“Glycogen can power a runner on its own for periods of short, intense exercise, but because it burns up fast, long-distance runners rely mostly on their supply of slow-burning fats and proteins. In order to turn fats and proteins into energy, though, most mammals need to also keep burning a small but steady supply of glycogen. And that turns out to be a serious problem, because the body’s glycogen storage tanks are pretty small, and converting carbs and sugars into glycogen takes a little while. So runners burn glycogen faster than they can make it - if they run for long enough, their supply will inevitably run out, and they’ll bonk. Sled dogs never bonk, because early in the race, their bodies somehow shift over to burning fat and protein without needing to use any glycogen at all. And since fat and protein can be turned into energy pretty much immediately, the dogs can refuel throughout the race without ever worrying about running out of glycogen.”
