The body naturally synthesizes vitamin D when the skin is exposed to sunlight. When winter comes, we are exposed to less sunlight.
Researchers are trying to determine if the vitamin might play a role in people's susceptibility to colds, flu and other respiratory infections during those months.
Some past research has indeed found that people with relatively lower vitamin D levels in their blood tend to have higher rates of respiratory infections than those with higher levels of the vitamin, said Dr. Ilkka Laaksi, of the University of Tampere in Finland. He and his team studied two sets of men -- those who took a vitamin D supplement and those who took a placebo pill.
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.
However, the study also found that men in the vitamin D group were more likely to have no days missed from work due to a respiratory illness.
Fifty-one percent remained healthy throughout the six-month study compared to only 36 percent of the placebo group.
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. Experts generally recommend vitamin pills for people who do not get enough of the vitamin from food.
You can read the study at www.journals.uchicago.edu/doi/abs/10.1086/654881