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.

1. A critical review with meta-analysis found that vitamin D's apparent protection against acute respiratory infections was confined to small trials: the summary odds ratio was 0.69 in trials of 25 to fewer than 248 patients but 0.98 in trials of 248 to 16,000 patients (interaction p = 0.0001), with strong evidence of publication bias.

   well supported · human data

   A meta-analysis of previously published randomised trials, not new experimental data; the acute respiratory infection trials were re-analysed from an earlier pooled dataset, and the analysis splits trials by size, which can be confounded by differences in populations, doses and outcome definitions. It shows the benefit signal is fragile, not that vitamin D is useless for people who are deficient.

   Read the breakdown →Vitamin D, acute respiratory infections, and Covid-19: The curse of small-size randomised trials. A critical review with meta-analysis of randomised trials. PLoS One 2025

2. Correcting a genuine vitamin D deficiency, common in northern winters, is worthwhile for bone and general health; topping up someone who is already replete is unlikely to improve strength or performance.

   preliminary · human data
   Read the breakdown →PubMed: vitamin D status, supplementation and muscle function / athletic performance (reviews and trials)

This page collects what our published, cited studies say; it is information, not medical advice. Have a different question? Ask it here.