USA Track and Field, the governing body for athletics and running in America, has issued new guidelines advising those who cycle, jog or power walk regularly not to drink huge amounts of water.

The new guidelines mark a significant shift in standard recommendations for fluid replacement. Up until now runners, athletes and gym users have often been told that you can’t drink too much. Avoiding dehydration has been a basic rule of working-out.

But experts say the need to keep drinking water during work-outs has been over-stressed. “Drinking water at every opportunity can cause serious problems, such as hyponatraemia or water intoxication,” says Dr Dan Tunstall-Pedoe of St Bartholomew’s Hospital in London and the medical director of the Flora London Marathon.

“That leads to diluted sodium and other body salts, or electrolytes, in the blood, which can cause dizziness and respiratory problems. Some people collapse because of it, as happens quite frequently in the marathon.”

The latest findings indicate that we don’t need to drink as much as we once thought. Tunstall-Pedoe advises consuming about “a quarter of a pint of fluid for every hour of exercise, which won’t replace everything you lose through sweat, but will maintain a healthy fluid balance. More than that is not really advisable, unless it is very hot.”

But Louise Sutton, a sports dietician and lecturer in health and exercise science at Leeds Metropolitan University says hyponatraemia is unlikely to be a problem for the average gym user.

“Ideally fluid replacement strategies should be individualised with runners determining fluid requirements by using weight checks to monitor fluid losses during training. A one kilogram weight loss is equal to a one litre fluid loss, and this needs to be replaced plus a little extra to cover increased fluid losses in the immediate post exercise period. Sodium losses should be easily replaced at the next meal. Other important aspects include starting exercise fully hydrated and monitoring urine colour and volume.” Details: www.usatf.org