if you lose over 2-3% of your body weight in sweat then your performance deteriorates a LOT. so as you know that 1kg water = 1 litre water then you can work out how much fluid you need to take on during strenuous exercise. (weigh yourself before and after the sweat-fest!)
for most of us this will not be a factor in anything less than a 90 minute RACE.
you need to replace the stuff you use in exercise so you drink an isotonic drink NOT HYPERTONIC or anything othertonic...isotonic.
lucozade sport type things are great...only problem is they are quite calorific so if you are trying to lose weight as well then beware of that!
but definately reaplce all the water and more that you sweat.
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Running 5k Better
More: http://the5krunner.com
Goals: 82% Age Graded 5k and an open water Triathlon.