You lose fluids while exercising because you sweat and you breathe. When exercising you must always ensure your water intake is sufficient. The colour of your urine is a good way of checking whether you are getting enough fluids. Don’t go for gold. It should be pale and almost translucent. If you don’t drink enough before exercising, chances are you’ll suffer muscle cramps and injuries.
You must drink one to 1.5 litres of water daily, in addition to the fluids you ingest in your food. As a guideline, you should drink an additional litre of water for every hour of exercise. Obviously the exact amount depends on the temperature in which you are exercising and the effort you are making.
So make sure you drink enough before, during and after sports. Don’t use your thirst cues as a guideline because you will already be dehydrated by the time you start to feel thirsty. Losing 3 to 5% of your body’s fluids can already reduce your performance capacity by 10 to 30%.