- Had poorer lung function
- Had higher levels of immunoglobulin E (IgE), an immune system protein the body makes in response to allergens that tells you the likelihood that you're allergic
- Were more likely to need inhaled and oral steroid medications to reduce airway inflammation and mucus production
- Were more likely to need long-acting beta-agonist drugs that relax muscles in the lung's airways, improving a patient's ability to breathe freely and reducing asthma symptoms.
Further studies in the lab showed that vitamin D has an anti-inflammatory effect on cells and enhances the activity of inhaled steroids.
About 21 million Americans suffer from asthma, which is caused by inflammation and swelling of the airways. The inflammation, in turn, can cause excessive mucus production and narrowing of the airways, resulting in asthma symptoms such as shortness of breath, coughing, and wheezing.
The findings were presented at the American Academy of Allergy, Asthma & Immunology annual meeting.
The study doesn't prove cause and effect. And it's not clear whether low vitamin D causes more severe asthma that requires treatment or whether more severe asthma lowers vitamin D levels, says study researcher Daniel A. Searing, MD, of National Jewish Health in Denver.
Also still unknown is whether vitamin D supplements would improve asthma control and lower the need for medication, he tells WebMD.
Still, a number of studies now suggest that low vitamin D levels are associated with allergies and asthma, says James Gern, MD, vice chair of the committee that chose which studies to highlight at the meeting and professor of pediatrics at the University of Wisconsin in Madison.
If a person has vitamin D insufficiency, "we need to correct it anyway. So it will be interesting to see if the supplements help improve asthma symptoms," he tells WebMD. Gern was not involved with the work.
In the study, vitamin D insufficiency was defined as levels below 30 nanograms per milliliter of blood.