If you have asthma, vitamin D supplements won't protect you against colds, new research suggests.

The study found that taking vitamin D supplements didn't reduce the number or severity of colds in adults with mild-to-moderate asthma.

More than 400 participants, all of whom had low vitamin D levels, took either vitamin D supplements or a placebo for 28 weeks. During that time, about half of them got at least one cold. Eighty-two percent of those in the supplement group had sufficient levels of vitamin D after 12 weeks, but that didn't boost their resistance to colds, the research revealed.

The study was published recently in the American Journal of Respiratory and Critical Care Medicine.

The findings surprised the researchers because they had previously shown that asthma flare-ups fell 40 percent in patients who took vitamin D supplements to increase low levels of the vitamin. Colds often cause asthma flare-ups, and the study authors thought vitamin D supplements would reduce the number and severity of colds in asthma patients.

"Other studies of vitamin D and colds have produced mixed results. 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," study leader Dr. Loren Denlinger, an associate professor of medicine at the University of Wisconsin, said in a journal news release.

Based on the findings, "we can't recommend vitamin D for the prevention of colds," Denlinger said.