Taking a simple vitamin D supplement could spare you suffering through a cold or flu this year, UK researchers now claim.
The sunshine vitamin is vital for healthy bones, but also has a role in the immune system.
The analysis, published in the British Medical Journal, argues food should be fortified with the vitamin.
Trials on using supplements to prevent infections have given mixed results, so the researchers pooled data on 11,321 people from 25 separate trials to try to get a definitive answer.
The team at Queen Mary University of London (QMUL) looked at respiratory tract infections – which covers a wide range of illnesses from a sniffle to flu to pneumonia.
Overall, the study said one person would be spared infection for every 33 taking vitamin D supplements.
The researchers say that is more effective than flu vaccination, which needs to treat 40 to prevent one case, although flu is far more serious than the common cold.
There were greater benefits for those taking pills daily or weekly – rather than in monthly super-doses – and in people who were deficient in the first place.
One of the researchers, Professor Adrian Martineau, said: “Assuming a UK population of 65 million, and that 70% have at least one acute respiratory infection each year, then daily or weekly vitamin D supplements will mean 3.25 million fewer people would get at least one acute respiratory infection a year.”
But Public Health England (PHE) says the infections data is not conclusive, although it does recommend supplements.
These, it says, should be taken for improved bone and muscle health.
The immune system uses vitamin D to make antimicrobial weapons that puncture holes in bacteria and viruses.
Professor Louis Levy, the head of nutrition science at PHE, also tells BBC.com that the latest vitamin D research is inconclusive on the score of being a cold cure.
“The evidence on vitamin D and infection is inconsistent and this study does not provide sufficient evidence to support recommending vitamin D for reducing the risk of respiratory tract infections.”
But Prof Martin Hewison, from the University of Birmingham and the Society for Endocrinology, said the findings were “striking”.
“I agree with the authors that this study supports a new indication for vitamin D beyond its established benefits for bone health,” he adds.