Hydration.

officially sucks. 16 cups daily for regular people .... 5 quarts daily for active people ....

and -

as much as 10 liters daily for endurance athletes. okay. I'm not an endurance athlete. but I'm also not a fish. drink 1 liter of water before every workout, rehydrate afterwards. ummm ... I might make it through the workout with my belly sloshing around ... but I'd drown during recuperation!

seriously. I thought a couple nalgenes a day was doing great.