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 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.
They 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.
The team included data from 20 clinical trials 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.
Accumulated data indicates that people given vitamin D supplements did not have a lower risk of severe asthma attacks compared to those given placebo (dummy medication). And vitamin D supplementation did not influence measurements of asthma control or breathing capacity; neither did it affect risk of serious harmful side effects.
Discussing the limitations of the review, the authors note that people with severe asthma and those with very low vitamin D levels prior to supplementation were poorly represented, so they could not assess whether vitamin D supplements might help these individuals.
They note that 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.
Implications for practice
The report concludes: "The null findings of this review are based on the largest pool of evidence analysed to date, and are consistent for primary and secondary outcomes.
"We deemed this evidence to be high quality for the primary outcome of reductions in asthma exacerbations requiring systemic corticosteroids, and also for several of the secondary outcomes. Nonetheless, participants with severe asthma and those with baseline 25‐hydroxyvitamin D (25(OH)D) concentrations < 25 nmol/L were poorly represented, and a protective effect of the intervention cannot be excluded in these groups.
"A single study investigating effects of calcidiol yielded positive results; further studies investigating effects of this metabolite are needed."
They add that further research is required to clarify potential effects of calcidiol on risk of asthma exacerbation, and to determine whether vitamin D supplementation may yet have an effect in people with severe asthma or those with the lowest levels of baseline vitamin D (25(OH)D < 25 nmol/L), in whom a significant protective effect cannot currently be excluded.
Source: Cochrane Library
"Vitamin D for the management of asthma"
Authors: Anne Williamson, Adrian R Martineau, Aziz Sheikh, David Jolliffe and Chris J Griffiths