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"Our findings suggest that low vitamin D levels are associated with worse asthma," said lead researcher Dr. E. Rand Sutherland, from the division of pulmonary and critical care medicine at National Jewish Health in Denver.
In addition, vitamin D levels predict how well "somebody is going to respond to steroidal asthma medications," he said. "It may be that vitamin D is acting as a modifier of the immune system or a modifier of steroid response in ways that are relevant to people with asthma."
The report is published in the Jan. 28 online edition of the American Journal of Respiratory and Critical Care Medicine.
For the study, Sutherland's team took the vitamin D levels of 54 asthmatics and assessed lung function, airway hyper-responsiveness, which is the prevalence of airway constriction, and response to steroid treatment.
People with low levels of vitamin D in their blood did worse on the tests that evaluated lung function and airway hyper-responsiveness, the researchers found.
In those with vitamin levels below 30 nanograms per milliliter (ng/ml), airway hyper-responsiveness almost doubled, compared to those with more D in their blood.
Low vitamin D levels were also associated with a worse response to steroid therapy and increased production of the pro-inflammatory cytokine, TNF-alpha. This raises the possibility that low vitamin D levels are tied to increased inflammation of the airways.
The heaviest participants had the lowest levels of vitamin D, the study noted. Asthma is associated with obesity, and this (lack of vitamin D) may be a factor linking the two conditions, Sutherland said.
"There is a potential that restoring normal vitamin D levels in people with asthma may help improve their asthma," Sutherland said.
But whether vitamin D supplements will help asthmatics isn't known, he added.
Current recommendations for vitamin D supplements for adults is 400 IU to 600 IU, depending on age, according to the U.S. National Institutes of Health.
"There is likely little harm in adhering to those guidelines," Sutherland said.
The Institute of Medicine is currently evaluating these levels and expects to announce new guidelines in May.
Sunlight, fatty fish and fish oils are also sources of vitamin D.
Dr. Michael F. Holick, director of the Vitamin D, Skin and Bone Research Laboratory at Boston University School of Medicine, called this "a very nice study that confirms previous observations that vitamin D enhances lung function."
"It is also known that glucocorticoids [steroids] increase the destruction of vitamin D, thus making patients with asthma at higher risk for vitamin D deficiency, which in turn decreases lung function and makes their disease worse," he said.
Holick thinks most people, asthmatic or not, get too little D and should take supplements.
"It's pretty clear that you need a minimum of 1,400 and up to 2,000 IU a day, and if you are obese, you probably need at least one and a half to two times as much, because the fat sequesters the vitamin D," Holick said. "We now recognize that you can take up to 10,000 IUs a day and not worry about any untoward toxicity."
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