Past research has suggested that for people with asthma, vitamin D may help reduce symptoms. But a new study published in JAMA finds that for asthmatics with low vitamin D levels, supplementation with the vitamin had no such effect.

Asthma is a chronic disease that affects more than 25 million people in the US. It is characterized by inflammation and narrowing of the airways that transport air to and from the lungs.

The research team, led by Dr. Mario Castro of the Washington University School of Medicine in St. Louis, MO, note that in children and adults who have asthma, low vitamin D levels have been associated with abnormal lung function, an increase in asthma flare-ups and lower response to treatment, such as corticosteroids.

Some studies have indicated that vitamin D supplementation may reduce inflammation and promote effectiveness of corticosteroids for asthma patients, but the Washington University team wanted to investigate further.

For their study, the researchers randomized 408 asthma patients with low vitamin D levels to one of two groups.

Findings 'do not support vitamin D supplementation as a treatment for asthma'

Participants in the first group were required to take an initial 100,000 IU dose of vitamin D3, before taking a 4,000 IU dose daily for 28 weeks. Subjects in the second group were required to follow the same routine with a placebo. All participants continued with their usual corticosteroid treatment - inhaled ciclesonide.

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Researchers found that vitamin D3 supplements did not reduce symptoms in asthmatics with low vitamin D levels, or improve treatment.

At the end of the study period, the research team found that taking vitamin D3 alongside ciclesonide did not reduce the rate of initial treatment failure, compared with patients who took the placebo.

Treatment failure was defined as a decline in lung function and increased use of systemic steroids, beta-antagonists and use of health care institutions.

In detail, 28% of patients taking vitamin D3 experienced at least one treatment failure within 28 weeks, compared with 29% in the placebo group. The most common cause of treatment failure was down to the need for increased or inhaled systemic steroids or an increase in frequency of asthma symptoms.

In addition, the team found that patients who underwent vitamin D3 supplementation did not see any improvements in asthma control, airway function, airway inflammation or quality of life.

Commenting on their findings, the researchers say:

"In adults with persistent asthma and lower vitamin D levels, treatment with vitamin D3 did not reduce the rate of first treatment failure or exacerbation. These findings do not support a strategy of therapeutic vitamin D3 supplementation in patients with symptomatic asthma."

The researchers note that their findings are subject to potential limitations. For example, they point out that although vitamin D supplementation did not reduce treatment failure rate in participants, they saw a lower-than-expected event rate in the placebo group that may have influenced the ability to identify a significant difference between groups.

Furthermore, they note that during the course of the study, the definition of asthma exacerbations in clinical trials changed. "Although this does not directly affect the internal validity of the trial, it does affect the generalizability of our findings," the researchers write.

This is the latest in a line of studies that have dismissed the potential benefits of vitamin D. Earlier this year, Medical News Today reported on a study from Columbia University Medical Center in New York, NY, which suggested that, contrary to past research, vitamin D supplements do not reduce depression.

Other research recently published in the BMJ found that, after an analysis of studies looking at the health benefits of vitamin D, there was "no clear evidence" to suggest that vitamin D can help treat certain medical conditions, such as osteoporosis.