Vitamin D pills did not cut coughs and colds in healthy people

Respiratory tract infections, the everyday coughs, colds and chest infections, are among the most common reasons people feel unwell, and vitamin D supplements are widely taken in the hope of fending them off. The idea is appealing because supplementation is simple, cheap and easy to scale as a preventive measure. But individual randomised trials testing whether vitamin D actually prevents these infections had produced inconsistent, inconclusive results. This review set out to pull all of that trial evidence together and ask whether, in healthy adults and children, taking vitamin D genuinely lowers infection risk.

The researchers systematically searched Medline, EMBASE, CENTRAL and CINAHL from inception to January 2016, adding grey literature, clinical trial registers and conference abstracts. They included only randomised trials comparing vitamin D against placebo or no treatment in healthy adults and children, with two reviewers independently selecting studies and extracting data. Study quality was formally assessed, and results were pooled using random-effects models, with sub-group and meta-regression analyses to explore why studies differed. From 2627 initial hits, 15 trials involving 7053 individuals qualified, all using oral cholecalciferol, the D3 form of the vitamin.

The headline result was a null: vitamin D3 supplementation was associated with a 6% reduction in the risk of clinical respiratory infections, but this did not reach statistical significance. The included trials disagreed with each other substantially, and overall study quality was rated low. There were too few studies to reliably judge whether vitamin D reduced laboratory-confirmed infections, a more objective outcome. The evidence was also insufficient to show a protective effect even in the sub-groups of participants who were vitamin D deficient at the start.

Before this review, the trial evidence on vitamin D and respiratory infections was genuinely unsettled, with some studies suggesting protection and others none. This pooled analysis tilts the picture towards no meaningful benefit in previously healthy people, though the authors are careful to note that the included studies varied in population, baseline vitamin D levels and study length, which complicates any firm conclusion. The biggest open question is whether people who are truly deficient respond differently, something these trials could not settle. The authors call for future trials focused on deficient individuals, using more objective and standardised outcome measures.

For a generally healthy person, this is a reason to be sceptical that a vitamin D supplement will meaningfully cut the number of colds and chest infections you get through the year. The finding applies to previously healthy adults and children, not to people with diagnosed deficiency, where the evidence was simply too thin to judge either way. It also says nothing about vitamin D's other roles in the body, only about infection prevention. The honest reading is that the popular cold-prevention rationale for vitamin D did not hold up when the trial evidence was pooled.