Vitamin D and colds: a major analysis of 25 trials

What does the research say?

Across 25 trials, vitamin D supplements slightly reduced the risk of acute respiratory infections such as colds and flu. The benefit was much bigger for people who were very low in vitamin D to start with, and for those taking it daily or weekly rather than in occasional large doses.

The science

Instead of pooling published summaries, the researchers analysed the raw data from each participant, which allowed them to see who benefited most.

Overall, supplementation reduced the odds of at least one respiratory infection by about 12%. In people with very low starting levels (below 25 nmol/L) taking daily or weekly doses, the reduction was around 70%. Large single ‘bolus’ doses showed no benefit.

Key findings

• About 12% lower odds of acute respiratory infection overall.
• Much greater benefit in people who were deficient at the start.
• Daily or weekly dosing worked; large infrequent doses did not.

Limitations

• Trials varied in dose, population and how infections were recorded.
• The overall effect was modest.
• Later trials have produced mixed results, so this remains an area of debate.

References

1. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. doi.org/10.1136/bmj.i6583