Fueling the Long Run: How Your Body Makes Energy for a Marathon

A marathon is not powered by willpower alone. Your muscles need a steady supply of energy, and that energy comes mainly from carbohydrates and fat. Understanding how your body uses them can make fueling feel much less mysterious.

Your muscles run on ATP

Muscles do not use food directly. They use a small energy molecule called ATP. Think of ATP as the spendable cash your muscles use to contract. Because your body stores only a small amount, it must constantly make more.

It has three overlapping energy systems for doing this.

Three systems, one race

The phosphagen system is your instant battery. It uses stored energy for a few seconds—for example, when you sprint away from the start line or climb sharply up a hill.

The glycolytic system produces energy quickly by breaking down carbohydrate. Carbohydrate is converted into glucose, and muscles store glucose as glycogen. This system helps during hard efforts lasting from several seconds to a couple of minutes.

The oxidative system is the marathon engine. It uses oxygen to turn carbohydrate and fat into large amounts of ATP. It works more slowly, but it can keep supplying energy for hours.

All three systems work at the same time throughout the race—they do not switch on and off like separate buttons. The balance simply shifts: the phosphagen system contributes most during a sudden acceleration, glycolysis contributes more during a hard climb or surge, and the oxidative system does most of the work during steady marathon running. The faster you run, the more heavily you depend on carbohydrate because it can provide energy more quickly than fat.

Why carbohydrate matters so much

Before and during a marathon, carbohydrate is your most useful high-intensity fuel. Stored muscle glycogen helps you maintain pace, while carbohydrate from drinks, gels, chews, or food helps preserve those limited stores.

When glycogen becomes seriously depleted, runners often experience the familiar “wall”: pace drops, effort rises, and the legs feel empty. This is not a lack of motivation. It is a sign that the body is struggling to supply enough carbohydrate quickly enough.

Where lactate fits in

During a surge, hill, or fast finish, carbohydrate may be broken down faster than the mitochondria—the cell’s energy-producing structures—can process it. The resulting substance, pyruvate, can be converted into lactate.

Lactate is not simply waste. Other muscles, the heart, and the liver can reuse it as fuel or convert it back into glucose. The burning sensation during very hard running is linked more to the rapid buildup of hydrogen ions and other changes in the muscle than to lactate alone.

For marathoners, the practical lesson is simple: running above your sustainable pace creates energy demand faster than your aerobic system can comfortably meet it. That is why pacing matters as much as fueling.

Turning biology into a fueling plan

• Begin the race fueled. Eat a familiar, carbohydrate-rich meal before the start and avoid experimenting on race morning.

• Start fueling early rather than waiting until you feel tired. A common starting range is roughly 30–60 grams of carbohydrate per hour; some experienced runners can tolerate more after careful training.

• Take small amounts regularly. Your gut absorbs fuel better when it is spread across the run.

• Practice your plan during long runs. The digestive system can be trained to handle carbohydrate, just like the legs can be trained to run.

• Drink according to conditions and your individual needs. Too little fluid can impair performance, while excessive drinking can also be dangerous.

• Keep the pace controlled early. No gel can fully compensate for starting too fast and burning through carbohydrate too quickly.

The big picture is this: carbohydrate supports pace, fat supports endurance, oxygen helps unlock both, and consistent fueling protects your limited carbohydrate stores. A well-fueled marathon is not about taking in as much as possible—it is about supplying enough energy, often enough, at a pace your body can sustain.