Half of the participants were randomized to receive 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 participant was getting.
All of the children had vitamin D levels low enough that supplements should have an effect — if vitamin D truly is beneficial for reducing severe asthma attacks — but the study excluded children with severe vitamin D deficiency because it would be unethical to withhold it in those cases.
Compared to placebo, vitamin D did not reduce the number of asthma attacks participants experienced or their reliance on inhaled steroids.
That’s different from what was seen in the past with observational studies in Costa Rica, the U.S. and Canada, and Puerto Rico, where children with naturally low vitamin D levels seemed to have worse asthma.
“With observational studies, you never know — is vitamin D causing asthma to be worse or do kids with worse asthma end up having lower vitamin D?” said Celedón, who also holds the Niels K. Jerne chair of Pediatrics at the University of Pittsburgh School of Medicine.
Even with the rigor of the present study’s design, Celedón acknowledges that he can’t draw conclusions about whether very low vitamin D levels contribute to asthma attacks, but he argues that those children would be supplemented either way because of known effects on bone health.