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Supplementing vitamin D to asthma treatment to improve breathing is beneficial to only patients who achieve sufficient levels of supplement in the blood, according to a new study. 

The ability to control asthma did not differ between a study group that received vitamin D supplements and a group that received placebo, the study noted.

"Previous studies suggested that if you have asthma and low levels of vitamin D in the blood, you have worse lung function, more asthma attacks and more emergency room visits than asthma patients with higher vitamin D levels," said Mario Castro, MD, the Alan A. and Edith L. Wolff Professor of Pulmonary and Critical Care Medicine, in the press release. "This is the first randomized controlled trial to investigate whether taking vitamin D supplements can improve asthma control."

The study involved 408 adult patients from nine major U.S. medical centers. Every considered patient had a diagnosis of mild to moderate asthma, and all had what is considered deficient blood levels of vitamin D. They were then assigned randomly to one of the two groups. The treatment group received a loading dose of 100,000 international units of vitamin D3 followed by daily doses of 4,000 units, while the placebo group received identical looking but inactive vitamin D capsule. 

The investigators found no differences between the two groups in all major measures of asthma control. The groups showed no significant differences in the number of treatment failures requiring patients to take more medication, no difference in the number of asthma attacks and no difference in their need for emergency care, the press release added. 

However the two groups differed in how successfully they were able to reduce their daily dosages of inhaled steroids. 

"The difference was small-15 micrograms of steroid per day-but statistically significant," said Castro, who treats patients at Barnes-Jewish Hospital. "Over the long term, even that small amount may have an important impact on reducing side effects of inhaled steroids. Although inhaled steroids work very well in controlling asthma, patients don't like them because they cause weight gain and increase the risk of diabetes and high blood pressure. Anything we can do to reduce the amount they need is important."

The study has been published in JAMA.