A Physicist on Everest: How Body and Mind Break Down at Elevation [Excerpt]: Scientific American: As a climber goes up even higher in altitude, into the so-called death zone, the dangerously thin air above 26,000 feet, there is so little oxygen available that the body makes a desperate decision: it cuts off the digestive system. The body can no longer afford to direct oxygen to the stomach to help digest food because that would divert what precious little oxygen is available away from the brain. The body will retch back up anything the climber tries to eat, even if it’s as small as an M&M.
The consequence of shutting down the digestive system is, of course, that the body can no longer take in any calories. Lacking an external fuel source, the body has no choice but to turn on itself. It now fuels itself by burning its own muscle—the very muscle needed to climb the mountain—at a rate of about two pounds per hour.