Vitamin D protects against respiratory infections, but only in steady doses

Trials of vitamin D for preventing coughs, colds and other acute respiratory infections had produced a muddle of positive and null results, leaving clinicians and supplement takers alike unsure whether the vitamin does anything for immune defence. Pooling individual participant data rather than study-level summaries offered a way to cut through that inconsistency, because it lets researchers test who actually benefits: people with low starting levels, people on particular dosing schedules, or nobody at all. With respiratory infections among the most common causes of illness across every age group, the answer matters to a very wide audience.

The team gathered randomised, double-blind, placebo-controlled trials of vitamin D3 or D2 that had specified acute respiratory infection as a prespecified efficacy outcome, identifying 25 eligible trials with 11,321 participants aged 0 to 95. They obtained individual participant data for 10,933 of them, 96.6% of the total. Study quality was assessed with the Cochrane risk of bias tool, and the analyses examined the overall effect on infection risk alongside prespecified subgroups by dosing pattern and baseline vitamin D status, as well as safety outcomes.

Overall, vitamin D supplementation reduced the odds of experiencing an acute respiratory infection. The protection was confined to people given daily or weekly doses without additional bolus doses; those receiving one or more large boluses saw no benefit. Among the regular dosers, the effect was dramatically larger in people whose baseline blood level sat below 25 nmol/l than in those starting above that mark. Supplementation did not increase the proportion of participants with serious adverse events, and the body of evidence was rated as high quality. The analysis had limited power to separate effects on upper versus lower respiratory infections.

This analysis goes a long way toward reconciling a previously contradictory trial literature: much of the disagreement appears to trace back to dosing schedule and baseline status, with bolus-dose trials diluting the apparent effect. It strengthens the case that vitamin D plays a genuine role in respiratory defence, particularly for deficient individuals, while undercutting the intuition that an occasional big dose is a convenient shortcut. Open questions remain about upper versus lower respiratory infections specifically, and the authors note that adding data from ongoing trials could sharpen those secondary analyses.

The practical picture here is about pattern, not just presence: in these pooled trials the benefit came from steady daily or weekly supplementation, and people with very low starting levels had the most to gain. For someone whose vitamin D status is already healthy, the expected benefit was much smaller. It is also worth knowing that supplementation was safe on the serious adverse event measure. None of this tells any individual what to take, but it explains why blanket claims that vitamin D does nothing, or that it prevents every cold, are both wrong.