Carbohydrates that a person eats are converted by the liver and muscles into glycogen for storage. Glycogen burns rapidly to provide quick energy. Runners can store about 8 MJ or 2,000 kcal worth of glycogen in their bodies, enough for about 30 km/18–20 miles of running. Many runners report that running becomes noticeably more difficult at that point. When glycogen runs low, the body must then burn stored fat for energy, which does not burn as readily. When this happens, the runner will experience dramatic fatigue and is said to "hit the wall". The aim of training for the marathon, according to many coaches, is to maximize the limited glycogen available so that the fatigue of the "wall" is not as dramatic. This is in part accomplished by utilizing a higher percentage of energy from burned fat even during the early phase of the race, thus conserving glycogen.

Carbohydrate-based "energy" gels are used by runners to avoid or reduce the effect of "hitting the wall", as they provide easy to digest energy during the run. Energy gels usually contain varying amounts of sodium and potassium and some also contain caffeine. They need to be consumed with a certain amount of water. Recommendations for how often to take an energy gel during the race range widely.
Alternatives to gels are solid candy, cookies, other forms of concentrated sugars, or any food high in simple carbohydrates which can be digested easily by the individual runner. Many runners experiment with consuming energy supplements during training runs to determine what works best for them. Consumption of food while running sometimes makes the runner sick. Runners are advised not to ingest a new food or medicine just prior to or during a race. It's also important to refrain from taking any of the non-steroidal anti-inflammatory class of pain relievers (NSAIDS, e.g., aspirin, ibuprofen, naproxen), as these drugs change the way the kidneys regulate their blood flow and may lead to serious kidney problems, especially in cases involving moderate to severe dehydration.

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