (Chuncheon and Seoul, Korea) randomized, double-blind, placebo-controlled trial in adults with type 2 diabetes mellitus. Participants aged 30–69 years with vitamin D deficiency or insufficiency (<20 ng/mL 25(OH)D) and HbA1c < 8.5%, ambulatory and with unchanged antidiabetic therapy for at least 3 months; 158 randomized (placebo n=79, vitamin D n=79); 129 completed protocol.

Vitamin D3 (cholecalciferol) 2000 IU daily plus 200 mg elemental calcium daily, oral, for 24 weeks.

Vitamin D supplementation raised 25(OH)D to 30.2 ± 10.8 ng/mL vs 15.6 ± 7.1 ng/mL with placebo (P<0.001) and more strongly suppressed PTH (-5.5 ± 9.8 pg/mL vs +1.4 ± 15.3 pg/mL; P=0.003). HbA1c (7.40 ± 0.90% vs 7.27 ± 0.87%; P=0.415) and HOMA-IR showed no between-group differences. Safety: no hypercalcemia or kidney dysfunction. Conclusion: high-dose vitamin D with calcium safely achieves vitamin D sufficiency but does not improve long-term glycemic control in vitamin D–deficient/insufficient Korean adults with T2DM over 24 weeks; not recommended for glycemic management based on this evidence; longer or higher-dose trials may be needed in severely deficient individuals.

Seasonal recruitment (winter–spring) may limit generalizability; only ~44% reached target 25(OH)D (>32 ng/mL) in the vitamin D group; calcium co-supplementation may confound effects; 24 weeks may be insufficient to detect glycemic changes; some participants changed antidiabetic medications; two-center design may limit applicability.