Dehydration during exercise occurs when you don’t replace the fluids lost by your body through sweat or urine. If severe enough, it can reduce the ability of your body to shed excess heat through sweating. In theory, this can lead to heat exhaustion or stroke, which is why plentiful fluid intake has been recommended for so long.
Research, though, shows that the body can handle dehydrationup to a certain pointwithout suffering negative consequences. A study of marathon runners in France found that the fastest runnersbelow three hourslost 3.1% of their body weight through sweat or urine. They were even more dehydrated than the slower runners.
A more recent study by Éric Goulet, a researcher at the University of Sherbrooke in Quebec, found that not only is mild dehydration safe, but waiting for your thirst to kick in can increase your performance. Goulet analyzed five studies involving cycling time trials. Cyclists who drank only when thirsty had the best performance compared to those who drank early or late. He also found that exercise intensity and duration had a greater impact on cycling performance than dehydration.
The most important thing during exercise is to listen to your body. Keep water nearby so when your thirst kicks in, you can easily replenish your fluids.