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There has been some debate in recent times about the association between vitamin D levels and the risk of developing certain conditions such as diabetes, colon cancer, arthritis and infections. Studies show conflicting results. But when it comes to asthma, vitamin D may indeed protect against severe bouts of the illness.

A Cochrane review published today shows that asthmatics given vitamin D had fewer severe asthma attacks that needed treatment with anti-inflammatory tablets, known as oral corticosteroids.

For those taking vitamin D, the average number of attacks per person per year went down from 0.44 to 0.22. This is a clinically relevant reduction, meaning it is significant enough to justify a change in treatment.

The Cochrane Library contains in-depth reviews, known as meta-analyses, that independently study the best available evidence generated through previous research in the field.

The Cochrane review also showed vitamin D reduced the likelihood of attending hospital for an acute asthma attack from six per 100 patients to around three per 100. However, vitamin D had little or no effect on day-to-day asthma symptoms or breathing tests.

No serious side-effects of vitamin D occurred at the doses tested and the evidence reviewed was graded as of high quality.

Previous trials

The meta-analysis included a comprehensive review of published and unpublished trials up to January 2016. The studies included compared children or adults with asthma who were randomly chosen to receive either vitamin D or identical dummy tablets (placebo) for at least 12 weeks.

Seven trials involving 435 children and two trials involving 658 adults contributed to a pooled analysis of the results. Most of the patients included had mild or moderate asthma.

Vitamin D comes in two forms in the body. Vitamin D3 (cholecalciferol) is made in human skin when exposed to sunlight or can be taken in the diet from eating oily fish or foods with added vitamin D or vitamin D tablets. The second form is vitamin D2 (ergocalciferol), which can be ingested in the diet, mainly by eating mushrooms.

Both forms are changed in the body to a chemical called 25-hydroxy-vitamin D, which can be measured in blood samples. Levels lower than 50 units (nmol/L) are widely accepted to indicate vitamin D deficiency. This is common in many populations, especially where sunlight levels are low or where people cover up.

Previous studies in both children and adults have found a link between low vitamin D levels and increased risk of asthma attacks. Individual studies have, however, used different definitions of asthma attacks and found differing results.

It was therefore important to use the available evidence in a standard way to assess the effects of vitamin D on the risk of severe asthma attacks. These were defined as those requiring treatment with oral medication of corticosteroids, used to treat the swelling in the air tubes in really bad asthma.

Why vitamin D?

Asthma, especially in younger people and children, is a result of the same type of inflammation as happens in allergies such as hay fever, food allergy and eczema.

Tissues in the body, such as the lungs, produce the active vitamin D form (1,25-dihydroxyvitamin D) in higher levels when they are inflamed or infected. The increased vitamin D seems to help fight the infection and have anti-inflammatory properties.

Given that viruses in the airways – during colds, for instance – trigger the majority of asthma attacks, it is likely the mechanism of action of vitamin D relates either to prevention of such infections, or to decreasing the resulting inflammation – or both.

The main limitations of the Cochrane review are that children and people with frequent severe asthma attacks were not well represented. Only 13 of the 305 children included experienced an attack that needed corticosteroids tablets, compared with 118 of 628 adults who needed the tablets.

Consequently, the finding that vitamin D protected against severe asthma attacks is based mainly on results from adults and may not be applicable to children. Also, the review does not provide evidence about the best dose of vitamin D or best level in the blood.

We need additional trials to establish if vitamin D can reduce the risk of severe asthma attacks in children and in people whose vitamin D levels are measured at the start and end of the trial.

The Conversation

Haydn Walters receives funding from NHMRC.

Julia Walters and Sean Beggs do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.