Long-distance runners could be risking their health by drinking too much water, experts have warned.
Long distance runners are at high risk
Most people are aware of the dangers of dehydration, and the need to keep drinking during exercise.
But drinking too much water, plus a loss of sodium, can cause a potentially fatal condition called hyponatremia, or water intoxication.
It is the keen amateur who is most at risk. Elite runners such as Paula Radcliffe move too fast to drink too much.
The need to keep drinking water has been stressed a bit too much
Dr Dan Tunstall-Pedoe, medical director for the Flora London Marathon
People who do an hour or two in a gym or go to an exercise class are not likely to develop hyponatremia - because they are exercising for a relatively short time and are unlikely to drink too much.
But experts say the need to keep drinking water during work-outs has been "over-stressed".
'Wrong, wrong, wrong'
Concerns over hyponatremia have led USA Track and Field, the body which governs athletes and running in America, to issue new guidelines for long-distance runners.
Dr David Martin, an exercise psychologist from Georgia State University, who studied joggers' drinking habits, said the change was overdue.
He examined the causes of illness in fun-runners since 1985 and found 70 cases of hyponatremia, many more than from dehydration.
He told a national newspaper: "We are very worried about the increasingly large group of people who are taking up running for the first time and who are told the party line Make sure you drink, You can't drink too much. Carry water with you or you will get dehydrated. Don't worry about the heat, just drink more'.
"But that's just wrong, wrong, wrong."
Dr Dan Tunstall-Pedoe, a cardiologist at St Bartholomew's Hospital, London and medical director for the Flora London Marathon, told BBC News Online: "In a marathon, you should drink about half a litre of water an hour.
"But in the gym, it's a totally different situation."
Dr Tunstall-Pedoe said: "Hyponatremia is not a problem if you're only exercising for an hour.
"The need to keep drinking water has been stressed a bit too much."
He said people may lose a couple of pints of water while working out at the gym or an exercise class, but they can replace that straight away once they had finished.