Does our body tell us when we can’t go any farther, or does our brain? A look at marathon runners and the science behind human endurance.

“A classic situation in which athletes believe they have hit a true physical limit is ‘bonking’ during a marathon: you stagger to a halt, ostensibly because your body runs out of carbohydrates. When Noakes started running in the 1970s, the standard advice was to drink only water during long races. Then, in the late stages of a sixty-four-kilometre race one year, he tried a few spoonfuls of corn syrup. ‘Five minutes later, I just started running. I finished that race faster than I ever finished,’ he recalls. ‘It was like the brain released something.” The discovery led to the first external funding (‘a thousand rand in a brown paper packet,’ he says) for his nascent sports science lab, to study the effects of corn syrup on participants in South Africa’s Stellenbosch marathon.

“The fact that the corn syrup worked seems to support the idea that the body is limited by its finite store of carbohydrates. But it almost worked too well, and Noakes began to question whether carbohydrates could even reach the muscles that quickly. Sure enough, recent experiments in Britain have shown that your brain picks up the presence of carbohydrates in your mouth via previously unknown sensors, anticipates that fuel is headed to your muscles, and allows you to go a bit faster — even if you trick it by spitting out the carbs rather than swallowing them to replenish your muscles.”