Taking Vitamin D supplements in addition to asthma medication appears to cut the risk of severe asthma attacks, a review of evidence suggests.
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