Shocker, the classic dogma might be wrong, that you should ‘drink ahead of thirst, otherwise it is already to late’. This is the message of a new British Journal of Sports Medicine meta-analysis, pointing out that many of the classic studies are time-to-exhaustion tests, rather that test where you cover a given distance as fast as possible.
The orthodox view is that if you lose more than 2% of your body mass in fluid loss, your endurance will be hurt. But reviewing five studies, Eric Goulet at the University of Sherbrooke found that loosing 2.2% of body mass actually improved performance 0.06%, compared to subjects who drank enough to roughly maintain body mass.
This agrees with real-world data like this study from a marathon in France, which found that sub-3:00 finishers lost 3.1% of their body weight, 3:00-4:00 finishers lost 2.5%, and slower-than-4:00 finishers lost only 1.8%.
Looks like our built-in system of thirst might have been right all along. These findings may have more effect on sports like running where you have to carry your own weight, than in swimming where additional water is just more water in the water. But if they are right, we can maybe stop harassing ourselves and others into drinking so much water, and I love it when nature wins :-)
Read more here on Sweat Science.