New research suggests vitamin D supplements do not reduce the number or severity of colds in asthma patients.

Researchers from the University of Wisconsin found that among African Americans in the study, those receiving supplemental vitamin D, rather than a placebo, experienced more colds.

The findings surprised researchers because previous studies have shown a 40 percent reduction in asthma exacerbations in patients with a vitamin D deficiency who achieved normal levels of the vitamin with supplements. Because colds often trigger exacerbations, they hypothesized that vitamin D supplementation would reduce colds and cold severity.

"Other studies of vitamin D and colds have produced mixed results," researcher Dr. Loren Denlinger said in a statement. "Most of those studies were conducted among healthy patients. We wanted to ask the same question of a patient population in which the impact of a cold carries greater risk."

For the study, researchers collected and analyzed data from more than 400 people with mild-to-moderate asthma whose vitamin D levels were insufficient or deficiency. Those enrolled had asthma symptoms despite low-dose ICS therapy. The patients were randomized to receive either vitamin D supplementation or placebo for 28 weeks. Neither the patients nor their physicians knew whether they received vitamin D or the placebo.

During that time, about half the participants experienced at least one cold. The severity of their colds was measured by the Wisconsin Upper Respiratory Symptom Survey-21 (WURSS-21).

The researchers analyzed separately the results of the 82 percent of participants receiving supplements who achieved vitamin D sufficiency within 12 weeks. Achieving sufficiency made no difference in number of colds or their severity this group experienced.

Researchers believe asthma patients with low vitamin D levels may be more likely to experience upper respiratory infections asymptomatically than those with normal levels of vitamin D, which is known to trigger an inflammatory response. This inflammatory response may, in turn, reduce the risk of lower airway infections, which are triggers for asthma exacerbations.

Although there are other reasons to recommend vitamin D supplements for asthma patients, including the fact that they are at greater risk for bone weakening, Denlinger said, "we can't recommend vitamin D for the prevention of colds."

The findings are detailed in the American Journal of Respiratory and Critical Care Medicine.