What is Bonking (Hitting the Wall)?

‘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).