Backpacking on calorie deficit and ideal energy distribution…

"If you're short on oxygen, that will manifest in lots of ways before effecting your metabolism."

One is the insidious route. When hypoxia starts to set in, one of the first things to be impaired is judgment, and next is appetite. You might forget to eat, and that will screw up your performance pretty quickly.

–B.G.–

http://www.marksdailyapple.com/forum/thread64390.html

"4) altitude. If I understand this process correctly. Fat needs oxygen to burn. If that assumption is correct then higher altitudes will limit oxygen and thus the ability for fat to be used as effector for fuel. Back before I really started to focus on fueling this was a very common occurrence on many of my trips at higher elevations in the Sierra. I can remember continuously bumping up to the Wall while I was on the SHR a few years ago. Someday I will have to do that trip again to see the difference in fueling."

True enough, in the sense that there are more C-H bonds to be oxidized in a fatty acid chain cleaved off a triglyceride molecule than in a molecule of glucose. Thus the higher you go, the more your body will tend to use glucose for fuel at any given level of exertion, if it is present. This can be mitigated to some degree by training to increase your VO2 max, thereby increasing your body's ability to deliver what O2 there is available at a given elevation to the muscle cells. Also, over time at altitude your red blood cell count will increase, which also results in improved O2 delivery. Not a major factor on something like the JMT, but on longer routes at altitude, the benefit would occur. The other way to increase your utilization of fat at altitude, assuming the presence of enough glucose to support its metabolism, is to decrease your level of exertion.

This will be controversial, but if you really want to do this, you should adapt your body to a low carbohydrate diet before you go out there. Then your body will be better able to use fat for energy rather than glycogen. You'll be able to fast relatively effortlessly and your bodyfat will provide your energy. Quite a few ultramarathon runners do this to get an edge since running and consuming calories often comes with nasty gastrointestinal issues. Not having to eat at all relieves this.