Vitamin D's immune reputation rests on just 13 trials

Vitamin D is one of the most familiar supplements on the shelf, and a common hope behind taking it is that it helps the body prevent or fight infection. Whether that hope is backed by controlled human trials is a separate question from whether it is plausible. Before this review, the trials scattered across six decades of research had never been systematically gathered and weighed together. The authors set out to do exactly that, and to say where the evidence was strong enough to justify serious follow-up.

The team searched PubMed and Ovid MEDLINE for randomised controlled trials published between 1948 and 2009 that tested vitamin D for treating or preventing infectious diseases in humans. Thirteen trials met the criteria. Ten were placebo controlled, and nine of those ten used a rigorous double-blind design. The reviewers then compared the trials' populations, sample sizes and vitamin D strategies, and noted design features that should guide future studies in the field.

The thirteen trials varied substantially in who was studied, how many people took part and how vitamin D was given, which made a single pooled answer impossible. Serious adverse events attributable to vitamin D supplementation were rare across all studies. On the evidence to date, the strongest case for further research was in tuberculosis, influenza and viral upper respiratory tract illnesses. Nowhere did the reviewers find evidence firm enough to recommend changing clinical practice.

This review is a map of a young field rather than a verdict on it. It confirms that vitamin D and infection is a live scientific question with encouraging leads, while making plain that the trial base is small and inconsistent. The authors call for more rigorously designed trials to pin down the relationship between vitamin D status and the immune response to infection. Until those arrive, confident claims about vitamin D as an infection fighter run ahead of the evidence.

If you take vitamin D partly in the hope of dodging winter bugs, this is a useful calibration: the controlled trial evidence behind that hope was limited, mixed and concentrated in a few conditions. It is equally worth noting what did not turn up, namely serious harm, which was rare across the trials reviewed. The reasonable reading is curiosity rather than conviction. Stronger trials were the review's main request, and that is the standard to hold newer claims to as well.