28 juin 2005

Vitamin C best in the cold

Roxanne Khamsi

Supplement protects only in chilly climes.
Upping your intake of vitamin C to ward off the common cold makes sense if you're an extreme skiier or mountaineer, researchers say.
But a review of 55 studies carried out over 65 years shows that prophylactic use of this vitamin in everyday circumstances has little effect. This latter find clashes with the conviction held by many that vitamin C supplements boost the immune system and ward off illness, an idea that gained great popularity during the 1970s thanks to famous chemist and Nobel laureate Linus Pauling.
Pauling advocated the consumption of megadoses of this vitamin. He noted that, unlike most animals, humans can't produce their own vitamin C. In many countries, including the United States, the current recommended daily allowance of vitamin C ranges between 60 and 90 milligrams. But Pauling calculated that the average adult should eat 1,000 milligrams or more.
Recently, large-scale scientific studies have cast doubt over how much vitamin C supplements protect people against the common cold. To tackle the issue, Robert Douglas of the Australian National University, Canberra, and Harri Hemilä of the University of Helsinki, Finland, decided to review the literature on this subject.
Chilling detail
Many of the studies in their review asked whether vitamin C reduces the incidence of the common cold. Pooling the data, Douglas and Hemilä found no significant protective effect overall. But they did find that marathon runners, skiers and soldiers exposed to icy conditions or physical stress experienced a 50% reduction in colds thanks to the vitamin.
They also found that use of the vitamin reduced the duration of colds by only 8% in adults and 14% in children. This, they say, provides poor justification for everyday mega doses of the nutrient. According to Hemilä, most adults catch only one common cold a year. Taking supplements every day to avoid this makes little sense, he says.
Douglas similarly plays down the potential protective benefits of this nutrient: "There is little doubt that vitamin C has some biological impact. Although in the main it is nothing like what Pauling predicted and has little place in therapy," he says.
"For the record, I do not use vitamin C personally at all. But I still remain open to the possibility that a very high dose very early after the onset of cold symptoms could be shown in the future to have a useful effect on reducing the severity of illness," Douglas says. The findings appear this week in the journal PLoS Medicine1.

1. Douglas, R. M. & Hemilä, H. PLoS Medicine Published online: doi:0.1371/journal.pmed.0020168 (2005).

Encore une croyance qui se révèle fausse. Nous avons ici un exemple frappant de l'appel à l'autorité: un savant (physicien, prix Nobel) nous donne son opinion dans un domaine qui n'est pas sa spécialité. Sa bonne réputation de scientifique vient au secours de son ignorance du sujet en question et impressionne le chaland.