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Cold, Flu, & Cough Health Center

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Vitamin D May Cut Risk of Flu

Study Shows Vitamin D May Help Prevent Respiratory Tract Infections

Watching for Signs of Illness

Participants in the study had blood samples drawn monthly using a sophisticated technique to accurately measure vitamin D levels. They didn't know that vitamin D was being measured, and even investigators didn't know until the end of the study.

All participants were asked to report signs of illness, such as nasal congestion, sore throat, cough with or without fever, chills, fatigue and general malaise.

Those reporting any symptoms were seen the same day at the study site by one of the infectious disease investigators.

People in the study kept a diary of symptoms and were called every one to three days during the illness to review any signs of symptoms until they were better. The investigators recorded the duration of each symptom, the total duration of the illness, and any antimicrobials administered.

Sabetta says the findings suggest that supplementing vitamin D to achieve a blood level of 38 nanograms per milliliter or higher could result in a significant health benefit by reducing odds of contracting viral infections of the respiratory tract.

But he says more studies are needed to determine the efficacy of vitamin D supplementation in the prevention of infections, including influenza.

The researchers conclude that the lower levels of vitamin D seen during the winter in temperate climates may contribute to the prevalence of influenza in colder months.

The findings, Sabetta says, have significant implications for public health and also may explain the seasonality of certain infections, and also the higher morbidity and mortality of such illnesses in people who are predisposed to lower concentrations of vitamin D.

Sabetta says vitamin D has known effects on the immune system, and the study reinforces the association between vitamin D deficiency and susceptibility to infections of the respiratory tract.

The study is published online in the journal Plos ONE.

Vitamin D levels depend "on how big you are, your skin color, your diet and how much sun exposure you get," Sabetta tells WebMD. "Individuals should get their vitamin D levels checked. If you are gardening a lot, you probably are fine, but people in an office all day may need supplements."

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