(Credit: EJ Hersom/Department of Defense) It's a familiar scene that played out most recently at the London marathon: An exhausted runner staggers and falls in the home stretch, unable to will their legs forward another step. It's an extreme example of a phenomenon endurance athletes come to know intimately, often called "hitting the wall," or sometimes by the more offbeat term "bonking." The proverbial wall appears when our bodies have run out of stores of glucose, a sugar molecule that is our main source of energy during strenuous exercise. Without energy, our muscles can't function and we run smack into what feels like a physical barrier. With training, athletes can push the wall back by conditioning their muscles to get better at burning fat as well, which gives them access to a second energy source.
The gradual process by which our bodies come to prefer using up fat reserves has ...
