Newswise — 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, led by James R. Sabetta, M.D., followed 198 healthy adults during the fall and winter 2009-2010. The rationale for the study was to determine if the declining concentrations of vitamin D seen in the fall and winter in a temperate climate could be a factor in the seasonal increased prevalence of respiratory viral infections such as the flu.

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).

Participants reporting any symptoms were seen the same day at the study site by one of the two board-certified Infectious Diseases investigators. Participants kept a diary of symptoms and were called every 1-3 days during the illness to review any signs or symptoms until asymptomatic. The duration of each symptom, the total illness duration, and any antimicrobials administered were recorded.

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.

The data in this study suggests that supplementing with vitamin D to achieve a blood level 38 ng/ml or higher could result in a significant health benefit by reducing the incidence of viral infections of the respiratory tract. Further studies are necessary to examine the efficacy of vitamin D supplementation in the prevention of specific infections, including influenza.

Indoor crowding is commonly thought to contribute to the influenza epidemics seen each winter in temperate zones. However, influenza epidemics do not occur in the summer in crowded workplaces or other gatherings, despite the presence of the virus and a multitude of nonimmune people. The lower levels of vitamin D seen in the winter in temperate climates may contribute to the prevalence of influenza in the winter.

Not only do the findings of this research have significant public health implications, but they may also help explain the seasonality of certain infections and the higher morbidity and mortality of some infections in individuals who are predisposed to lower concentrations of vitamin D, such as pregnant women, dark-skinned individuals, and obese patients.

Vitamin D, which we get to some extent from exposure to natural sunlight, has known effects on the immune system. The association between vitamin D deficiency and susceptibility to infections of the respiratory tract has been suggested for many years, and Dr. Sabetta’s study helps to solidify this association. Importantly, the study demonstrated a beneficial effect of a concentration of vitamin D only slightly higher than the lower limit of 30 ng/ml currently considered to represent a sufficient level. Many experts recommend at least 800 to 1,000 IU of vitamin D daily, especially during the winter months in northern latitudes when limited sunlight reduces the amount of the vitamin produced naturally in the skin. Vitamin D is also known to support the body’s absorption of calcium to prevent osteoporosis, and has been linked to cardiovascular health.

The study, published in the peer-reviewed open access journal PloS ONE, is available online at

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PloS ONE (14-Jun-2010)