Vitamin D Supplements Did Not Ward Off Respiratory Infections

CATEGORY: FOODS & NUTRIENTS

A vitamin D capsule is a common fixture of a winter routine, taken in the hope of fending off colds. The largest pooled analysis of randomized trials yet assembled found no statistically significant protection against colds, flu or other respiratory infections. It drew on 40 studies covering 61,589 people and appeared in The Lancet Diabetes & Endocrinology.¹ Roughly half of participants came down with at least one respiratory infection during their trial, on the supplement or on placebo. The share barely differed. The odds were about 6% lower among supplement takers, a gap small enough that chance alone could account for it.

A Celebrated Finding That Shrank as the Data Grew

This revises one of the best-known results in supplement research, and it comes from the same team that produced it. In 2021 these researchers pooled 43 trials and reported that people given vitamin D had about 8% lower odds of a respiratory infection, modest protection but statistically significant.² The new analysis adds six newer trials, several of them large. The protection no longer clears the bar of significance. The authors note that the estimate itself barely moved while its uncertainty now spans no effect, and they point to a familiar pattern called small-study effects: the smaller trials in the pool tended to report bigger benefits, while the largest and most recent found essentially none.

Sixty Thousand Randomizations, One Question

Every study in the pool was a randomized trial. Children and adults from countries around the world took vitamin D or an identical-looking placebo, sorted by lot into one group or the other, the design that can establish cause and effect rather than mere association. Researchers then recorded who developed at least one acute respiratory infection, a category running from the common cold through flu to bronchitis and pneumonia, over follow-up periods lasting from a few weeks to several years. Earlier analyses had suggested the benefit might concentrate in particular groups, such as people taking the vitamin daily rather than in occasional large doses, or those starting out with low blood levels. That did not hold up either. Comparisons by age, by starting blood level, by dosing frequency and by dose size showed no statistically significant differences between subgroups. No slice of the population turned up in which the supplement reliably worked.

What the Verdict Leaves Alone

The finding is about preventing respiratory infections in the general population. Bone is a different question. Vitamin D’s established role in building and maintaining bone, where it helps the body absorb calcium, is untouched by this result, as is the treatment of diagnosed vitamin D deficiency. And because the analysis pooled randomized trials rather than observational studies, its answer carries the weight that only random assignment can give. The absence of protection is not an artifact of who chooses to take supplements.

Key Takeaways

• Across 40 randomized trials with 61,589 participants, vitamin D supplements did not significantly reduce the risk of colds, flu or other respiratory infections, and roughly half of participants got at least one infection with or without the supplement. No age group, dose size, dosing schedule or starting blood level showed convincing protection.
• The same team’s 2021 analysis had found modest, statistically significant protection; adding six newer trials, several of them large, erased that significance. The result concerns prevention of respiratory infections and leaves untouched vitamin D’s role in bone health and the treatment of diagnosed deficiency.

References

1. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology. 2025;13(4):307–320. link
2. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. The Lancet Diabetes & Endocrinology. 2021;9(5):276–292. link