Do not wait until you are thirsty to drink, because thirst is a sign that dehydration has already begun, and aim for regular hydration of 300 to 500 ml per hour during exertion.

noli's read

The idea that thirst is a late signal is nuanced by research. Meta-analyses (e.g., Journal of the International Society of Sports Nutrition) emphasize that drinking 'to thirst' (ad libitum) is often sufficient for the majority of runners, as the body regulates its fluid balance very well. The assertion that thirst arrives 'too late' is a view inherited from older recommendations that feared dehydration at all costs. However, for long and intense efforts, the recommendations of 300-500ml/h are consistent with observational studies on the prevention of excessive weight loss. What is exaggerated is the universal injunction: drinking too much, without listening to one's body, can lead to hyponatremia (dilution of salt in the blood), a real risk documented in case studies of marathon runners. The advice remains relevant for avoiding performance drops, but it would benefit from being less rigid.

the sources

1. 1Maughan RJ et al. 2016
2. 2Hew-Butler T et al. 2015