Everyone knows the importance of keeping hydrated. Whether it鈥檚 a water bottle or a sports drink, athletes and gym bunnies are rarely seen without a source of fluid close at hand. Common advice is to deliberately drink beyond what thirst dictates, or 鈥減ush fluids鈥�, to combat dehydration and keep performance up to scratch.
Usually that鈥檚 a waste of time, and just occasionally it can be fatal. Exercise-associated hyponatraemia (EAH) is a dangerous condition that occurs when people have drunk so much that the concentration of sodium in their blood falls too low. This leads to excess water moving into the tissues of the brain, causing brain swelling. Symptoms include nausea, vomiting and confusion. In rare cases 鈥� 12 have been recorded worldwide 鈥� the victim has died.
Slower marathon runners, who tend to drink more over the several hours it takes them to complete the course, are one group more likely to develop EAH, and women seem to be more at risk than men. The condition can even be caused by sports drinks claimed to be 鈥渋sotonic鈥� 鈥� meaning that they contain the same concentration of dissolved substances as normal body fluids 鈥� as they tend to contain sugar but very little salt.
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Cases of EAH rose in the US in the 1990s. , director of the exercise science and sports medicine unit at the University of Cape Town, South Africa, who was the first to describe the condition, blames the rise on marketing activities by the makers of sports drinks, which he says promote overdrinking (). He also claims that guidelines issued by the American College of Sports Medicine and other bodies have been influenced by sponsorship from the manufacturers of Gatorade, formerly Quaker Oats, now PepsiCo.
Since awareness of EAH has grown, most guidelines now warn athletes not to overdrink. Although some examples of advice to drink 鈥渁s much as possible鈥� remain, most advice now sets lower and upper limits on how much to drink. , a physiologist and sports nutrition specialist at Loughborough University in the UK, says blanket guidelines are flawed because people vary in how much they sweat. He recommends that people weigh themselves before and after their exercise to find out how much they sweat, and drink enough to maintain their body weight.
But this is overdoing it, says Noakes. Drinking to satisfy your thirst is all that is needed. 鈥淭he easiest way to lower your performance is to overdrink, not underdrink.鈥