Asthma risk unaltered by taking extra vitamin D
Taking vitamin D supplements does not reduce the risk of asthma attacks in children or adults, research shows.
The findings are in contrast to a previous review that indicated a reduction in asthma attacks among people taking vitamin D.
Researchers from the University of Edinburgh and Queen Mary University of London did not examine other potential health benefits, but stress there is no harm in taking vitamin D.
Experts analysed the results of 20 randomised controlled trials that included data on 1,155 children and 1,070 adults. The majority of patients in the trials had mild to moderate asthma.
Researchers compared outcomes for patients who took a vitamin D supplement with others who took a placebo.
The team found no significant difference in the number of people who experienced an asthma attack requiring treatment with a course of steroid tablets.
The review – published by Cochrane – did not find any effect of taking vitamin D on asthma control even if people were vitamin D-deficient when they joined the studies.
Neither was there any effect with different doses of the supplement, nor in people of different ages.
Most of the trials included in the review involved patients taking the typical form of vitamin D supplement, which is called cholecalciferol.
One trial that used calcidiol – a compound that the body can make from vitamin D – reported an improvement in asthma control in patients taking this supplement.
The reviewers say further research is needed to confirm whether this form of vitamin D has benefits for people with asthma.
Our updated systematic review, which brought together evidence from a total of 20 trials undertaken in children and adults, found that vitamin D did not reduce the risk of asthma exacerbations.
There were limited numbers of people with very severe asthma and with very low levels of vitamin D prior to supplementation, so we are not in a position to come to any clear conclusions as to whether such individuals might be helped by vitamin D supplementation.
Vitamin D deficiency has been linked to an increased risk of severe asthma attacks and our previous Cochrane review, published in 2016, found that vitamin D reduced the risk of asthma attacks.
However, more studies have been published since then and when we included the extra data in our updated review, the overall results changed.
The trials we looked at did not include many people with severe asthma or people with very low levels of vitamin D in their blood, so these are areas where more research is still needed.
Read the full review on Cochrane Library
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