Glucose Metabolism Effects of Vitamin D in Prediabetes: The VitDmet Randomized Placebo-Controlled Supplementation Study

Randomized, placebo-controlled, three-arm supplementation study in older overweight adults with prediabetes from eastern Finland. Eligible participants were at least 60 years old, had BMI > 25 and < 35, and had impaired fasting glucose or impaired glucose tolerance without type 2 diabetes; the trial was conducted during winter 2011/2012.

Oral vitamin D3 was given daily for 5 months at two doses: 40 μg/day (1,600 IU/day) or 80 μg/day (3,200 IU/day). Participants were randomized to vitamin D3 or placebo; the active regimens used two 20 μg tablets daily to reach the assigned dose.

Vitamin D3 clearly increased circulating 25(OH)D3 and lowered PTH, but it did not meaningfully improve glucose metabolism in this prediabetic population. Change in 120 min glucose differed across groups (placebo -0.5 [1.6], 40 μg/day 0.7 [1.7], 80 μg/day 0.5 [1.8]; P_K-W 0.039), but fasting glucose, fasting insulin, HbA1c, HOMA2 indices, and insulin sensitivity were not materially improved. A borderline reduction in IL-1 receptor antagonist suggested a possible modest anti-inflammatory effect, consistent with the authors' conclusion that wintertime high-dose vitamin D3 did not enhance glucose homeostasis. Overall, the intervention altered vitamin D status and mineral metabolism rather than glycemic control.

Small active-arm sample sizes and a short 5-month duration limit power to detect modest metabolic effects. The cohort was restricted to older, overweight Finnish adults studied in winter, which reduces generalizability. Multiple metabolic and inflammatory outcomes were tested, so isolated borderline findings should be interpreted cautiously.