Vitamin C supplements are practically useless in a bid to avert colds, researchers concluded in analysis published on Wednesday.

Branding the widely-held belief a myth, they found that only individuals under extreme stress, such as soldiers and marathon runners, benefited at all from the supplements.

They were 50 percent less likely to catch a cold if they took vitamin C daily, the Finnish and Australian scientists said.

But for everyone else, the benefit was so slight that it was hardly worth bothering, concluded the review of data from 30 studies involving more than 11 000 people.

"It doesn’t make sense to take vitamin C 365 days a year to lessen the chance of catching a cold," said Professor Harri Hemila, from the University of Helsinki’s Department of Public Health.

The research appeared in The Cochrane Library, published by the Cochrane Collaboration, an international organisation based in Oxford, southern England, that evaluates medical research.

The scientists collated studies spanning several decades which looked at the effect of taking daily supplements of at least 200 milligrams of vitamin C.

The recommended daily intake of vitamin C is 60 milligrams — less than that found in a glass of orange juice.

The researchers acknowledged that vitamin C supplements could have health benefits other than warding off colds for those under extreme stress.