After "hitting the wall" in the New York Marathon, Benjamin Rapoport, an MD/PhD student in the Harvard-MIT Division of Health Sciences and Technology, decided to take a rigorous approach to calculating how fast
any runner can reasonably hope to run a marathon, and just how much carbohydrate individual runners need to fuel their 26.2-mile races. The result is a new model, published October 21 in the open-access journal PLoS Computational Biology, which allows any runner to calculate those targets using an estimate of his or her aerobic capacity.

Of the hundreds of thousands of people who run a marathon each year, more than 40 percent hit the figurative wall. "Hitting the wall" occurs when carbohydrates, which the body relies on for energy during
intense aerobic exercise, are completely depleted in the liver and leg muscles. This depletion forces the body to start burning fat, but fat metabolism uses oxygen less efficiently than carbohydrate metabolism and
so runners are forced to slow down. Many runners also experience pain and fatigue with the abrupt transition to fat metabolism.

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