Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
‘Bonking’ or ‘hitting the wall’ is the sudden, profound fatigue that occurs when muscle and liver glycogen stores are depleted during prolonged endurance exercise — classically around the 30km (20 mile) mark of a marathon. With glycogen exhausted, the body must rely on slower fat metabolism, causing a dramatic drop in pace, heavy legs, mental fog and sometimes dizziness. Prevention: adequate pre-event carbohydrate loading, in-race fuelling (30-90g carbs/hour) and training the body to use fat efficiently (fat adaptation).
- Severe fatigue from glycogen depletion
- Classically ~30km/20 miles in a marathon
- Body forced onto slower fat metabolism
- Prevented by carb loading + in-race fuelling
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The 'bonk' or 'wall' is a real physiological event, not just fatigue — it happens when glycogen stores in the liver and muscles run critically low, forcing the body to rely on a much slower fat-burning metabolism that can't match the energy demand, which is why proper carbohydrate fuelling during long endurance events (not just beforehand) is essential to avoid it.
FAQ
How can I avoid bonking during a marathon or long ride?
Consuming 30-90g of carbohydrate per hour during the effort (depending on duration and intensity), rather than relying solely on pre-event fuelling, is the standard strategy to prevent glycogen depletion.
Does fitness level prevent bonking?
Better-trained athletes can delay the bonk by storing more glycogen and burning fat more efficiently, but even elite athletes will bonk if they under-fuel relative to the duration and intensity of the effort.