Taking an oral vitamin D supplement in addition to standard asthma medication could halve the risk of asthma attacks requiring hospital attendance, according to research led by Queen Mary University of London.

This is particularly good news for the 15 to 20 million asthma sufferers in India — many of whom are likely vitamin D-deficient unknowingly — heading into the holiday and fireworks season, when the already polluted air quality of metros takes a turn for the worse. Vitamin D is thought to prevent such attacks by boosting immune responses to respiratory viruses and dampening harmful airway inflammation.

The new study, funded by the National Institute for Health Research, and published in The Lancet Respiratory Medicine, collated and analysed the individual data from 955 participants in seven randomised controlled trials, finding a vitamin D supplement led to:

• a 30% reduction in the rate of asthma attacks requiring treatment with steroid tablets or injections
• a 50% reduction in the risk of experiencing at least one asthma attack requiring emergency care and/or hospitalisation

The researchers found no side effects of supplementation, for instance, no participants developed excessively high calcium levels or renal stones, and serious adverse events were evenly distributed between participants taking the vitamin D supplement and those on placebo.

“These results add to the ever growing body of evidence that vitamin D can support immune function as well as bone health,” said lead researcher Adrian Martineau. “On average, three people in the UK die from asthma attacks every day. Vitamin D is safe to take and relatively inexpensive so supplementation represents a potentially cost-effective strategy to reduce this problem.”

In particular, the vitamin D supplement was found to have strong and statistically-significant protective effect in participants who had low vitamin D levels to start with. These participants saw a 55% reduction in the rate of asthma exacerbations requiring treatment with steroid tablets or injections.

Study authors caution that participants were mostly adults with mild to moderate asthma; the findings aren’t necessarily transferable to children and adults with severe asthma, they said, who were underrepresented in the group of 955 people studied.