Daily vitamin D beats occasional megadoses at fending off respiratory infections

Respiratory tract infections, from the common cold through to chest infections, are among the most frequent reasons people feel unwell. Low blood levels of 25-OH vitamin D have repeatedly been associated with these infections, which raises an obvious question: does supplementing actually prevent them, or do low levels merely travel with poor health? Individual randomised trials have returned inconclusive and conflicting answers. This review pooled those trials to estimate whether vitamin D offers genuine protection, and whether the way it is dosed changes the answer.

The authors pre-registered their protocol on the PROSPERO register, then gathered randomised placebo-controlled trials of vitamin D for the prevention of respiratory tract infection. From 1137 citations retrieved, 11 studies covering 5660 patients qualified for the analysis. Odds of infection were pooled using a random-effects model, the risks of bias within trials and from selective publication were assessed, and meta-regressions and subgroup analyses probed which factors influenced trial outcomes, including whether vitamin D was given as a small daily dose or as occasional large bolus doses.

Overall, vitamin D showed a protective effect against respiratory tract infection, with pooled odds of 0.64 against placebo. The included studies disagreed with each other substantially, however. The most striking pattern came from dosing: trials using once-daily supplementation showed stronger protection than trials using bolus doses, with odds of 0.51 versus 0.86, a statistically significant difference. The authors also found some evidence that publication bias may have flattered the overall picture, and they explicitly advise that the results be interpreted with caution.

This pooling strengthens the case that the link between vitamin D and respiratory infection is more than a pattern seen in people who happen to have low levels, since it draws only on randomised placebo-controlled trials. At the same time, it does not settle the matter: the disagreement between studies and the possibility of unpublished negative results both cut against confidence. The dosing signal, favouring daily over bolus regimens, is one of the more useful leads to emerge from the analysis and invites dedicated testing in future trials.

For anyone who takes vitamin D through the darker months, the most practical detail here is not simply whether it works but how it was given: steady daily dosing looked considerably more protective than infrequent large doses. The overall protective effect is plausible but not watertight, given how much the trials disagreed. This is information to weigh rather than an instruction; whether supplementation makes sense for you depends on your own levels, diet and sunlight exposure.