[snuxoll]

Flying dehydrates you - the cabin is pressurized by constantly pumping outside air into the pressure vessel that is the fuselage, and the air at cruising altitudes is extremely dry in addition to having less oxygen than we are accustomed to at ground level. We increase our respiration rate due to the lower oxygen content, and lose a fair amount of water in the process as the dry air drains it from us.

I've never been on a flight long enough that I needed to eat, but even on a regional flight I find myself wanting at least something to drink because my throat feels a little dry.