When you exercise, your body sweats as it tries to return to its optimal temperature. As sweat evaporates from your skin, it removes heat from the body, but you also lose body fluid. 

So, you need to drink fluid during exercise to replace the fluids you lose when you sweat. That way, you’ll reduce the risk of heat stress, maintain normal body function, and maintain performance levels. The general rule is: if you’re sweating, you need to be drinking fluids.

It is possible to drink too much during exercise. To avoid over- or under-hydration, it can be useful to know your sweat rate. That way, you can work out exactly how much you should be drinking.