It is essential for protecting cells and absorbing iron from food but it cannot cure a cold, even when taken in mega-doses.

Vitamin C is the most widely promoted supplement against colds and flu, but its protective effect is a myth, according to new research.

A review of 30 studies involving a total of 11,000 people has found no evidence that, for the average person, taking extra Vitamin C can stop sneezes, sniffles and coughs.

The only people who may benefit from swallowing supplements of the vitamin are those who endure extreme physical stress, including soldiers, skiers and marathon runners.

They were half as likely to catch a cold if they took daily Vitamin C, the researchers from Australia and Finland found. For the rest, buying the supplements was not worth the effort or expense.

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

The new analysis appears in the latest issue of The Cochrane Library, which is published by the Cochrane Collaboration, an international organisation that evaluates medical research.

Scientists pooled information from studies spanning several decades, which looked at the effect of taking daily supplements of at least 200mg of vitamin C. This kind of "meta-analysis" is often more sensitive than a stand-alone study and is considered better able to spot subtle trends.

Controversy has surrounded the protective effects of vitamin C since its discovery in the 1930s.

The belief that vitamin C can cure the common cold took hold in the 1970s, largely thanks to the American Nobel Prize-winning chemist Dr Linus Pauling, who championed the vitamin.

His book, Vitamin C and the Common Cold, encouraged people to take large vitamin C doses of 1,000 mg daily. The current recommended daily allowance of vitamin C is only 60mg. Just one 220ml glass of orange juice contains more than that - about 97mg.

Despite Dr Pauling's almost religious faith in the vitamin, the evidence supporting the medicinal value of taking more than dietary amounts of vitamin C has been mixed.

However, the Cochrane researchers acknowledge that vitamin C supplements, whether taken alone or with other substances, might have health benefits other than keeping adult colds at bay. Professor Hemila said that he wanted to see more studies on colds in children, and the effect of the vitamin on pneumonia.

"Pauling was overly optimistic, but he wasn't completely wrong," he added.

Staying healthy

Take Echinacea

The popular herbal remedy can cut the risk of catching a cold by more than half according to a review of 14 studies published in The Lancet Infectious Diseases.

Drink Hot Honey And Lemon

The best treatment for a cold is to drink plenty of fluids and take paracetamol every four hours.

Avoid Shaking Hands

It is not coughs and sneezes that spread diseases but hands. The commonest way of catching a cold from someone is by shaking hands with them and then rubbing your eyes or mouth.

Buy A Scarf

The nose dries out in low temperatures or air-conditioning and respiratory viruses become trapped and start to reproduce. Wrapping up warm, especially protecting the nose, can stop you catching a cold.