Beck Weathers

“One of the body’s most important physiological adaptations to high altitude is the millions and millions of extra oxygen-bearing red blood cells that your bone marrow produces in response to chronic oxygen deprivation. The extra oxygen-carrying capacity is critical. Still, you thirst for air when high on the big mountains. Breathing is such hard work that 40 percent of your total energy output is devoted to it. Each day you can blow off an amazing seven liters of water through your lungs alone. That leaves you constantly dehydrated. Also, you can no longer sleep or eat. Once in the Death Zone, above 25,000 feet, the thought of food becomes repugnant to most people. Even if you can force yourself to chew and swallow something, your body will not digest it. Yet you are burning about twelve thousand calories a day, which means you’re consuming your own tissue—about three pounds of muscle a day—in order to stay alive.”
― Beck Weathers, Left for Dead: My Journey Home from Everest