Fueling the Long Haul: The Energy Source for Sustained Long-Term Activity Like Endurance Running

December 3, 2025 •

4 min read

Over 540,000 people finished a marathon in 2023, showcasing the power of human endurance. For sustained long-term activity like endurance running, the body primarily relies on the aerobic energy system, which efficiently uses both fat and carbohydrates to produce a steady supply of ATP. This process allows athletes to maintain a submaximal pace for hours on end, avoiding rapid fatigue.

Quick Summary

The body primarily uses the aerobic energy system for prolonged endurance activities, burning a combination of carbohydrates and fats for fuel. Fat serves as the most abundant energy reserve for longer distances and lower intensities, while glycogen stores from carbohydrates provide a faster, more efficient fuel, crucial for higher-intensity bursts and to start the activity. The balance of these fuels shifts based on intensity and duration.

Key Points

Aerobic System is Key: For sustained long-term activities like endurance running, the body's aerobic (oxidative) energy system is the primary source of ATP.
Carbohydrates for Intensity: Glycogen from carbohydrates is the faster-burning fuel, used preferentially for higher-intensity running and to start the activity.
Fat for Endurance: The body's fat reserves are a vast, long-term energy source, mainly used during lower-intensity, longer-duration aerobic exercise.
Crossover Point: As exercise intensity increases, the body shifts from burning primarily fat to burning primarily carbohydrates. Endurance training moves this crossover point, allowing athletes to burn fat more efficiently at higher speeds.
Strategic Fueling Prevents 'Hitting the Wall': Since glycogen stores are limited, consuming carbohydrates during longer events is critical to maintain blood sugar and delay fatigue caused by glycogen depletion.

The Body's Energy Systems

To understand the energy source for sustained long-term activity, it's essential to first grasp the three main energy systems the body uses to create Adenosine Triphosphate (ATP), the universal energy currency for all cellular functions.

The Phosphagen System: This is the immediate, explosive energy system, fueled by stored ATP and creatine phosphate. It provides energy for high-intensity, short-duration activities lasting up to about 10 seconds, such as a quick sprint. It is anaerobic, meaning it does not require oxygen.
The Anaerobic Glycolytic System: This system uses stored glycogen (from carbohydrates) to produce ATP without oxygen. It powers high-intensity efforts lasting from 30 seconds to approximately 3 minutes. It's faster than the aerobic system but less efficient and produces lactate, which can lead to fatigue.
The Aerobic (Oxidative) System: This is the primary system for sustained, long-term activity. Requiring oxygen, it produces a large and steady amount of ATP by breaking down carbohydrates, fats, and, in some cases, protein. It becomes the dominant energy provider after about two to three minutes of exercise as breathing and heart rate stabilize.

The Role of Carbohydrates and Fats in Endurance

For sustained long-term activity like endurance running, the aerobic system leverages both carbohydrates and fats, but the proportions depend heavily on the intensity and duration of the exercise.

The Critical Role of Carbohydrates

Carbohydrates are the body's most readily available fuel source for aerobic metabolism. They are broken down into glucose, which is stored as glycogen in the muscles and liver.

Initial fuel: At the start of a run and during periods of higher intensity, the body preferentially burns carbohydrates because they can be metabolized more quickly than fat to produce ATP.
Glycogen limitation: The body's glycogen stores are limited, typically providing enough fuel for about 90 minutes of high-intensity running. When these stores are depleted, a runner can experience a sudden and severe loss of energy, a phenomenon known as "hitting the wall".
Fueling during activity: To prevent glycogen depletion during races like a marathon, runners must consume carbohydrates (e.g., gels, sports drinks, chews) during the event to replenish blood glucose levels.

The Endless Supply of Fat

Fat represents the body's largest energy reserve. Even the leanest athletes have a virtually inexhaustible supply of fat to fuel prolonged exercise.

Low to moderate intensity: During lower-intensity running, the aerobic system is highly efficient at using fat for fuel. A trained endurance athlete can rely on fat metabolism to conserve precious glycogen stores.
Energy density: At 9 kcal per gram, fat is more than twice as energy-dense as carbohydrates (4 kcal per gram), making it a highly efficient long-term fuel source.
Oxygen dependence: The process of breaking down fat for energy (beta-oxidation) requires more oxygen than carbohydrate metabolism. This is why fat becomes the more dominant fuel source at lower intensities, where the body can supply enough oxygen to the working muscles.

