A while back I wrote To Run Further - Fill the Tank First.
Today I was on a training climb that was intended to be a shake down of a new backpack and my experience is still the same. An hour before, I ate lunch a veggie burger patty, and tortellini pasta. I did a 1300' climb with a 30 pound pack pushing pretty hard. I didn't eat at all during the climb. I had plenty of energy and did not bonk at all which would often happen under the same circumstances.
I haven't tried this at altitude or over a longer period of time.
Altitude is especially tricky as altitude depresses appetite for me at least (others too). I've been reading (kinda sorta) that there really is something to "carb loading," but that more has to do with eating a lot of carbs the night before but that you can store extra energy in your muscles for a little while.