RCT2019 · NEJM · Pittas AG et al.
D2d: vitamin D doesn't prevent diabetes — except in the deficient
Who, how long
2,423 US adults with prediabetes, 4,000 IU D3 daily vs placebo, 2.5 years
What they found
In the whole group diabetes incidence fell 12%, not significant. In those with low baseline levels (< 12 ng/ml), though, –62%; the 2023 individual-data meta-analysis (3 RCTs, n=4,190) found –15% progression to diabetes in prediabetes when serum levels rose above 50 ng/ml.
⚠ Caveat
80% of participants started with sufficient vitamin D — the “give everyone” model failed again, while “test, then supplement” was vindicated.
Related in the knowledge base
Last reviewed: 6 September 2026
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