How the Body Switches Energy Sources While Running

When running, the body generates energy by using carbohydrates and fats as its main fuel sources. The preferred source of energy depends on several factors, including the intensity and duration of the activity, as well as the athlete’s training status. While carbohydrates provide quick but limited energy, fats offer an almost endless supply, though at a slower rate. This article explains the mechanisms of energy production and the influence of exercise intensity and training level.

Carbohydrate Metabolism: Quick Energy from Glycogen and Glucose

Carbohydrates play a central role in energy production, especially during high-intensity exercise. The body stores them as glycogen in the muscles and liver. During physical activity, this glycogen is converted into glucose and used as fuel.

Glucose can be processed in two ways. During very intense efforts, when oxygen demand is high, it is broken down via anaerobic glycolysis. This process rapidly produces large amounts of ATP (short for adenosine triphosphate — the body’s universal energy currency), but also generates lactate as a byproduct, which can lead to muscle fatigue when it accumulates.

During moderate to low-intensity exercise, energy is primarily supplied through the aerobic breakdown of glucose. This process occurs in the mitochondria and is much more efficient, as it produces significantly more ATP per molecule of glucose than anaerobic glycolysis. Additionally, it does not produce lactate, which allows for sustained performance over longer periods.

Fat Metabolism: Slow but Enduring Energy Supply

Fats represent an almost limitless energy source, but they must be processed through a comparatively more complex and slower pathway. The body stores fats as triglycerides in fat tissue, which are broken down into free fatty acids via lipolysis when needed. These fatty acids enter the bloodstream and are absorbed by the muscles, where they are broken down in the mitochondria through beta-oxidation.

Since this process depends on a sufficient supply of oxygen, fat metabolism works especially well at low to moderate intensity levels. As intensity increases, the use of fatty acids decreases because they cannot supply ATP quickly enough to meet the rising energy demands. In such cases, the body increasingly relies on carbohydrates as a fuel source.

Differences in Availability and Speed of Energy Supply

A key difference between these two energy systems lies in the speed and availability of the energy provided. Carbohydrates deliver quickly accessible energy but are limited in storage capacity. Glycogen stores can hold only around 400–600 grams of carbohydrates, which is sufficient for only a few hours of high-intensity activity.

Fats, on the other hand, are available in virtually unlimited quantities — even lean individuals have enough fat reserves to theoretically complete several marathons in a row. However, because energy production from fat is much slower, it is less suitable for high-intensity efforts.

Impact of Training Status on Energy Supply

Regular endurance training leads to various metabolic adaptations that allow the body to rely more efficiently on fat as an energy source. One of the most important changes is an increase in mitochondrial density in muscle cells. Mitochondria are the “powerhouses” of cells, where fatty acids and glucose are used to generate ATP in the presence of oxygen. The more mitochondria there are, the better the body can oxidize fat and preserve carbohydrate stores.

Another effect of training is the increased activity of fat-metabolizing enzymes. Enzymes such as carnitine palmitoyltransferase 1 (CPT-1) and hormone-sensitive lipase (HSL) play a critical role in mobilizing and processing fatty acids. With higher enzyme activity, the body can release stored fats more quickly and utilize them in the mitochondria. Studies have shown that this enzymatic adaptation is promoted by long-term endurance training, allowing for more efficient use of fat reserves (Holloszy & Coyle, 1984).

In addition to these adaptations, regular training shifts the so-called crossover point — the moment at which the body switches from primarily burning fat to predominantly using carbohydrates. While untrained individuals begin to rely more on carbohydrates even at moderate intensity, trained athletes can continue to derive significant energy from fats at higher intensities. This allows them to conserve carbohydrates for more intense phases, such as final sprints or uphill sections in a race.

Gender Differences in Energy Supply

Scientific research suggests that women have a higher rate of fat oxidation than men, meaning they derive more energy from fats, especially during submaximal efforts. A study by Tarnopolsky (2008) found that women use up to 25% more fat as a fuel source than men during moderate-intensity exercise.

One key factor in this difference is the influence of the hormone estrogen. Estrogen plays an important role in energy metabolism by promoting greater reliance on fat stores rather than carbohydrates. It does so, among other mechanisms, by increasing the activity of fat-metabolizing enzymes responsible for mobilizing and oxidizing fatty acids.

Additionally, women tend to have a higher proportion of type-I-muscle fibers. These slow-twitch fibers are particularly suited for aerobic energy production, as they contain a high density of mitochondria and efficiently use fatty acids to generate ATP. Because type I fibers are optimized for endurance efforts, this may help explain why women are better able to use fat as an energy source during long, moderate-intensity exercise.

Conclusion

While running, the body uses both carbohydrates and fats as energy sources. Which one dominates depends on exercise intensity, duration, and the athlete’s training status. Carbohydrates provide fast but limited energy, whereas fats offer an almost inexhaustible supply, though at a slower rate.

Endurance training enhances fat metabolism by increasing mitochondrial density and activating fat-burning enzymes. As a result, trained individuals can utilize fats more efficiently even at higher intensities and conserve carbohydrates for peak efforts.

Due to hormonal differences, women tend to have a higher rate of fat oxidation than men, which supports their adaptation to prolonged endurance activities. This knowledge can be used to tailor training toward more efficient energy utilization and improved endurance performance.

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