A study conducted in Mongolian schoolchildren supports the possibility that daily vitamin D supplementation can reduce the risk of respiratory infections in winter. In a report that will appear in the journal Pediatrics and has received early online release, an international research team found that vitamin D supplementation decreased the risk of respiratory infections among children who had low blood levels of vitamin D at the start of the study.
"Our randomized controlled trial shows that vitamin D has important effects on infection risk," says Carlos Camargo, MD, of Massachusetts General Hospital (MGH), the study's corresponding author. "In almost 250 children with low blood levels of vitamin D during winter, we found that taking a daily vitamin D supplement cut in half the risk of a respiratory infection."
Several recent investigations have suggested that vitamin D - best known for its role in the development and maintenance of strong bones - has additional important roles, including in immune function. Studies led by Camargo and other researchers have associated higher vitamin D levels with reduced risk of respiratory infections such as colds or flu, but such observational studies cannot prove that the vitamin actually protects against infection. That kind of evidence must come from randomized controlled trials comparing two similar populations that either do or do not receive an intervention such as vitamin D supplementation. The first such trial, in Japanese schoolchildren, had equivocal results, showing a reduction in the risk of one type of influenza but no effect on another type, so many organizations have called for further randomized trials to settle the issue.
Since vitamin D is naturally produced by the body in response to sunlight, maintaining adequate levels in winter is particularly challenging in areas such as the northern U.S. and Canada that have significant seasonal variations in daily sunlight. The current study analyzed data from the Blue Sky Study, conducted in Ulaanbaatar, Mongolia, by a team led by Harvard investigators in collaboration with local health researchers. Mongolians are known to be at high risk for vitamin D deficiency, especially during winter, and the Blue Sky Study followed schoolchildren, all of whom were found to have low blood levels of 25-hydroxyvitamin D (25OHD), which is considered the best measure of vitamin D status, at the study's outset.