Key takeaways
- Bonking ('hitting the wall') is sudden, severe fatigue from running out of carbohydrate fuel (glycogen).
- Symptoms: profound weakness, heavy legs, dizziness, irritability, and mental fog.
- Prevent it by fueling with carbs consistently during long efforts and starting well-fueled.
- Recover by taking in fast carbohydrates and easing the pace until energy returns.
What bonking is
Bonking — ‘hitting the wall’ — is the sudden, severe fatigue that strikes when your body depletes its stored carbohydrate (glycogen), the primary quick fuel for your muscles and brain during hard, prolonged exercise. When the tank runs dry, performance can collapse dramatically and abruptly.
How it feels
The bonk brings overwhelming fatigue and heavy, unresponsive legs, often with dizziness, irritability or low mood, and mental fog. It genuinely feels like ‘running out of gas’ — because, in carbohydrate terms, you have.
A runner who skipped fueling on a long run suddenly feels their legs turn to lead and their mind go foggy at mile 18 — a classic bonk. They take a gel and slow to a walk, but it’s 15 minutes before the carbs revive them, a lesson in fueling earlier next time.
Prevention and recovery
Prevent the bonk by starting well-fueled and eating carbs consistently during long efforts — roughly 30–60g per hour via gels, food, or drinks — rather than waiting until you feel low. Pair carbs with electrolytes on hot, long days. If you bonk, take fast carbs and ease the pace, but recovery takes time — which is why steady fueling and a smart pacing strategy matter so much in ultras.
The bottom line
The bonk is your body hitting empty — the sudden, crushing fatigue when carbohydrate stores run dry mid-effort. It's largely avoidable: start well-fueled and take in carbs steadily (roughly 30–60g/hour) on long runs, rather than waiting to feel weak. Once you've bonked, fast carbs and a slower pace are the only fix, and they take time — so fuel early and often.
Frequently asked questions
What is bonking?
Bonking, or 'hitting the wall', is the sudden, severe fatigue that hits when your body runs out of its stored carbohydrate (glycogen) — the primary quick fuel for your muscles and brain during hard, prolonged exercise. Without it, you experience profound weakness and a dramatic drop in performance, often quite abruptly.
What does bonking feel like?
It comes on as overwhelming fatigue and heavy, unresponsive legs, often with dizziness, lightheadedness, irritability or low mood, and mental fog or difficulty concentrating. It can feel like you've suddenly 'run out of gas' — because, in terms of carbohydrate fuel, you have.
How do you prevent and recover from the bonk?
Prevent it by starting well-fueled and eating carbohydrates consistently during long efforts (a common guideline is roughly 30–60g per hour, via gels, food, or drinks) rather than waiting until you feel low. If you do bonk, take in fast-acting carbohydrates immediately and ease your pace — it takes time for the fuel to kick in, so prevention is far better than the cure.
Sources
- Endurance fueling — American Council on Exercise
- Trail running nutrition — American Trail Running Association