Low levels of vitamin D in the blood have already been linked to an increased risk of having asthma attacks.
They examined results of studies of 435 children and 658 adults from the U.K., Canada, India, Japan, Poland, and the U.S. They were an ethnically diverse group, reflecting a range of geographic settings.
Most people taking part had mild to moderate asthma with a smaller number having severe asthma.
Studies lasted 6-12 months and usually involved vitamin D being taken alongside usual asthma medication.
Taking vitamin D supplements was associated with fewer hospitalizations or emergency room visits for severe asthma attacks -- from 6% to around 3%.
Taking vitamin D supplements also reduced the number of asthma attacks needing steroid treatment.
There was no increase in side effects with the vitamin D doses given.
"This is an exciting result, but some caution is warranted,” study researcher Professor Adrian Martineau from the Asthma UK Centre for Applied Research, Queen Mary University of London, says in a statement.
"First, the findings relating to severe asthma attacks come from just three trials: most of the patients enrolled in these studies were adults with mild or moderate asthma. Further vitamin D trials in children and in adults with severe asthma are needed to find out whether these patient groups will also benefit.
"Second, it is not yet clear whether vitamin D supplements can reduce risk of severe asthma attacks in all patients, or whether this effect is just seen in those who have low vitamin D levels to start with. Further analyses to investigate this question are ongoing, and results should be available in the next few months."
Commenting on the findings in a statement, Dr. Erika Kennington, Asthma UK’s head of research, says: "While this research shows promise, more evidence is needed to conclusively show whether vitamin D can reduce asthma attacks and symptoms. With so many different types of asthma it could be that vitamin D may benefit some people with the condition but not others. Asthma UK’s research centres are working hard to discover how and why vitamin D affects asthma symptoms and if it could be a potential treatment in the future."