The “Crossover Point”: A Fuel Shift in Motion

As exercise intensity increases, the body shifts its fuel preference. This physiological shift, known as the "crossover point," is where the body transitions from using fat as its primary fuel to using carbohydrates. For most untrained individuals, this occurs at a relatively low intensity. However, endurance training improves the body's ability to burn fat at higher intensities, shifting the crossover point to the right. This allows a trained runner to run faster for longer before tapping heavily into their limited glycogen reserves.

Comparison of Energy Sources for Endurance


Feature	Carbohydrates (Glycogen)	Fats (Triglycerides)	Protein (Amino Acids)
Energy Production	Fast and efficient, high power output.	Slower, but very efficient, lower power output.	Used primarily for repair, minimal use as fuel.
Fuel Source	Stored in muscles and liver (limited).	Stored in adipose tissue and muscle (virtually unlimited).	Not a primary fuel source, except in extreme conditions.
Speed of Use	Rapidly metabolized for quick energy.	Slowly metabolized, ideal for steady-state exercise.	Negligible speed for fuel purposes.
Oxygen Requirement	Aerobic and anaerobic pathways.	Aerobic pathway only.	Aerobic pathway only.
Primary Use in Run	Higher intensity efforts, start of race, and sprints.	Lower to moderate intensity, longer duration runs.	Not for primary fuel; vital for recovery.

Maximizing Your Fuel Strategy

For optimal performance in endurance running, a strategic approach to nutrition is vital. This involves fueling before, during, and after exercise to manage glycogen stores and promote recovery.

Carbohydrate loading: For races over 90 minutes, runners often practice "carbo-loading" in the days before to maximize their muscle and liver glycogen stores.
During-race fueling: Consuming easily digestible carbohydrates like gels, chews, or sports drinks every 45-60 minutes during long runs helps maintain blood glucose and spares glycogen.
Strategic training: Some runners use a "train low" strategy during certain low-intensity training sessions to improve their body's ability to burn fat more efficiently, while still fueling adequately for high-intensity efforts and races. For more insights into advanced endurance fueling, check out this guide on the science of fuelling for the long run.

Conclusion

In essence, the aerobic energy system is the engine that powers sustained long-term activity like endurance running. It is a highly versatile system that can burn both carbohydrates and fats to meet the body's energy demands. While carbohydrates provide the quick, high-intensity fuel and are essential for topping off energy stores, fat is the massive, long-lasting reserve that allows a runner to cover great distances without running out of power. Understanding the interplay between these two fuel sources, and developing a strategic nutrition plan, is the key to unlocking true endurance potential and avoiding the dreaded bonk.

Frequently Asked Questions

The primary energy source for long-distance running is the aerobic energy system, which utilizes both carbohydrates (glycogen) and fats, depending on the exercise intensity and duration.

Carbohydrates are converted into glucose and stored as glycogen in the muscles and liver. During exercise, this glycogen provides a rapid and efficient fuel for the aerobic system, especially during higher-intensity efforts.

The body starts using fat for energy during exercise once the aerobic system becomes the dominant energy provider, typically after a few minutes of steady activity. As the run becomes longer and lower in intensity, the body's reliance on fat increases.

A runner hits the wall when their muscle and liver glycogen stores become depleted. Without the body's preferred fast fuel source, performance drops significantly, and the runner is forced to rely almost entirely on the less efficient fat metabolism.

While protein can be metabolized for energy, it is not a primary fuel source during endurance running. The body only turns to protein for fuel in very small amounts during extremely long duration activities or when carbohydrate stores are severely low.

For optimal endurance performance, it is ideal to burn a mix of both. Burning fat efficiently at lower intensities helps conserve limited carbohydrate stores, which can then be used for higher-intensity pushes or toward the end of a race.

Training with a strategic approach, such as performing long, low-intensity runs, can improve your body's metabolic efficiency and its ability to burn fat more effectively for fuel, a concept sometimes referred to as 'fat adaptation'.