Researchers in Finland randomly assigned 164 male military recruits to take either 400 international units of vitamin D or inactive placebo pills every day from October to March. That's the time of year when people's vitamin D stores typically decline and respiratory infections typically peak.
Fifty-one percent of the men who took vitamin D remained healthy throughout the six-month study, compared to 36 percent of the placebo group.
Researchers say larger clinical trials are still needed before the vitamin can be recommended to curb the risk of respiratory infections.
The study is in the Online Journal of Infectious Diseases.