Daily vitamin D supplements lowered the risk and severity of asthma attacks, according to a new review of nine clinical trials.

The analysis, published in the Cochrane Database of Systematic Reviews, involved 435 children and 658 adults, most of whom had mild to moderate asthma.

The reviewers found that oral vitamin D supplements ranging from 400 to 4,000 units a day reduced the risk of attacks requiring medication by 37 percent. The number of attacks requiring emergency intervention decreased by more than 60 percent among vitamin D users.

At the same time, taking vitamin D did not appear to have a meaningful effect on daily symptoms as measured by a lung function test and questionnaires. The review found no effect on FEV1, the commonly used test of lung function, or on scores on the Asthma Control Test, a questionnaire that uses a 25-point scale to measure how well a person’s asthma is controlled.

The reason for the effect is unclear, but the authors suggest that vitamin D triggers antiviral and anti-inflammatory responses that might decrease the risk for lung infection.

“We don’t yet have the evidence to say that everyone should take it,” said the lead author, Adrian R. Martineau, a professor of respiratory infection at Queen Mary University of London.