The sunshine nutrient made by the skin in sunlight also reduced by nearly a third the number of asthmatics needing steroids after suffering breathing difficulties.
It adds further evidence the nutrient also found in oily fish such as salmon, sardines, herring, mackerel and fresh tuna, red meat, liver, egg yolks and fortified foods such as most fat spreads and some breakfast cereals boosted the body’s defences.
The incurable long-term condition affects around 5.4 million Britons – one in every 12 adults and one in every 11 children.
Yet it kills three people die every day but two in three asthma deaths are preventable - writesgolifehacks.info.
The attacks are commonly triggered by viral upper respiratory infections.
Scientists believe the vitamin protects against such attacks by boosting immune responses to respiratory viruses and dampening down harmful airway inflammation.
The study by Queen Mary University of London analysed the individual data from 955 participants in seven randomised controlled trials, which tested the use of vitamin D supplements.
Vitamin D supplementation resulted in 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.
It also reduced by 50 per cent the risk of experiencing at least one asthma attack requiring a visit to casualty or being admitted from six per cent of people experiencing such an event to three per cent.
Vitamin D supplementation was also found to be safe at the doses administered and no instances of excessively high calcium levels or renal stones were seen.
Serious adverse events were evenly distributed between participants taking vitamin D and those on placebo.
Lead researcher Professor Adrian Martineau 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.”
And because it looked at so many individuals, the researchers could see the extent to which different groups respond to vitamin D supplementation, in more detail than previous studies.
In particular, vitamin D supplementation was found to have a strong and statistically-significant protective effect in participants who had low vitamin D levels to start with.
They saw a 55 per cent reduction in the rate of asthma attacks requiring treatment with steroid tablets or injections – from 0.42 events per person per year to 0.19.
Because of the relatively small numbers of patients within sub-groups, researchers cautioned they did not find definitive evidence to show that effects of vitamin D supplementation differ according to baseline vitamin D status.
First author Dr David Jolliffe said: “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 National Institute for Health Research 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.”
The study was published in The Lancet Respiratory Medicine.