Research synthesisModerate evidenceMixed effect size9 studies · 6 beneficial · 3 neutral · 0 harmful
Across 9 studies, 6 reported beneficial effects of vitamin D supplementation on insulin sensitivity, with effect sizes ranging from small to moderate; the remaining 3 studies found neutral effects. The majority of beneficial findings come from meta-analyses and RCTs in clinical populations such as women with PCOS, gestational diabetes, or obesity, with effects more apparent in vitamin D-deficient individuals. Median study duration was 63 days, suggesting effects may take 8-12 weeks to observe.
- Effective dose range: 1000-4000 IU/day
- Studied populations: women with PCOS, women with gestational diabetes, patients with diabetes/prediabetes, obese/overweight children and adolescents, epilepsy patients, sarcopenic obesity
Caveats: Available evidence is overwhelmingly positive — clinical literature in this area is subject to publication bias (null-result studies are less likely to be published or indexed). Many of the included studies did not reach statistical significance — effect may be smaller than the predominant direction suggests. Benefit appears strongest in vitamin D-deficient populations, and effects are often modest and inconsistent in replete individuals.
Generated Jun 14, 2026