Vitamin D Supplements Did Not Ward Off Respiratory Infections

CATEGORY: FOODS & NUTRIENTS

Taking a vitamin D supplement did not provide statistically significant protection against colds, flu, or other respiratory infections in the largest pooled analysis of randomized trials yet assembled—40 studies covering 61,589 people, published in The Lancet Diabetes & Endocrinology.¹ Roughly half of participants came down with at least one respiratory infection during their trial whether they had been assigned to vitamin D or to a placebo. 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 result revises one of the best-known findings in supplement research—and it comes from the same team that produced it. In 2021, these researchers pooled 43 trials and reported that people assigned to vitamin D had about 8% lower odds of a respiratory infection, a modest but statistically significant protection.² The new analysis adds six newer trials, several of them large, and the protection no longer clears the bar of statistical significance. The authors note that the estimate itself barely moved but 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 trials found essentially none.

Sixty Thousand Coin Flips, One Question

Every study in the pool was a randomized trial: participants—children and adults from countries around the world—were assigned by chance to take vitamin D or an identical-looking placebo, 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 that runs from the common cold through flu to bronchitis and pneumonia, over follow-up periods ranging from a few weeks to several years. Pooling trials this way works like stitching dozens of small photographs into one panorama: a pattern too faint to make out in any single frame either sharpens into view—or, as happened here, fades into the background.

No Dose, Schedule, or Blood Level Stood Out

Earlier analyses had suggested the benefit might concentrate in particular groups—people taking vitamin D daily rather than in occasional large doses, or those starting out with low levels of the vitamin in their blood. The updated analysis found no convincing evidence of that either: comparisons by age, by starting vitamin D blood level, by dosing frequency, and by dose size showed no statistically significant differences in protection between subgroups. The analysis could not identify any slice of the population in which the supplement’s protection reliably appeared.

What the Verdict Covers, and What It Leaves Alone

The finding is about preventing respiratory infections in the general population—the reason many people add the supplement to a winter routine. It does not bear on vitamin D’s established role in building and maintaining bone, where the vitamin helps the body absorb calcium, nor on the treatment of diagnosed vitamin D deficiency, which remains a separate medical question. And because the analysis pooled randomized trials rather than observational studies, its answer carries the weight that only chance 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; roughly half of participants got at least one infection with or without the supplement.
• The same research team’s 2021 analysis had found modest, statistically significant protection; adding six newer trials, several of them large, erased that significance.
• No age group, dose size, dosing schedule, or starting blood level showed convincing protection in the updated analysis.
• The result concerns preventing respiratory infections; it does not bear on vitamin D’s established role in bone health or on treating 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