I’ve been at altitude starting my 9-day acclimation period for the Everest Challenge. Observe that on August 17th upon arising my weight plummeted to 174.0 exactly 36 hours later*, and no, this is not dehydration—it is the natural result of acclimatizing to altitude. I see exactly the same physiological change every time I go to ~10,000' elevation (with the reverse effect upon returning to sea level, unfortunately!). It seems to be the required change for the body to dump a bunch of new red blood cells into the bloodstream (my educated guess, I call this the spleen dump*).
Yesterday I rode Upper Rock Creek and beat last year’s time by 4 minutes. Yay! I was heavier, only at altitude ~14 hours (and I could feel it), but well rested and the temps were cool. Whereas last year I was much lighter and it was the last climb of the race. Still, it was not a max effort and so I am quite hopeful that I might do OK this year, endurance and recovery being the concerns, since I am sure I have more power and muscle mass in my quads.
* The rule of thumb for me is that going from sea level (500') to 10,000', that I will lose ~3 pounds of body weight right around 36 hours at altitude, and this corresponds to a marked adaptation for aerobic activity (I call this the “spleen dump” of red blood cells being released). At 8-9 days later, there is another notable “bump up”, apparently an infusion of matured cells from bone marrow, resulting in another performance boost.
Happens like clockwork for me every time I go to altitude. It is not dehydration in any normal sense, as I drink plenty, urine is clear as water, and the loss continues as long as I stay at altitude.