Asthmatics with higher blood levels of vitamin D have better lung function than those with lower levels, according to new research from National Jewish Health, in Denver.
The findings have been published online ahead of print publication in the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine.
E. Rand Sutherland, MD, MPH, the lead author and his colleagues enrolled 54 nonsmoking asthmatics, assessed their levels of serum vitamin D [25(OH)D] and tested their lung function and airway hyper-responsiveness (both hallmarks of asthma.) They also tested the subjects’ response to steroid treatment.
"Our goal was to assess whether vitamin D levels were associated with asthma severity in adults," said Dr Sutherland, who is chief of the pulmonary division at National Jewish Heath.
They found that with decreasing levels of vitamin D, subjects performed more poorly on tests of lung function and airway hyper-responsiveness.
"We showed that in adults with asthma, lower vitamin D levels were associated with lower lung function, an increased propensity for bronchospasm and poorer steroid response," said Dr Sutherland.
Furthermore, the effect seemed to be dose dependent: The lower the vitamin D level, the worse the subjects tended to perform on tests of lung function and airway hyperresponsiveness. Airway hyper-responsiveness nearly doubled in subjects with serum levels of vitamin D below the "sufficient" threshold level of 30 ng/mL, when compared to subjects with higher levels.
In addition to its association with lower lung function, increased airway hyper-responsiveness, and poorer response to steroids, a lower level of vitamin D was associated with increased production of the pro-inflammatory cytokine, TNF-α, by immune cells in the blood, raising the possibility that low vitamin D levels could be linked to enhanced inflammation in asthma.
Vitamin D levels were also inversely correlated with body mass index (BMI) – the higher the BMI of a subject, the lower their vitamin D levels tended to be. "Asthma is known to be associated with obesity. The lower levels of vitamin D in obese subjects may illuminate one factor that ties obesity and asthma together," said Dr Sutherland.
"Our findings suggest that vitamin D levels influence a number of important features of asthma, including lung function, bronchospasm and therapeutic response to steroids," said Dr Sutherland. "The next question to answer is whether giving supplemental vitamin D will lead to clinical improvements in patients with asthma, highlighting the need for clinical trials in this area."
(Source: American Thoracic Society: American Journal of Respiratory and Critical Care Medicine: February 2010)