Vitamin D fails to fend off coughs and colds in trials of 30,263 people

Vitamin D is one of the most widely taken supplements, and warding off coughs, colds and other respiratory infections is one of the most common reasons people reach for it. The scientific backing has always been contested: previous systematic reviews and meta-analyses of randomised trials reported inconsistent results, with some suggesting protection and others finding none. That inconsistency matters, because millions of people base a daily habit on it. This meta-analysis set out to pool the randomised trial evidence afresh and test whether the protective effect survives scrutiny.

The researchers searched PubMed, EMBASE and the Cochrane Library in June 2021 for randomised controlled trials of vitamin D supplementation and acute respiratory infections. From 390 trials identified in the search, 30 met the inclusion criteria, together involving 30,263 participants. Results were pooled to estimate the relative risk of infection with supplementation versus without. The team then ran subgroup analyses by dosing pattern, including daily and short-term supplementation, and by study quality, judged with two separate established rating tools, and checked for publication bias.

Across all 30 trials, vitamin D supplementation showed no significant effect on preventing acute respiratory infections, with a relative risk of 0.96. In subgroups, daily supplementation and short-term supplementation each looked protective, with a relative risk of 0.83 in both cases. But those benefits disappeared when the analysis was restricted to high-quality studies: the relative risks were 0.89 across 10 trials rated by one quality scale and 0.87 across 4 trials rated by another, neither significant. The researchers also detected publication bias, suggesting trials with unfavourable results were less likely to appear in the literature.

This finding pushes against earlier reviews that reported protection from vitamin D, particularly with daily dosing, and offers an explanation for the disagreement: the apparent benefit concentrates in lower quality trials and may be inflated by publication bias. It does not address vitamin D's other roles, such as bone health, and it cannot rule out benefit in specific groups the subgroups did not isolate. The open question is whether any future high-quality trial in a deficient population would find an effect, but on the evidence pooled here the immune protection story looks weak.

If a daily vitamin D capsule sits in your routine specifically as cold and flu insurance, this analysis suggests that job is not being done: across 30,263 trial participants there was no significant protection, and the flattering subgroup results dissolved in the strongest studies. That is useful information rather than an instruction, since people take vitamin D for several reasons this study says nothing about. It is mainly a reason to be sceptical of immunity marketing on supplement labels.