It's been mentioned a time or two that weight loss from radiation treatment is usually lean muscle loss, not fat loss. I'm having a hard time getting my head around this based on what I know about metabolism.

I used to run marathons. We all knew what hitting the wall (usually at mile 20) meant. Your stores of glycogen are gone and your fuel needs are in excess of what the slower metabolism of fat can provide, so you start to eat some of your own muscles.

I can understand that in the higher energy world of marathon running. But in cancer treatment I'm not running a marathon. I'm at a much lower energy requirement mode. I sit around a lot, I read, I sleep, I eat, I walk some (but certainly no running).

Doesn't the body use the fuel source that easiest to burn, glycogen first, fat second (unless you're running a marathon), muscle last.

I know the radiation is causing higher energy needs, but is there something else going on that causes fat metabolism to be bypassed in favor of muscle metabolism?

If I'm not asking the right question, please educate me. I'm an engineer, we need to know the WHY of things, not just do it because I say to do it.

thanks, Tony