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Can vitamin D help prevent a cold?

A town in Wales, UK, is depicted in this January 23, 2008, photo. Some scientists believe that gloomy and wet weather can cause a increase in depression due to low levels of vitamin D3, which is produced naturally in the body in sunlight.
Photo by Matt Cardy/Getty Images

Because they are not patentable molecules, naturally occurring vitamins such as ascorbic acid (commonly known as vitamin C) and cholecalciferol (commonly known as vitamin D3) have not been the subject of the type and number of randomized clinical trials that their pharmaceutically created peer substances have been. Indeed, despite numerous peer-reviewed papers published by two-time Nobel Prize-winner Linus Pauling and his colleagues supporting the use of vitamin C to reduce the severity and duration of the common cold -- as well as additional work supporting a role for vitamin C supplementation in cancer therapy -- there still remains a great deal of skepticism on the part of the general public as well as the scientific community about this nutrient. Some researchers point to the inaccuracy of reviews that dismiss vitamin C's benefits, and others believe that the definitive vitamin C trial is yet to come.

Now, a team of Canadian researchers reports the results of a randomized controlled trial investigating the possible role of vitamin D3 supplementation and gargling on preventing upper respiratory tract infections (URTIs).

Vitamin D3 is synthesized by the body in the presence of sunlight, so during the months of the year when the sun's rays strike a region less directly, the people in that region have lower levels of this substance. Additionally, a population that spends the bulk of its time indoors has lower levels of vitamin D3 than a group at a similar latitude that spends more time outdoors. People with darker skin require more sunlight to make the same amount of vitamin D3 as people with lighter skin. The US government provides reference intakes (recommended amounts) of vitamin D3, although there scientists are far from consensus regarding these figures, and the government's own (lower) pre-2010 figures are still promulgated online.

The Canadian research team studying vitamin D3 and gargling with respect to the prevention of URTIs randomized individuals into four different groups: supplementation with 10,000 IU (international units) of vitamin D3 per week plus gargling with water, a placebo plus gargling with water, supplementation with 10,000 IU (international units) of vitamin D3 per week without gargling, and placebo without gargling. The researchers report that "Vitamin D3 treatment was associated with a significantly lower risk for laboratory-confirmed URTI and with a significantly lower mean viral load." Although fewer participants who gargled had lab-confirmed URTIs, the difference was not statistically significant. Believing that vitamin D3 is a "promising intervention" for preventing upper-respiratory viral infections, the Canadian team recommends additional trials with more participants, varied dosing regimens, and longer follow-up periods.

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