Does taking vitamin D help ward off the flu?

You may have heard that vitamin D, the “sunshine vitamin,” when taken in sufficient amounts may provide added protection against wintertime colds and the flu. The important word here is may.

Vitamin D (which is actually a hormone) has become a hot topic in recent years as a result of increased awareness of its positive effects on health including support of bone mineralization, heart and blood vessel function, anti-cancer actions and a general boosting of immunity. A significant proportion of the population is relatively deficient in vitamin D, which is normally formed in the skin with exposure to the sun’s ultraviolet rays. The problem is compounded during the winter months when people spend more time indoors. Researchers took note of this, and also considered the fact that the annual flu season generally runs from October through early May. They began to investigate supplemental vitamin D as a preventive agent against colds and the flu.

To date the research results have been mixed. Published data suggest that if supplementing with vitamin D has any significant role in helping to prevent the flu it is most noteworthy for people who have low circulating vitamin D levels to begin with. You can have your doctor perform a blood test to determine your vitamin D status. Labs usually report a normal 25-hydroxy vitamin D level as anything greater than 30 ng/mL, but most experts believe levels should be between 40-75 ng/mL for optimal health. Ask your doctor to be sure in your own unique health circumstances.


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It is difficult to get adequate vitamin D through the diet, and while a little daily sun exposure (15-20 minutes) goes a long way towards optimizing vitamin D levels the concerns raised by our dermatologists have to be considered. Supplementation makes sense for most people.

Supplemental vitamin D comes in two forms - vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). While D2 can be effective, D3 is preferable because the body uses it more efficiently. Conservative physicians may recommend a dose of 400-800 IU per day, but I generally advise my patients to take 1,000-2,000 IU of vitamin D3 daily. People who are significantly deficient in vitamin D require a higher dose.

Should everyone take supplemental vitamin D this winter? The evidence suggests doing so may not help protect you against the flu, but with most people being relatively deficient in vitamin D, especially during the winter months, supplementation may help optimize your health in other ways and should be considered together with your doctor.

About the author

Russell Greenfield, M.D. is Assistant Clinical Professor with The UNC-Chapel Hill School of Medicine,and Visiting Assistant Professor at The University of Arizona College of Medicine. He was the founding medical director of Carolinas Integrative Health, a freestanding center in Charlotte, NC owned and operated by the Carolinas HealthCare System.

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