Low blood levels of vitamin D (the 'sunshine vitamin') have been linked to an increased risk of severe asthma attacks, defined as those requiring oral (systemic) steroid medications.

A previous Cochrane Review on this topic in 2016, by the same authors of the current updated review, found that data suggested vitamin D reduced the risk of asthma attacks. Yet debate has continued, and some subsequent trials have found vitamin D to have no effect. 

The team or researchers, from Queen Mary University of London, UK, therefore conducted an updated meta‐analysis to include data from new trials completed since their last review.

They aimed to find out if vitamin D supplementation: reduces the risk of severe asthma attacks; improves control of asthma symptoms; leads to any negative side effects.

Their review of 20 clinical trials, amounting to 2,225 participants, concludes: "In contrast to our previous Cochrane Review on this topic, this updated review does not find that vitamin D offers protection against severe asthma attacks or improves control of symptoms.

"Further trials are required in people with frequent severe asthma attacks and those with very low baseline vitamin D status, and into the potential for calcidiol (a particular form of vitamin D) to offer protective effects."

The review

The authors searched for randomised controlled trials that assessed the effect of vitamin D supplementation on the risk of severe asthma attacks and asthma symptom control. We compared and summarised the results of the studies and rated our confidence in the evidence, based on factors such as study methods.

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