Your liver also stores a significant amount of glycogen that it can release into the bloodstream. "Bonking" is a term from endurance events, for example in a marathon bonking typically takes place around 20 miles into the race when runners have been running for more than 2 hours, which fully depletes their glycogen stores. Modern runners typically refuel with carbohydrates, drinks/gels, to delay bonking, but depending on pace and time you typically can't consume enough calories to fully replenish what you have used and will bonk eventually. Then you will need to continue at a slower pace burning fat for fuel.

I’m an ex cyclist but occasionally I like a bit of nostalgia. This recalls to me one conversation with a coworker who pointed out that while exerting yourself you can only absorb around 600 calories an hour, but as we know, you can burn thousands.

You have what’s in your muscles, liver, stomach, and whatever your body can cannibalize from fat and muscle. If you go long enough (without overdoing it) you start burning muscle, which is a really good example of why recovery and pace matter so much. And why runners and cyclists have those wiry little arms.

Friday99 gave a pretty good explanation but to offer a little insight into the experience: your brain runs on glycogen too. The Bonk is what happens when your glycogen stores crash so far that your mental capacity is diminished.

Smart communities train their members to watch for heat exhaustion, heat stroke, and bonking, how to tell the difference, and how to care for people in these situations.