[cimmanom]

Some Olympic-level athletes deliberately train at high altitudes because the body responds by making more red blood cells in order to make the most of what oxygen is available. This then gives them an advantage when competing at lower altitudes.

So basically, if you moved to Denver and maintained your current activity level, your body would adapt within a few weeks.

Also, Denver for one is not all that high, even at 5k feet. Visiting there for a few days after several years spent at sea level didn’t result in any noticeable altitude sickness symptoms for me, just somewhat decreased stamina for aerobic activity. (Whereas a few hours spent driving over the continental divide at nearly triple the altitude did trigger very obvious altitude sickness symptoms such as dehydration, sleepiness, and headache.)