Giving people with asthma and low vitamin D levels extra doses of the vitamin doesn't do much to help their lungs, a new study suggests.
Previous research had linked low vitamin D levels to more asthma-related problems, like weaker lungs and more asthma exacerbations.
But it wasn't clear if vitamin deficiencies were causing those problems, or if giving patients extra vitamin D was the answer.
Lead author Dr Mario Castro from Washington University School of Medicine in St Louis, Missouri, said as many as two-thirds of his asthma patients are lacking in vitamin D.
"Our thought was that vitamin D would potentially make . . . the standard therapy, inhaled corticosteroids, work better in these patients, and that it would reduce their asthma exacerbations," he told Reuters Health.
To test that theory, the researchers studied just over 400 adults with asthma and low vitamin D levels. They randomly assigned half of the participants to take vitamin D3 supplements every day, and the other half to take a vitamin-free placebo pill.
All of the patients also took corticosteroids to help keep their asthma in check.
Over the next six months, Castro and his team tracked so-called treatment failures - the times patients were sent to the emergency room or hospitalised for asthma, had declines in lung function or had to increase their use of asthma medication.
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They found between one-quarter and one-third of participants in both the vitamin D and placebo groups had a treatment failure during the study. The only benefit linked to vitamin D was that patients in the vitamin group needed slightly lower doses of corticosteroids to control their asthma.
When the researchers looked only at patients whose vitamin D levels significantly improved from the supplements, they found benefits in some areas compared to the placebo group - such as fewer patients with an asthma exacerbation - but not in other areas.
Inexpensive and minimal side effects
The findings don't support using vitamin D as a general treatment strategy for people struggling with asthma symptoms, the authors write in the Journal of the American Medical Association. They also presented their results Sunday at the American Thoracic Society International Conference in San Diego.
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Castro said he thinks it is still appropriate to treat some asthma patients with vitamin D while more research is being conducted. He prescribes it to people with low vitamin D levels who also have uncontrolled asthma symptoms, and monitors them to make sure their levels come up.
"I think there may be some benefit, and vitamin D is inexpensive and has (minimal) side effects," he said.
Vitamin D3 supplements cost a few dollars per month for the dose used for most of the study, about 4 000 international units.
Dr Ken Kunisaki from the University of Minnesota in Minneapolis said there are still some questions, such as what is a big enough dose of vitamin D to give people who are deficient.
But he said that in general, the new findings were "disappointing."
Regarding the role of vitamin D in asthma, "It's been a fairly consistent story from the observational studies," Kunisaki, who specializes in pulmonary and critical care medicine, told Reuters Health.
Those studies relied on measuring people's vitamin D levels and asthma symptoms, rather than instructing some patients to take extra vitamin D. In observational studies, it's possible another underlying factor could explain the link between low vitamin D and asthma-related problems, he said.
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"It goes to show why we do randomised clinical trials," said Kunisaki, who wasn't involved in the new research.
"This study would say that at least in this population, there's no role for routinely looking at vitamin D levels, at least for asthma-control purposes," he concluded.
The study was funded by the National Institutes of Health, and the asthma medications used were provided free of charge by their manufacturer, Sunovion Pharmaceuticals.
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