Vitamin D supplements don't prevent severe asthma attacks in at-risk children, according to a study that challenges previous research.

"The reason that's important is there are colleagues around this country and worldwide who are testing vitamin D levels for kids with asthma and giving them vitamin D," said study lead author Dr. Juan Celedón. He's chief of pediatric pulmonary medicine at UPMC Children's Hospital of Pittsburgh.

"As a system, it costs a lot of money to run all these tests and give the supplements. We've shown no benefit for children with moderately low vitamin D levels," Celedón said in a hospital news release.

While past observational studies suggested that vitamin D could reduce asthma-attack severity, this is reportedly the first placebo-controlled clinical trial to assess if that's true.

The three-year study included nearly 200 children, aged 6 to 16, across seven U.S. hospital systems. All had at least one asthma attack during the year before the study began.

Half of the children received 4,000 IU of vitamin D per day, and the other half got placebo pills. No one involved in the study knew which type of pill each child was getting.

All of the children had vitamin D levels low enough that supplements should have had an effect if vitamin D truly is beneficial for reducing severe asthma attacks. But kids who took the supplements did not have fewer asthma attacks or less reliance on inhaled steroids than those who took placebo pills.

The study was published Aug. 25 in the Journal of the American Medical Association.

Randomized controlled studies such as this one are considered the gold standard of research.

Previous observational studies found that children with low vitamin D levels seemed to have worse asthma.

But with "observational studies, you never know—is [low] vitamin D causing asthma to be worse or do kids with worse asthma end up having lower vitamin D?" Celedón said.