‘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).
Key points
- 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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