I know people that live at altitude control their weight better.

I also know top athletes train in hyperbaric chambers (extra oxygen).

I'm not remotely at that level but I can see the difference between my performance in local terrain vs Death Valley. There's a decent amount of stuff down there bumpy enough to be interesting but not so bumpy as to make it hard to get through. But, on the flip side, I have never been able to find a good, sustainable pace in the mountains.