Mike Mayo gives some Physio pointers on Hydration at altitude (NO GATORADE!)

Regarding Gatorade (don't touch the stuff myself).
There is a marked physiological difference between standing around in
the sun on a hot summer day at Byron and summer cross-country flying in
the mountains.
At Byron, sweating profusely to keep body temperature from rising, we do
lose electrolytes as well as water. Gatorade can replace those
electrolytes but it is not really necessary because our exposure time is
short enough that normal food will restore them soon enough. Proactively
replacing water is essential, though.
Flying in the mountains in summertime we are usually close to freezing
level. We don't sweat very much once up there. Hanging about on the
ground, of course, we may sweat quite a bit but usually we spent a good
amount of that time in the shade where it is significantly cooler than
at Byron. But up high, even though the outside air is frigid, our lungs
are at the same old temperature. The air we breathe in has hardly any
water in it whereas the air we breathe out contains plenty of water.
Thus we unsuspectingly lose water more rapidly than usual. But not
electrolytes. Hence it is necessary to drink plenty of water even
though feeling cold. And plain water is best. We don't really need to
replace anything else in flight. Few of our flights are long enough to
require food before we get home. It is advisable to have a gallon of
drinkable water on board. You might drink only a quarter of that in
flight but the rest of it is worthwhile in case of landing out and
having a long wait for a retrieve. Even then plain water is all you
need. A few hours of electrolyte loss may slow down an athlete's
performance but won't have a noticeable effect any of us.

--MikeM