| What it is | Sudden energy crash from glycogen depletion |
| Feels like | Weakness, lightheadedness, 'empty' |
| Cause | Running out of carbohydrate fuel |
| Prevent with | Regular carbs during effort; fuel early |
To bonk (or ‘hit the wall’) is to suffer a sudden, severe drop in energy during prolonged exercise when the body’s carbohydrate stores (glycogen) become depleted, leaving you weak, lightheaded, and struggling to continue. Distinct from general fatigue, it’s primarily a fueling failure, and it’s prevented by eating carbohydrates regularly during long efforts and starting well-fueled.
Fuel to avoid it
Prevented by carb loading beforehand and taking a gel and electrolytes during ultra efforts.
Frequently asked questions
What does it mean to bonk?
Bonking, or hitting the wall, is a sudden and dramatic loss of energy during long endurance exercise, caused mainly by depleting your body's glycogen (stored carbohydrate) fuel. It leaves you feeling weak, heavy-legged, lightheaded, and sometimes shaky or foggy, making it very hard to maintain pace or even keep going.
How do you prevent bonking?
Start the effort well-fueled (and carb-load before long events), then eat carbohydrates regularly during the activity — commonly via gels, chews, or real food — before you feel depleted, since it's hard to recover once you've bonked. Consistent fueling and hydration, including electrolytes, keep your energy steady over hours.
What's the difference between bonking and just being tired?
General fatigue builds gradually from accumulated effort, while bonking is a relatively sudden, fuel-driven crash from running out of glycogen — often described as the body 'shutting down.' The fix differs too: ordinary fatigue needs rest and pacing, whereas a bonk specifically calls for taking in carbohydrates to restore blood sugar and energy.
Sources
- Hitting the wall — American Council on Exercise
- Endurance fueling — American Trail Running Association