‘Drink as much as you can to avoid dehydration” has become a mantra for gym-goers and top athletes alike. For years the message from nutritionists has been that you can’t get enough liquid when you are exercising. But some sports medicine experts now believe that drinking too much water when exercising poses a far greater risk to health.

Overdosing on water causes changes in blood dilution that trigger the potentially fatal condition hyponatremia. A study published in last month’s New England Journal Of Medicine (NEJM) confirms the problem is on the rise. Researchers claimed that, during the 2002 Boston marathon, 13% of the runners drank enough to send their blood salt levels plummeting to an abnormal low.

So how does the danger arise? During intense exercise, the kidneys can’t excrete excess fluid, so if someone keeps drinking the water is leeched into the body’s cells. The more they drink, the more is retained, especially in the highly water-absorbant brain cells. With no room to expand, the brain pushes against the skull, compressing the brain stem. Eventually, primary functions such as breathing and heartbeat are shut down. “Water intoxication can result in dizziness, respiratory problems or worse. Some people collapse into a coma. In the worst cases, it can kill,” says Dr Dan Tunstall-Pedoe of St Bartholemew’s hospital, London, and medical director of the London marathon.

In the 2002 Boston marathon, a 28-year-old woman died as a result of gorging on water. Feeling terrible and thinking she was dehydrated, she chugged a litre of fluid on the run when she had already drunk too much. Tunstall-Pedoe says there were 14 cases of hyponatremia in the 2003 London event. Like those in the NEJM study, they tended to be fun runners who took four hours or longer to complete the distance – providing ample opportunity for sipping water en route.

Weighing yourself before and after exercise to see how much fluid you lose (and therefore how much you need to drink to replace that loss) is a sensible approach, suggests Louise Sutton, head of health and exercise science at Leeds Metropolitan University. “Taking care not to overdrink after you finish a workout is important, too. Wait until you start urinating for a sign that your body is no longer retaining water. When your urine is a pale straw colour, you have drunk enough.”

Dr Tim Noakes, who has conducted extensive research into water intoxication at the University of Cape Town, agrees, saying that bottled water manufacturers have “made dehydration a medical illness to be feared”. Last year, new guidelines were issued in the US to urge people not to fall into the trap of thinking more water is better.

So what if you do become parched at the gym? Surely a bottle of water is the best way to top up your fluids? Apparently not. In studies on endurance athletes and chronically dehydrated patients in hospitals, drinks containing small amounts of body salts and a little carbohydrate were more effectively absorbed from the gut than plain water. Sutton says a sports drink, or even orange squash with a pinch of table salt, will get you back into positive fluid balance far more quickly.

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