Why do marathon runners hit the wall?

Because the body runs out of its most accessible fuel. "The wall" is the sudden, severe fatigue that many marathon runners experience somewhere after 18 to 22 miles, and it has a specific physiological cause.

Muscles are powered mainly by two fuels: glycogen, a stored form of carbohydrate, and fat. Glycogen burns efficiently and can support a fast pace. Fat is far more abundant but converts to usable energy more slowly, so it cannot sustain the same speed.

A trained runner stores roughly 1,500 to 2,000 calories of glycogen in the muscles and liver. A marathon burns considerably more than that. When glycogen runs low, the body is forced to rely disproportionately on fat, and pace collapses — often abruptly rather than gradually. Runners describe legs that will not respond, and because the brain also runs on glucose, mental fog and low mood are common alongside it.

The distance at which it strikes is not coincidental. It falls close to where stored glycogen typically runs out at marathon pace, which is why the last few miles have a reputation quite separate from the first twenty.

The standard countermeasures all address the fuel gap: carbohydrate loading in the days before, taking on carbohydrate during the race in gels or drinks, starting at a sustainable pace rather than banking time early, and training the body to use fat more efficiently through long slow runs.

Dehydration and electrolyte loss produce overlapping symptoms and often occur at the same time, which is why the two are frequently confused.

Anyone attempting a marathon should build up over months and take medical advice before starting if they have any relevant health condition.