Does vitamin D actually do anything?
What the studies we have published say — strongest evidence first. Every claim below links to the paper it came from.
A pooled analysis of 115 studies covering 199,055 healthy people found that nutrient supplements had little to no effect on preventing viral respiratory tract infections. The only signals were narrow: zinc appeared protective for children in Asia, and vitamin D modestly reduced infections in adults, mainly in North America.
well supported · human dataAs strong as this kind of evidence gets: results pooled from 115 studies covering 199,055 people in 37 countries, though the vast majority were children. The two positive signals, zinc and vitamin D, were regional and the studies behind them disagreed with each other, so even those deserve caution.
Across one population study and seven clinical studies, lower blood vitamin D3 levels correlated with higher COVID-19 mortality, and a regression through the pooled data pointed to a theoretical zero-mortality level near 50 ng/mL. That is a correlation and an extrapolation, not proof that raising vitamin D prevents deaths.
preliminary · human dataTreat this as a pattern, not proof: it pools one population study and seven hospital studies where lower vitamin D went with more deaths, and following people like this cannot show that raising vitamin D would have saved anyone. The zero-deaths level at 50 ng/mL is a line extended beyond the data, never tested in a trial.
Vitamin D supplementation produced no significant change in IGF-1, a growth factor tied to muscle, bone and ageing, in a meta-analysis of six randomised trials totalling 773 participants.
preliminary · human dataA fair verdict from results pooled across six trials with 773 people: no meaningful change in IGF-1 overall, and none in any subgroup. The trials disagreed with each other considerably, so a small effect in specific groups cannot be entirely ruled out.
A meta-analysis of 30 randomised trials with 30,263 participants found vitamin D supplementation had no significant effect on preventing acute respiratory infections. Apparent benefits from daily and short-term dosing disappeared when only the highest quality trials were analysed, and publication bias was detected.
not supported · human dataA convincing null: pooling 30 trials covering 30,263 people found no overall protection against respiratory infections, and the hints of benefit from daily dosing vanished in the best-run studies. Signs that unflattering results went unpublished weaken even those hints further.
Vitamin D supplementation reduced the risk of acute respiratory infections in pooled individual data from 25 trials covering 11,321 people. The protection appeared only with daily or weekly dosing, not with large bolus doses, and was strongest in people who started out severely deficient.
well supported · human dataAbout as solid as supplement evidence gets: individual records pooled from 25 trials in 11,321 people aged 0 to 95, and the evidence was rated high quality. The catch is that the benefit vanished with occasional large-dose regimens and was concentrated in people who were deficient to begin with.
Pooled individual data from 10 randomised trials in 677 people of Black or South Asian descent living at latitudes of 40 degrees north or higher suggest that keeping winter blood levels above the 25 nmol/L deficiency threshold takes 23.9 to 24.1 µg of vitamin D a day, substantially more than the equivalent estimates for White individuals and above what several agencies currently recommend.
well supported · human dataStrong for estimating intake needs, unproven for health outcomes: pooled individual data from 10 trials in 677 Black and South Asian children and adults living at northern latitudes. The targets are blood levels rather than demonstrated benefits, and the authors say much more work is needed in these populations.
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This page collects what our published, cited studies say; it is information, not medical advice. Have a different question? Ask it here.