The study – published in Pediatrics – tested the effects of vitamin D supplementation on the risk of developing acute respiratory tract infections in a randomised controlled trial in Mongolian school children, after previous research suggested higher levels of blood serum vitamin D – 25(OH)D – to be linked incidence of respiratory infection.
Led by Professor Carlos Camargo of Harvard Medical School, and Massachusetts General Hospital, USA, the international research team found that vitamin D supplementation decreased the risk of such respiratory infections among children who had low blood levels of vitamin D at the start of the study.
The authors suggest their findings support the possibility that daily vitamin D supplementation can reduce the risk of respiratory infections in winter.
"Our randomized controlled trial shows that vitamin D has important effects on infection risk," said Camargo. "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 studies have suggested that vitamin D – often referred to as the sunshine vitamin – has several important roles in the human body, including modifying in immune functions.
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, they noted.
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 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(OH)D at the study's kick-off.
In the new study, Camargo and his colleagues compared the number of winter respiratory infections among a group of children who received daily doses of 300 IU vitamin D added to locally produced milk with that of a control group receiving the same milk without the additional vitamin D.
Based on reports from their parents, the children receiving vitamin D had about half the incidence of respiratory infections that the control group had.
While blood samples taken at the outset of the study revealed vitamin D deficiency in all participants – the average 25(OH)D levels at around 7 ng/ml (17 nmol/L) in both groups – by the end of the seven-week supplementation period key differences were found in blood levels of vitamin D, with those receive vitamin D averaging 19 ng/ml (47 nmol/L).
"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," said Camargo.
"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,” he said.
“The key question for future research is at what initial vitamin D level would children no longer receive benefit from winter supplementation?"
Published online ahead of print, doi: doi: 10.1542/peds.2011-3029
“Randomized Trial of Vitamin D Supplementation and Risk of Acute Respiratory Tract Infection in Mongolia”
Authors: Carlos A. Camargo Jr, Davaasambuu Ganmaa, A. Lindsay Frazier, Franca F. Kirchberg, et al