"In nature the energy budget is primal law, the absolute bottom line. Energy supply, energy availability, governs every single living thing. Humans are no different, once you take away our civilized veneer of overprotection. Put us in a truly elemental situation -- as on mountaineering, or exploratory expeditions, or even in ultra-distance running -- and the energy budget becomes the driving force in our lives, too. It immediately replaces sex, money, power, religion, politics, or any of those other motivations we've developed to keep our metaphorical motors running. The calculations that govern success or failure for that level of effort have precisely to do with calories in and calories out. It's the metabolic equivalent of cash flow -- and when you're out there on the edge, bankruptcy doesn't mean embarrassment and inconvenience, it means death."
