Research synthesisModerate evidenceMixed effect size8 studies · 5 beneficial · 3 neutral · 0 harmful
Across 8 studies, 5 reported beneficial effects of vitamin D supplementation on insulin sensitivity, with 3 showing neutral results. Effect sizes were mixed, ranging from small to large, but the most consistent findings suggest small benefits in specific populations, such as vitamin D-deficient individuals. The median study duration was 84 days (12 weeks), based on one trial, and the most studied dose was 2,000–4,000 IU/day, though dose reporting was limited.
- Effective dose range: 2,000–4,000 IU/day
- Studied populations: vitamin D-deficient women with PCOS, obese/overweight children and adolescents, patients with diabetes or prediabetes, epilepsy patients on enzyme-inducing ASMs
Caveats: Many of the included studies did not reach statistical significance — effect may be smaller than the predominant direction suggests. The highest-quality evidence (systematic review of PCOS, n=1063) showed modest and inconsistent improvements, especially in vitamin D-deficient subgroups. Most studies did not report baseline vitamin D status, making it difficult to generalize to replete populations. Evidence base is small (only 8 studies) — conclusions should be considered preliminary.
Generated Jun 11, 2026