Research synthesisHigh evidenceMixed effect size14 studies · 12 beneficial · 2 neutral · 0 harmful
Across 14 studies, 12 reported beneficial effects of vitamin D supplementation on increasing 25-hydroxyvitamin D levels, with effect sizes ranging from small to large (most commonly small to moderate). The predominant evidence comes from randomized controlled trials and meta-analyses in clinical populations (e.g., older adults, people with HIV, children), with a median study duration of 90 days. Doses varied widely but commonly ranged from 400 to 4000 IU/day, with some studies using higher weekly or single-dose regimens.
- Effective dose range: 400–4000 IU/day
- Studied populations: older adults, children, people with HIV, patients with vitamin D deficiency, kidney transplant recipients, athletes
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). Two studies (in phenylketonuria patients and kidney transplant recipients) found neutral effects, suggesting that in certain clinical contexts or with certain co-treatments, the expected increase may not occur. Most studies lasted 8–12 weeks or longer; shorter supplementation periods may not produce the same magnitude of effect.
Generated Jun 11, 2026