Heart and lungs

Vitamin D 'protects against severe asthma attacks'

"Vitamin D supplements could halve risk of serious asthma attacks," The Guardian reports. A review of previous data found that vitamin D supplements could have a protective effect against serious asthma attacks when taken alongside normal asthma treatment.

Vitamin D is made in the skin when it's exposed to sunlight. Many people in the UK have low levels of vitamin D, especially during the winter when sunlight is weak. People with low levels of vitamin D seem to be more likely to have asthma attacks. The researchers wanted to pool data to get a clearer picture of whether vitamin D supplements could help.

Any review of this sort is only as good as the studies fed into it. While the studies were judged to be of a good quality, the reviews' authors warn that there were "relatively few" studies included; seven in total. But the main conclusions were based on just three studies which mainly involved adults with mild or moderate asthma. This means the results may not apply to people with severe asthma, or to children.

Unfortunately, the review can't tell us whether everyone with asthma would benefit from taking vitamin D, or only people whose vitamin D levels are low. Also, it doesn't give us a recommended dose. 

Public Health England recently recommended everyone in the UK considers taking a 10microgram dose of vitamin D daily, especially in winter. However doses in some of these asthma studies were much higher.

So it is unclear whether there is such a thing as an optimal dose, and even if there was, what that would be.

Where did the story come from?

The study was carried out by researchers from several different universities, all of whom are part of the Cochrane Collaboration, an international network of researchers who review medical evidence according to high quality rules. It had no external funding. The study was published in the peer-reviewed journal Cochrane Database of Systematic Reviews. As with all Cochrane publications, the study is open-access, so you can read it for free online.

The UK media covered the study reasonably accurately. The Guardian and BBC News gave clear summaries of the research, pointing out the limitations of the study as well as the top findings.

The Mail Online stated that "as long as patients do not take a huge dose, there is no risk of side effects," which is not entirely true. Some people in one study of low-dose vitamin D were found to have too much calcium in their urine, which over time can damage the kidneys.

What kind of research was this?

This was a systematic review and meta-analysis of double-blind, randomised, placebo-controlled trials. These are the most reliable types of studies – the gold standard – when seeing if a drug or other treatment works. However, the researchers only found seven studies that met their criteria for inclusion, and only three studies (one including 22 children and two including 685 adults) contributed to the main findings.

What did the research involve?

The authors did searches for completed randomised controlled studies of people with clinically-diagnosed asthma, which tested the effects of vitamin D supplements. They were mainly interested in the effects on asthma attacks, defined as an attack needing to be treated with steroid pills, but they looked at other outcomes too, including emergency hospital visits, day-to-day asthma symptoms, lung function tests and time off school or work. They pooled the results to see how vitamin D affected the chances of having any of these outcomes.

They also examined the studies for anything that might have biased the results, and graded their findings as based on high-quality, moderate-quality or low-quality evidence. The studies were chosen by two researchers working independently, which helps reduce the risk of bias.

The researchers then used standard statistical methods to calculate the risk of having each outcome measured, with and without vitamin D, and tested their results for reliability and over-reliance on one study or another.

What were the basic results?

People with asthma who took vitamin D supplements had on average 0.22 attacks needing treatment with steroid pills each year (approximately one every four years), compared to 0.44 attacks (approximately one every two years) for those people taking placebo supplements (rate ratio 0.63, 95% confidence interval (CI) 0.45 to 0.88).

People taking vitamin D were also less likely to need to go to hospital at any time with an asthma attack. Three people in every 100 who took vitamin D in the studies had to go to hospital with an asthma attack, compared to six in every 100 who took placebo supplements (odds ratio 0.39, 95% CI 0.19 to 0.78).

However, vitamin D did not seem to have any effect on people's day-to-day asthma symptoms, tests of lung function, or time taken off school or work. No-one in the studies died of an asthma attack, so it's not possible to say whether vitamin D reduces the risk of a fatal asthma attack.

How did the researchers interpret the results?

The researchers said: "Vitamin D is likely to offer protection against severe asthma attacks." However, they add that they need to see more research done on children and people who get severe asthma attacks, before they can give "definitive clinical recommendations" about who should take vitamin D.

In interviews given to journalists, they suggest people consider taking vitamin D, and one researcher said people should request a test to see if they have low vitamin D levels, then ask their GP or pharmacist for advice.


Asthma attacks are frightening for adults and children alike, and can be fatal. A treatment that can help people avoid having an asthma attack, especially a severe attack that needs hospital treatment, has long been an aim of asthma research. If a simple vitamin supplement, already recommended for use, can help reduce the risk of attacks, that's excellent news.

There are a few important points to remember, however:

  • People with asthma should not stop taking their normal asthma medication. Everyone in the studies took vitamin D as well as their asthma treatment, not instead of it.
  • The information for reducing numbers of childhood asthma attacks relies on one study of just 22 children. We need better information from a bigger study to be sure it helps children.
  • We don't know whether everyone with asthma would benefit, or just people already low in vitamin D.
  • We don't know the best dose to take for people wanting to reduce their risk of asthma attacks.

It's unlikely that taking the Public Health England recommended dose of 10 micrograms of vitamin D daily will cause any harm, although we don't know if it's enough to help prevent asthma attacks. More than 100 micrograms could be harmful. Vitamin D can encourage the body to absorb more calcium than it needs, which over time could damage the kidneys, heart and bones.

If you are considering adding vitamin D supplementation to your, or your child's, asthma medication then it is a good idea to discuss it with the doctor in charge of your care. 

NHS Attribution