Vitamin D may help reduce the incidence and severity of viral respiratory tract infections including influenza, according to a new study conducted by investigators at Greenwich Hospital and Yale University School of Medicine.

The study followed 198 healthy adults during the fall and winter of 2009-2010.

In the double-blinded study, participants had blood samples drawn monthly using a 25-hydroxyvitamin D test to accurately measure vitamin D levels. Participants did not know that vitamin D was being measured, and the investigators did not know the levels until the end of the study. All participants were asked to report any evidence of an acute respiratory tract infection (nasal congestion, sore throat, and/or cough with or without fever, chills, fatigue and general malaise).

Of the 18 participants who maintained vitamin D levels of 38 ng/ml or higher during the study period, only three (16.6 %) developed viral infections. Of the 180 other participants, 81 (45 %) developed viral infections. Those with the higher vitamin D levels also experienced a marked reduction in the number of days ill.

 PloS One – June 14, 2010 ;Epub.
http://dx.plos.org/10.1371/journal.pone.0011088