“Hitting the wall” is the abrupt loss of pace and rising effort that some runners experience late in a marathon. It is often explained as simply “running out of glycogen,” but the physiology is more complicated. Glycogen availability matters, yet fatigue during a marathon also reflects pacing, muscle damage, thermoregulation, hydration, motivation and the brain’s regulation of effort.
What is the marathon wall?
Muscle and liver glycogen are limited. As a long race continues, carbohydrate availability falls and a greater fraction of energy comes from fat oxidation. Fat is an important endurance fuel, but it cannot support the same high rate of ATP production as carbohydrate during hard running. If pace is too aggressive for the runner’s carbohydrate supply and fitness, maintaining speed becomes increasingly difficult.
This does not mean that every late-race slowdown is caused by complete glycogen depletion. “The wall” is a useful description of the experience, not a single laboratory diagnosis.
Do dehydration and electrolytes cause muscle cramps?
The older version of this article treated dehydration and sodium or potassium loss as the main explanation for marathon cramps. Current evidence does not support such a simple model.
Prospective studies of distance runners have repeatedly found that athletes who cramp are not consistently more dehydrated and do not consistently have lower serum sodium or potassium than runners who do not cramp. A 2026 systematic review likewise found little consistent support for commonly proposed hydration or electrolyte predictors. Neuromuscular fatigue, individual susceptibility and previous cramping history appear to matter.
That does not make fluid and sodium irrelevant. Severe dehydration, heat illness and exercise-associated hyponatremia are genuine medical problems. They are simply not interchangeable with ordinary exercise-associated muscle cramps.
Is marathon running dangerous?
Marathons create substantial physiological stress, and rare serious events do occur. But saying that regular marathon runners are generally at “high risk of serious cardiovascular changes” is too broad. Transient rises in cardiac biomarkers and temporary changes in cardiac function can occur after prolonged endurance events; in healthy, adequately prepared runners these findings do not by themselves mean permanent heart damage.
The sensible risk message is narrower: people with symptoms, known cardiovascular disease or important risk factors may need medical assessment before high-intensity endurance events, and untrained people should not jump directly into a marathon.
Fueling for a marathon
For long races, carbohydrate availability is one of the most modifiable performance factors. Runners commonly increase carbohydrate intake before an event and consume carbohydrate during prolonged exercise. The exact amount depends on duration, tolerance and training.
Race-day fueling should be practiced in training. Trying unfamiliar gels, drinks or large carbohydrate loads for the first time during the race is a reliable way to turn a marathon into a gastrointestinal research project.
Hydration: neither too little nor too much
The aim is not to replace every gram of body mass lost through sweat. Overdrinking can dilute blood sodium and cause exercise-associated hyponatremia, which can be life-threatening. Individual sweat rate, temperature, pace and race duration all change fluid needs.
For many recreational runners, drinking to thirst is a reasonable starting strategy, adjusted for known individual needs and environmental conditions.
How to reduce the chance of hitting the wall
- Build endurance gradually over months, not weeks.
- Choose a pace that matches your actual training rather than your optimistic race-morning self.
- Practice carbohydrate intake during long training sessions.
- Practice your hydration strategy in similar weather.
- Do not treat salt tablets as a universal cramp prevention method.
- Stop and seek medical help for chest pain, fainting, confusion, severe shortness of breath or other alarming symptoms.
Bottom line
The marathon wall is real, but it is not just a tank suddenly becoming empty. Glycogen availability is important, while pacing, fatigue, muscle damage, heat and other factors also contribute. Muscle cramps are not reliably explained by dehydration or electrolyte loss, and both underdrinking and overdrinking can cause problems. Training and a practiced fueling strategy matter more than a single “hack.”