I usually run for an hour in hot weather every Saturday (I'm in the tropics, so it's usually 30 degrees or above, with around 90% humidity). I weigh myself before and afterwards, and usually I'll lose about a kilo in weight in that time - which I assume is mostly water - I sweat like a pig.
(Currently I'll drink about 600 ml of water while running, plus another 400 ml when I'm done, so I guess I'm actually losing about 2 kilos through dehydration during that hour. I weigh about 75 kg, so that's roughly 2.5% of my body weight that I've lost in that time.)
The people I run with don't drink anything while running. They're possibly better habituated to the environment than I am (mostly locals, while I've only been out here for a couple of years) - what could I do to reduce my fluid intake while I'm running?