New Cochrane review: Vitamin D supplements do not reduce the risk for asthma attacks

According to an updated Cochrane review, vitamin D supplements do not reduce the risk of asthma attacks in children and adults.

These results are in direct contrast to the findings of a Cochrane review which indicated an increase in asthma attacks in those who took vitamin D. However it did not examine any other health benefits.

Researchers from Queen Mary University of London as well as the University of Edinburgh conducted the review. Adrian Martineau, Clinical Professor of Respiratory Immunity and Immunity at Queen Mary University of London said that vitamin D deficiency is linked to severe asthma attacks. Our previous Cochrane review published in 2016 found that vitamin D reduces the risk of developing asthma attacks. There have been many more studies published since then, and the overall results were altered when we added the additional data to our updated review. Vitamin D supplements did not have any effect on asthma symptoms or the risk of developing asthma attacks, as compared to a placebo.

Professor Martineau and his associates analysed the results from 20 randomised controlled trials, which are the gold-standard in medical research. This included data on 1,155 children with asthma and 1,070 adults. This compares with nine trials that involved 1,093 people, whose data were used in the previous review. The majority of patients who participated in the trials had mild-to-moderate asthma.

Researchers compared patients who received a vitamin D supplement to patients who received a placebo (dummy medication) and found no statistically significant differences in the number who had an asthma attack that required treatment with a course steroid tablets.

The review found no effect of vitamin D on asthma control, even in those who were vitamin D-deficient at the time of the study, or in those of different ages or with different dosages.

Professor Martineau stated that, "Contrary to the previous Cochrane review on the topic, this updated review doesn't find that vitamin D offers protection from asthma attacks or improves asthma symptoms control." The trials we reviewed did not include people with severe asthma or those with very low levels vitamin D. These areas are where more research is needed.

We are unsure why the updated review produced a different result than our original study in 2016.

This could be because people with asthma are receiving better treatment. It could also be that vitamin D deficiency rates have declined over time due to increased intake of supplements and fortified foods. Either one of these factors could negate the potential benefits of vitamin D supplements. These latest findings are likely to prove valid for people with asthma today, regardless of the reason. This is why it's important to update reviews whenever new research is published.

Today's review contains data from 20 clinical trials, as opposed to nine in the 2016 review. Children with asthma are better represented than ever before. The review team stated that they applied stricter criteria to which studies would be included than in other reviews. They excluded studies that did no comparison of vitamin D with a placebo or that did not monitor patients for at most 12 weeks.

The majority of the reviewed trials involved patients who were taking cholecalciferol (a common form of vitamin D supplement). One study that used calcidiol (a compound the body can make from vitaminD) reported an improvement in asthma control for patients who took this supplement. Reviewers believe that further research is needed in order to confirm whether this form vitamin D is beneficial for people with asthma.

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