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Too good to be true: Vitamin D won’t fight colds, study says

With cold and flu season about to get in full swing, many Canadians are stocking up on vitamins and supplements they believe will prevent or shorten the length of illness.

In recent years, one of the go-to supplements in this category has become vitamin D.

But according to a new study published in the Journal of the American Medical Association, the claims that vitamin D can ward off respiratory illness are too good to be true.

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Researchers from New Zealand tested the theory by comparing the frequency and severity of colds among 322 healthy adults without a vitamin D deficiency. The adults randomly received either high doses of vitamin D or a placebo. At the end of 18 months, the researchers found that everyone in the group developed the same number of infections (just under four each) and that there was no noticeable difference in the number of workdays missed or severity of illness.

The study's author, David Murdoch, head of pathology at the University of Otago in Dunedin, New Zealand, told JAMA it's still unclear whether vitamin D supplementation can prevent colds in those who are deficient in the vitamin.

That doesn't mean you should stop supplementing with vitamin D. Mounting evidence suggests vitamin D deficiency may be linked to a number of diseases, most notably cancer, and that supplementation could be an effective way to reduce the risk. Although humans produce vitamin D through exposure to the sun, countries like Canada, that have lengthy winters and limited sunshine during those months, are increasingly advised to take vitamin D supplements.

A 2010 report from Statistics Canada found that few Canadians are truly vitamin D deficient. But most of us don't have high enough levels that many researchers believe are necessary to protect against cancer and other illnesses.

Are you taking vitamin D supplements in hopes of avoiding a cold?

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About the Author

Carly Weeks has been a journalist with The Globe and Mail since 2007.  She has reported on everything from federal politics to the high levels of sodium in the Canadian diet. More

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