https://www.scientificamerican.com/article/how-to-stay-hydrated-during-exercise/

When I first started running semi-seriously, I diligently carried a water bottle on my belt, as instructed by all the running magazines and websites. Then, as my (ahem) thirst for the sport grew, I graduated to an embarrassing fanny-pack-style beverage bladder. But, oddly enough, by the time I achieved the level of having a shoe sponsorship and coaches, I wasn't carrying anything with me at all. A few sips at random water fountains or the on-course aid stations worked just fine.

There's a reason I tell that story. For many years, the rule-of-thumb for exercise hydration has been that if you sweat out more than two percent of your body mass, your performance will decline. That's based on military research meant to prepare U.S. soldiers for hot climate combat during the Second World War. In practical terms, the rule means that a man who weighs 80 kilograms (176 pounds) can only sweat out 1.75 litres (about 60 fluid ounces) of sweat before he will start to fall apart performance-wise.