I'm with Dru ... i think I read somewhere that the
scientific explanation for the altitude/latitude thing
was bunk. But from what I've been told it's subjectively
true that a given altitude is 'harder' at high latitudes
than low. Eg, I was told 5000m on Mt Logan feels like
6000m in the Cordillera Blanca. My guess is that the
extra cold and dryness and remoteness play a role. Also I
would bet that some of this 15% body mass loss on short
expeditions involves a lot of water loss, not fat or muscle.