A promising study in Pediatrics found that making sure kids have sufficient Vitamin D during winter's darkest months may decease the incidence of acute respiratory tract infections by 50%.

Vasily Fedosenko/Reuters

PROBLEM: Vitamin D (the "sunshine vitamin") is known to be important in developing and strengthening bones, but it may also have positive immunological benefits. It's naturally produced by the body in response to sunlight, so people tend to have lower levels of vitamin D in northern regions (where winter days are shortest). This study assesses the vitamin's role in protecting children from acute respiratory tract infections (ARIs, AKA colds) -- and tests whether supplements are an effective preventative measure for individuals with vitamin D deficiencies.

METHODOLOGY: The sample population consisted of schoolchildren in Mongolia, a country known for its harsh, dark winters and concurrent vitamin D deficiencies. Blood tests administered prior to the double-blind experimental period confirmed that the kids already had low levels of vitamin D. From January to March, the 143 children's daily glasses of milk were fortified with 300 IU of vitamin D (undetectable to their taste). One hundred four others received ordinary milk. Incidences of ARIs throughout the three-month period were reported by the children's parents.

RESULTS: The kids who drank the "special" milk had higher blood levels of vitamin D, as one would expect. This was indicated by their average blood levels of 25-hydroxyvitamin D, which, at 19 ng/mL, were not fully restored but nonetheless represented a significant increase over the control group, which maintained the initial baseline level of 7 ng/mL. These same children reported half as many incidences of ARIs, even when adjusting for age, gender, and "history of wheezing."

CONCLUSION: Supplementing the diets of kids with low vitamin D levels is strongly associated with a decreased incidence of acute respiratory tract infections (colds).

IMPLICATIONS: The authors point out that while the study was conducted internationally in a unique environment, the children in this study could be considered analogous to other at-risk populations.  A 2009 study in the Archive of Internal Medicine reported that nearly 3 out of 4 adolescents and adults in the United Staes have insufficient vitamin D levels. African American children living in northern states have the highest rates of deficiency. Breast-fed infants and the elderly also tend to suffer from vitamin D deficiencies.

While the study suggests the people with deficiencies would benefit immunologically from taking supplements, it remains unclear whether children with already-healthy levels of vitamin D could also reduce their risk of ARIs. At the time this experiment was conducted, the recommended dosage of vitamin D was below 300 IU, but it has since been increased to 400 IU. 

The full study, "Randomized Trial of Vitamin D Supplementation and Risk of Acute Respiratory Tract Infection in Mongolia," is published in the journal Pediatrics.