An independent review by the Cochrane research body of nine clinical trials found it also cut the rate of asthma attacks needing steroid treatment.
But researchers say it is unclear whether it only helps patients who are vitamin D deficient.
They say more studies are needed before they can give patients official advice.
They recommend talking to a GP or pharmacist to get advice before taking a vitamin D supplement.
The Cochrane review’s lead author, Professor Adrian Martineau, said they found vitamin D “significantly reduced the risk of severe asthma attacks, without causing side effects”.
They found taking vitamin D reduced the risk of severe asthma attacks requiring a hospital admission or a visit to A&E from 6% to 3%.
They also found the rate of asthma attacks needing steroid treatment dropped from 0.44 to 0.28 attacks per person per year.
But they found that vitamin D did not improve lung function or day-to-day asthma symptoms.
Known as the “sunshine” vitamin, it is found in food and is made in the body when the skin is exposed to sunshine
One in five adults and one in six children in England are thought to have low levels of vitamin D
Limited amounts of the vitamin are found in foods such as oily fish, eggs and fortified cereals
For most people the bulk of their vitamin D comes from sunlight
Low vitamin D levels can lead to brittle bones and rickets in children
Vitamin D can boost immunity and dampen down inflammation
It is possible to overdose from vitamin D – but that would be five times the amount of vitamin D that was given in these trials
Q&A: Vitamin D
The researchers looked at nine recent clinical trials – seven involving 435 children and two studies involving 658 adults, lasting up to a year.
Prof Martineau called the review “an exciting result” but acknowledged “some caution is warranted” and further study is needed.
The trials were mainly carried out on adults with mild or moderate asthma so further testing is needed to see the affect on children and those with severe asthma “to find out whether these patient groups will also benefit”, he said.
He said further analyses were on-going and results should be available in the next few months.
In July Public Health England recommended that everyone should consider taking vitamin D supplements in autumn and winter.
An extensive review of evidence suggested everyone over the age of one needs to consume 10 micrograms of vitamin D each day in order to protect bone and muscle health.
And public health officials said, in winter months, people should consider getting this from 10 microgram supplements, if their diet is unlikely to provide it.
The level of vitamin D taken in these clinical trials was much higher than this recommendation at 25 to 50 micrograms per day.In the UK, 5.4 million people are being treated for asthma – that is one in 11 of the population.
Every day there are 185 hospital admissions and three deaths because of the condition.
Dr Erika Kennington, Asthma UK’s head of research, said: “While this research shows promise, more evidence is needed to conclusively show whether Vitamin D can reduce asthma attacks and symptoms.
“With so many different types of asthma it could be that Vitamin D may benefit some people with the condition but not others. Asthma UK’s research centres are working hard to discover how and why Vitamin D affects asthma symptoms and if it could be a potential treatment in the future.”
‘Visit your GP’
Prof Martineau pointed out that in the study, vitamin D was added on to asthma medication the patients were already taking. He explained: “We don’t want people giving up taking their asthma treatment.”
He also warned against taking vitamin D without advice.
“Going to see your GP is a key part of the message we want to give – I don’t think it would be appropriate to just start taking vitamin D without knowing whether you have vitamin D deficiency or not and we don’t yet know what the threshold of vitamin D is below which you will have a benefit.”
Dr Rebecca Normansell, a GP from the Cochrane body, said asthma patients are not routinely tested for their vitamin D levels, but following further study “it may be that that will be something that we should be considering as a reason to test vitamin D”.
“Talking with your pharmacist or GP is a great place to start as there may be other things that could be done for you as well to improve your asthma beyond thinking about your vitamin D,” she said.
Dr Imran Rafi, from the Royal College of General Practitioners (RCGP), called the research “encouraging”.
“However, more work still needs to be done in gathering the evidence, particularly around effectiveness for young people and children – especially as it currently affects as many as one in 11 children.”
He said he looked forward to seeing the results of further clinical trials to get a better understanding of this potential method of treatment.
“It is important to remember that not every drug is suitable for every patient and if a patient has asthma, they shouldn’t make any changes to their medication without first discussing it with their family doctor.”
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