& Joannie Dobbs Saturday
November 18, 2006
Holiday flu could be caused by a shortage of vitamin D
The holiday season is a time for sharing. Frequently, this includes sharing the influenza virus that causes the flu.
But, holiday closeness is not likely the reason that flu season peaks in December and January.
Question: How does the season affect susceptibility to the flu?
Answer: Often, cold weather is blamed for flu season. But attempts to prove this are not convincing. Another theory with growing evidence has been proposed by Dr. John Cannell and his colleagues who think that seasonal changes in vitamin D status may be the main factor increasing susceptibility to the flu during winter.
It is well known that exposing skin to sunlight stimulates the formation of vitamin D in the body. Typically, vitamin D levels in the body plummet during the winter months due to limited exposure to sunlight unless the diet provides plenty of the vitamin.
Q: Is it possible that vitamin D can protect against viral infections?
A: In the past few years, scientists have learned that vitamin D plays important roles in the function of the immune system. One of vitamin D's roles is to trigger the production of special antimicrobial peptides that can inactivate the influenza virus. Also, there is evidence that groups of people with higher levels of blood vitamin D have a lower incidence of viral respiratory infections than those with low levels.
At this time, Cannell states that the evidence is too limited to recommend vitamin D supplementation for the treatment or prevention of the flu. However, future clinical trials with vitamin D should provide more definitive answers. Meanwhile, Cannell's group encourages health-care providers to consider monitoring the vitamin D status of their patients.
Q: How much vitamin D does the body require?
A: The amount of vitamin D you need depends on many factors. If you wear sunscreen, avoid the sun, and/or have dark skin, you are more likely to need a source of vitamin D from the diet or supplements.
Recommended intakes of vitamin D set almost 10 years ago are now considered to be too low for people who get little sun exposure.
However, since excessive intake of vitamin D can lead to toxic effects over time, Cannell's group urges increased use of blood tests measuring a form of vitamin D called 25-hydroxyvitamin D to determine who needs vitamin D supplements and at what dose.
Alan Titchenal, Ph.D., C.N.S. and Joannie Dobbs, Ph.D., C.N.S.
are nutritionists in the Department of Human Nutrition, Food and Animal Sciences,
College of Tropical Agriculture and Human Resources, UH-Manoa.
Dr. Dobbs also works with the University Health Service
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