Can vitamin D supplements prevent respiratory infections?

Vitamin D clearly helps build and maintain strong bones. And in recent years a number of articles have suggested, though not proved, that insufficient levels of the vitamin are also linked to everything from breast cancer and tuberculosis to depression and multiple sclerosis. Now a new study adds some more evidence for the vitamin's role in preventing respiratory infections such as sinusitis and pneumonia to the list. 

In the study, published this month in the Journal of Infectious Diseases, Finnish researchers divided a group of 164 young male military recruits into two groups. Half received a placebo and half got 400 international units (IU) of vitamin D. After six months, there was no difference between the groups in the percentage of people who reported symptoms, such as a cough, runny nose, or sore throat. But those who received the vitamin D supplement were significantly less likely to have missed duty because of respiratory infections than those who got a placebo.

Although more convincing studies are needed, what I found particularly interesting was the improved outcome with a fairly low dose of 400 IU a day. Last year, the Institute of Medicine raised the recommended levels of vitamin D to 600 IU a day for most adults and 800 a day for those 70 and older. Some experts recommend even higher amounts, up to 3,000 IU a day for people living in areas such as the Northeast, where people might not be able to make enough of the vitamin from exposure to ultraviolet light.

I would hesitate to recommend those higher doses, since the risks are still unclear, especially in patients with heart disease. Instead, I would stick with the new IOM guidelines. Of course, the best way to get vitamin D is through dietary sources, such as fortified milk and fatty fish such as salmon or sardines. Limited exposure to sunlight can also provide vitamin D, but talk with your doctor first, to assess your risk of skin cancer. Testing can make sense if you have symptoms your doctors suspects might be related to low levels of vitamin.

— Joseph Mosquera, M.D., is a board-certified physician also trained in integrative medicine, and a ConsumerReportsHealth.org adviser  

—Aaron Bailey