Taking vitamin D supplements can halve the risk of severe asthma attacks, experts have found.
Experts believe taking the pills helps reduce inflammation and wards off viruses.
The British study, funded by the research arm of the NHS, found people who took the daily tablets alongside their normal asthma treatment were 50 per cent less likely to suffer attacks so severe that they had to be taken to A&E.
And they saw a 30 per cent reduction in attacks requiring treatment with steroids. Some 5.4million people in the UK suffer with asthma. In New Zealand, 521,000 people take medication for asthma, according to Statistics NZ.
Three people die every day in the UK because of asthma attacks, and two of these deaths are usually preventable.
Vitamin D is already known to strengthen the bones and muscles.
But the new study, published last night in the Lancet Respiratory Medicine journal, adds to growing evidence that it also protects against respiratory problems.
Nearly a third of the British population is deficient in the vitamin, thanks to our modern diets, indoor lifestyle and grey weather.
Lead researcher Professor Adrian Martineau of Queen Mary University London, said: "These results add to the ever growing body of evidence that vitamin D can support immune function as well as bone health.
"On average, three people in the UK die from asthma attacks every day.
"Vitamin D is safe to take and relatively inexpensive so supplementation represents a potentially cost-effective strategy to reduce this problem."
Public Health England last July published new guidance advising everyone in the country to take 10micrograms of vitamin D over the winter to avoid bone diseases such as rickets.
During the spring and summer, the skin makes vitamin D when it is exposed to the sun.
But in the autumn and winter Britain's gloomy weather, and our indoor lifestyles, means most people have to rely on their diet to get enough of the vital vitamin.
They can do that by eating liver, eggs, red meat and plenty of oily fish - but millions of people who do not eat enough of these foods should take supplements instead.
The new research, funded by the National Institute for Health Research, collated individual data from 955 participants in seven previous randomised controlled trials.
The scientists found a 30 per cent reduction in the rate of asthma attacks requiring treatment with steroid tablets or injections - from 0.43 events per person per year to 0.30.
And they saw a 50 per cent reduction in the risk of experiencing at least one asthma attack requiring hospitalisation - from 6 per cent to 3 per cent.
Co-author Dr David Jolliffe from QMUL added: "Our results are largely based on data from adults with mild to moderate asthma - children and adults with severe asthma were relatively under-represented in the dataset, so our findings cannot necessarily be generalised to these patient groups at this stage.
"Further clinical trials are on-going internationally, and we hope to include data from them in a future analysis to determine whether the promise of today's results is confirmed in an even larger and more diverse group of patients."
Professor Hywel Williams, director of the NIHR Health Technology Assessment Programme, said: "The results of this NIHR-funded study brings together evidence from several other studies from over the world and is an important contribution to reducing uncertainties on whether Vitamin D is helpful for asthma - a common condition that impacts on many thousands of people worldwide."