Exercise is thirsty work. Whenever you exercise you lose fluid, not only through sweating but also as water vapour in the air that you breathe out. Your body’s fluid losses can be very high and, if the fluid is not replaced quickly, dehydration will follow. This will have an adverse effect on your physical performance and health. Exercise will be much harder and you will suffer fatigue sooner.
During 1 hour’s exercise an average person could expect to lose around 1 litre of fluid.
MARATHON – If you are planning on running the marathon drink no more than 800ml per hour.
During more strenuous exercise in warm or humid conditions such as marathon running, you could be losing as much as 2 litres per hour.