Researchers have been interested in whether the vitamin might play a role in people’s susceptibility to colds, flu and other respiratory infections as these illnesses rise during winter when people get little exposure to sunlight.
The body naturally synthesizes vitamin D when the skin is exposed to sunlight.
Researcher Dr. Ilkka Laaksi of the University of Tampere in Finland said some past studies found people with relatively lower vitamin D levels in their blood tended to have higher rates of respiratory infections than those with higher levels of the vitamin.
Laaksi said along with that evidence, recent lab research had shown that vitamin D may play an important role in the body’s immune defenses against respiratory pathogens.
“However there is a lack of clinical studies of the effect of vitamin D supplementation for preventing respiratory infections,” he told Reuters Health.
For the latest study, Laaksi’s team randomly assigned 164 male military recruits to take either 400 international units (IU) of vitamin D or inactive placebo pills every day for six months from October to March, the months when people’s vitamin D stores typically decline and when respiratory infections peak.
After six momths, the researchers found no clear difference between the two groups in the average number of days missed from duty due to a respiratory infection which included bronchitis, sinus infections, pneumonia, ear infections and sore throat.
On average, men who took vitamin D missed about two days from duty because of a respiratory infection, compared with three days in the placebo group. That difference was not significant in statistical terms.
But men in the vitamin D group were more likely to have no days missed from work due to a respiratory illness.
Overall, 51 percent remained “healthy” throughout the study cmpared to 36 percent of the placebo group, the researchers reported in the Journal of Infectious Diseases (http://link.reuters.com/dan53n).
Laaksi said the findings offer some evidence of a benefit from vitamin D against respiratory infections but the extent of the benefit was not clear.
While recruits in the vitamin group were more likely to have no days missed from duty, they were no less likely to report having cold-like symptoms at some point during the study period.
Laaksi said larger clinical trials looking at different doses of vitamin D are still needed before the vitamin can be recommended for curbing the risk of respiratory infections.
Food sources of vitamin D include milk, breakfast cereals and orange juice fortified with vitamin D, as well as some fatty fish, like salmon and mackerel.