Carlos Camargo, MD, of Massachusetts General Hospital (MGH) says:
"Our randomized controlled trial shows that vitamin D has important effects on infection risk. 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."
Numerous recent studies have indicated that vitamin D has a significant impact on the body's immune function, in addition to its role of developing and maintaining strong bones.
Research led by Camargo and other researchers have linked higher vitamin D levels to a reduced risk of respiratory infections, including colds or flu. However, these studies were only observational and can therefore not provide evidence that the vitamin does indeed protect against infection. Researchers therefore conducted a randomized controlled trial in Japanese schoolchildren, which demonstrated equivocal results by showing a risk reduction in one type of influenza yet not in another type. For this reason, organizations have called for further randomized trials to be conducted.
The body naturally produces vitamin D in response to sunlight, which means that it can be difficult to maintain adequate vitamin D levels in winter in certain parts of the northern U.S. and Canada because of the great seasonal variations in daily sunlight. For the new study, the researchers assessed data from the Blue Sky Study, which was conducted in Ulaanbaatar, Mongolia by a team of Harvard and local health researchers.
Mongolians have a high risk of vitamin D deficiency, particularly during the winter months. The Blue Sky Study evaluated schoolchildren with low blood levels of 25-hydroxyvitamin D (25OHD), at the start of the study. 25OHD is considered the best measure to establish vitamin D status.
In the new blind study, Camargo and his team compared the incident number of winter respiratory infections in the group of children who received daily doses of vitamin D added to locally produced milk with the control group of children who received the same milk but without vitamin D. At the outset of the study the team obtained blood samples taken from all children to identify their level of vitamin D deficiency.
The average 25OHD levels were around 7 ng/ml (17 nmol/L) in both groups at the start of the study. However, at the end of the seven-week therapy the blood test revealed significant differences between the two groups, with those in the vitamin D group averaging 19 ng/ml (47 nmol/L), and even though this is still low, it was considerably higher than at the outset of the trial. Based on parent's reports, those children in the vitamin D group had around 50% less incidences of respiratory infections compared with those in the control group.
Camargo, a professor of Medicine at Harvard Medical School explains:
"Our study design provides strong evidence that the association between low vitamin D and respiratory infections is causal and that treating low vitamin D levels in children with an inexpensive and safe supplement will prevent some respiratory infections. The large benefit was undoubtedly related to the low baseline vitamin D levels of these children, so I would not expect the supplement to provide similar benefit in children who start with healthy levels of vitamin D. The key question for future research is at what initial vitamin D level would children no longer receive benefit from winter supplementation?"
The team highlights that they used 300 IU's of vitamin D daily, which was higher than the recommended daily dosage at the start of the study. However, since then, the U.S. Institute of Medicine has raised the recommended dose for children to 400 IU, whilst other groups recommend daily dosages as high as 1,000 IU for children at risk for vitamin D deficiency. The team also underlines the fact that the low baseline vitamin D levels seen in study participants is also relatively common in some groups of American children, as for example African-American children living in northern states, even though Mongolia may not seem to have much in common with the U.S. otherwise.
Written by Petra Rattue