Unless you run marathons, you probably won't get much protection from common colds by taking a daily supplemental dose of vitamin C, according to an updated review of 30 studies.

Conducted over several decades and including more than 11,000 people who took daily doses of at least 200 milligrams, the review also shows that vitamin C (ascorbic acid) does little to reduce the length or severity of a cold, according to the researchers at the Australian National University and the University of Helsinki.

However, they found that people exposed to periods of high stress -- such as marathon runners, skiers and soldiers on sub-arctic exercises -- were 50 percent less likely to catch a cold if they took a daily dose of vitamin C.

For most people, the benefit of the popular remedy is so slight when it comes to colds that it is not worth the effort or expense, the authors say. "It doesn't make sense to take vitamin C 365 days a year to lessen the chance of catching a cold," said co-author Harri Hemilä, a professor in the Department of Public Health at University of Helsinki in Finland.

The review appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

Since the discovery of vitamin C in the 1930s, controversy regarding its efficacy in treating ailments from lung infections to colds has surrounded it. In the 1970s, Nobel Prize-winning chemist Linus Pauling popularized its regular use. His book, "Vitamin C and the Common Cold," encouraged people to take 1,000 milligrams of the vitamin daily.

The current recommended daily allowance of vitamin C is 60 milligrams. An eight-ounce glass of orange juice has about 97 milligrams of vitamin C.

Despite early mixed results and later evidence against its efficacy, charismatic Pauling became the world's vitamin C champion. "Pauling never recanted and never backed down," said Wallace Sampson, founding editor of the Scientific Review of Alternative Medicine and emeritus professor of medicine at Stanford University.

Regardless of the evidence against it, vitamin C remains popular because many people --including those funding studies -- want to believe that it works, said Sampson, who debated Pauling on the radio and in letters.

These days, there is less interest in studying vitamin C and the common cold, said Hemilä, who has studied the vitamin for more than 25 years. The Cochrane Review was originally published in 1998 and updated in 2004 and this year. The latest update includes a single new study on the Vitamin C-cold connection.

However, researchers continue to examine vitamin C alone and in combination with other vitamins and substances, such as Echinacea, for its efficacy in preventing and treating diseases and conditions, including cancer. This is not necessarily a good thing, Sampson said. "It's broadside quackery."

Hemilä said he sees little use in further study for colds for adults. However, he would like to see more studies on vitamin C and colds in children and vitamin C and pneumonia. Vitamin C is not a panacea, but it is not useless either, Hemilä said. "Pauling was overly optimistic, but he wasn't completely wrong."

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FOR MORE INFORMATION:

Health Behavior News Service: Lisa Esposito at (202) 387-2829 or hbns-editor@cfah.org.

Douglas RM, et al. Vitamin C for preventing and treating the common cold. (Review). Cochrane Database of Systematic Reviews 2007, Issue 3.