Why too much water may prove fatal for exhausted athletes

People who exercise in hot weather should watch what they drink because too much fluid could be fatal, doctors say.

The belief that athletes must keep drinking to replace all the fluid lost during exercise is a myth that is not based on scientific evidence, according to Timothy Noakes, professor of exercise and sports science at the University of Cape Town in South Africa.

Athletes have been taught to assume that the greatest threat to their wellbeing during prolonged exercise, especially in the heat, is dehydration. They have been advised that the sensation of thirst understates their real need for fluid and they should drink the maximum amount that is tolerable.

But doing so can have lethal effects. A woman competitor in the 2002 Boston marathon died after drinking an excessive amount of a sports drink before and during the race. She died of hyponatraemic encephalopathy - a condition in which the blood becomes so diluted that the salt level falls and fluid is drawn into the brain under osmotic pressure, causing it to swell.

The same condition caused the actor Anthony Andrews to collapse last month after he had consumed large quantities of water while rehearsing My Fair Lady. He was taken to hospital but recovered.

Leah Betts was not so lucky. The 17-year-old from Essex died after swallowing half a tablet of ecstasy in 1996, triggering national panic about the dangers of recreational drugs. It is widely, but wrongly, thought that the drug killed Ms Betts, but tests showed it was actually the large quantity of water she drank. A number of other cases of teenagers dying from over-consumption of fluid have been recorded after all-night, often drug-fuelled dancing in overheated clubs.

Professor Noakes says that at least seven deaths and more than 250 cases of hyponatraemic encephalopathy caused by drinking too much fluid have been described among athletes, soldiers and hikers in medical literature. Reported cases are likely to represent a small proportion of the total.

Writing in the British Medical Journal, he says that until the late 1960s athletes were advised not to drink during exercise because it was believed that ingesting fluid in mid-race would impair performance.

But in 1969 an article entitled "The danger of an inadequate water intake in marathon running" was published which triggered a change of view. Although the article failed to show what its title claimed, it provided the incentive for a series of studies, many funded by a fledgling sports drinks industry, which culminated in guidelines specifying how much athletes should drink. The studies did not provideevidence to back the guidelines.

Professor Noakes says athletes and others taking exercise should forget what they have been told about fluid replacement and follow their instincts. "The best advice is that drinking according to the personal dictates of thirst seems to be safe and effective," he says.