Key messages

1) 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.

2) 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.

Why did we think that vitamin D might benefit patients with asthma?

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.

Our previous Cochrane Review on this topic in 2016 found that vitamin D reduced the risk of asthma attacks, yet debate has continued, and some subsequent trials found vitamin D to have no effect. We therefore conducted an updated meta-analysis to include data from new trials completed since our last review.

What did we want to find out?

We wanted 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.

What did we do?

We 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.

We also analysed whether effects of vitamin D supplementation differed according to baseline vitamin D status, the dose or form of supplements administered, how often people took the supplements, or the age of participants.

What did we find?

We included data from 20 clinical trials in this review that involved a total of 2225 people; nine of these were included in the previous Cochrane Review on the topic and 11 were published since then. Of the 20 studies, 15 reported data on severe asthma attacks. The trials lasted between three and 40 months, and all but two investigated a particular form of vitamin D called cholecalciferol or vitamin D3. This is the most common form of vitamin D tablet.

• People given vitamin D supplements did not have a lower risk of severe asthma attacks compared to those given placebo (dummy medication).

• Vitamin D supplementation did not influence measurements of asthma control or breathing capacity; neither did it affect risk of serious harmful side effects.

What are the limitations of the evidence?

• People with severe asthma and those with very low vitamin D levels prior to supplementation were poorly represented, so we cannot assess whether vitamin D supplements might help these individuals.

• A single study investigating effects of calcidiol, an alternative form of vitamin D, showed a protective effect. Further investigation of this form of vitamin D is needed.

How up to date is this evidence?

This review updates our previous review. The evidence is up to date to September 2022.