What is the major food fuel during exercise of long duration?

January 4, 2026 •

4 min read

While the body uses a combination of carbohydrates and fats for energy, for exercise lasting longer than 90 minutes, carbohydrates are the most critical fuel source. This is because the body's glycogen stores are limited and can become depleted during sustained activity, leading to fatigue.

Quick Summary

The body primarily uses carbohydrates, stored as glycogen, and fat for energy during exercise. For long-duration activities, carbohydrates become the dominant fuel as intensity increases, playing a critical role in sustaining performance and delaying fatigue. Strategic fueling ensures glycogen stores remain available for high-intensity efforts.

Key Points

Carbohydrates are the dominant fuel for high-intensity exercise: As exercise intensity increases, the body preferentially burns carbohydrates due to their rapid energy release.
Glycogen depletion causes fatigue: Fatigue during prolonged exercise, often called "hitting the wall," occurs when muscle and liver glycogen stores are depleted.
Fat fuels low-to-moderate intensity and spares glycogen: During lower intensity activity, the body relies more on its vast fat reserves, preserving limited carbohydrate stores for when they are most needed.
Metabolic flexibility is key for endurance: Well-trained endurance athletes are more efficient at switching between using fat and carbohydrates, allowing them to better manage their fuel stores.
Fueling during exercise extends performance: Consuming carbohydrates every hour during long-duration activities helps maintain blood glucose levels and delays the onset of fatigue.
Protein is not a primary fuel source: The body only turns to breaking down muscle protein for energy in situations of glycogen depletion, which negatively impacts performance and recovery.

The Body's Energy Systems: A Dynamic Process

During any physical activity, the body uses adenosine triphosphate (ATP) for energy, which is generated from macronutrients like carbohydrates, fats, and, to a lesser extent, protein. The proportion of these fuels used shifts based on exercise intensity and duration.

Immediate Energy (First few seconds): For a short sprint or lifting a heavy weight, the body uses stored ATP and creatine phosphate for immediate, powerful bursts of energy.
Anaerobic Glycolysis (First few minutes): As exercise continues, the body turns to anaerobic glycolysis, breaking down muscle glycogen and blood glucose to produce ATP quickly without oxygen. This system can sustain high-intensity efforts for a few minutes before fatigue sets in due to lactic acid buildup.
Aerobic System (Long-Duration Exercise): For prolonged activity, the body switches to the aerobic system, using oxygen to produce a far greater amount of ATP from carbohydrates and fats. The balance between these two sources is a key determinant of endurance performance.

Carbohydrates: The Primary Fuel for High-Intensity and Endurance

Carbohydrates are stored in the body as glycogen in the muscles (around 300-500g) and liver (around 80g). During long-duration, moderate-to-high-intensity exercise, muscle glycogen is the most readily available fuel source.

Here's why carbohydrates are so crucial:

Faster energy release: The body can convert carbohydrates into usable energy faster than fat. As exercise intensity rises, the demand for quick energy outpaces what the slower fat metabolism can provide, making carbohydrates the body's preferred and most efficient fuel source.
Maintains blood glucose: Liver glycogen plays a vital role in maintaining stable blood glucose levels, which are critical for brain function. During prolonged exercise, as muscle glycogen is depleted, the liver releases glucose into the bloodstream to prevent a drop in blood sugar (hypoglycemia), which is a major cause of fatigue.
Delays fatigue: The dreaded "wall" that marathon runners hit is a direct consequence of muscle glycogen depletion. Continuing exercise becomes difficult as the body's primary high-intensity fuel is exhausted. Consuming carbohydrates during exercise helps to spare these limited glycogen stores and extend performance.

The Role of Fat in Endurance Fueling

Fat is a vast and virtually limitless energy reserve in the body, providing more than twice the calories per gram compared to carbohydrates. During long-duration, low-to-moderate-intensity exercise, fat is the body's dominant fuel source. A trained endurance athlete has a greater ability to use fat for fuel, an adaptation known as metabolic efficiency or flexibility. This helps spare precious glycogen stores for higher-intensity efforts later in the race. However, even during fat-burning, some carbohydrates are still required to facilitate the complete breakdown of fat for energy.

Comparison of Energy Sources for Long-Duration Exercise


Feature	Carbohydrates	Fat	Protein
Primary Fuel Source	High-intensity and later stages of prolonged exercise	Low-to-moderate intensity exercise	Used only when other fuel stores are low
Energy Density	4 calories per gram	9 calories per gram	4 calories per gram
Storage	Limited glycogen stores in muscles (~500g) and liver (~100g)	Nearly unlimited adipose tissue and intramuscular fat	Limited use from muscle breakdown
Availability	Rapidly accessible, especially with intake during exercise	Slow to access and transport to muscles	Used only in energy deficit
Glycogen Sparing	Direct fueling helps spare limited glycogen reserves	Higher utilization at lower intensities spares glycogen for later use	None; muscle cannibalization occurs when glycogen is low
Performance Impact	Essential for sustained high output; prevents "hitting the wall"	Primarily for sustained, steady efforts; supports ultra-endurance	Negative impact on performance and recovery; muscle damage

Protein's Supporting Role

While protein is not a primary food fuel during long-duration exercise, it plays a vital supporting role. The body relies on protein for muscle repair, recovery, and other critical functions. Only in situations of depleted glycogen stores, such as after two or more hours of sustained effort or with insufficient daily energy intake, will the body begin to break down muscle protein for energy through a process called gluconeogenesis. This process is inefficient and detrimental to performance and recovery.

Strategic Fueling for Peak Performance

Endurance athletes employ a variety of fueling strategies to maximize performance and delay fatigue. These strategies ensure a constant supply of carbohydrates to meet the body's energy demands.

Before Exercise (Carbohydrate Loading)

Athletes can significantly increase their muscle glycogen stores by following a high-carbohydrate diet in the days leading up to an endurance event. This provides a larger initial fuel tank, delaying the point at which fatigue from glycogen depletion occurs.

During Exercise

For activities lasting more than an hour, consuming carbohydrates during the event is crucial. This helps top up blood glucose levels, preventing the dramatic energy drop known as "bonking". Athletes consume sports drinks, gels, or easily digestible snacks with a combination of glucose and fructose to maximize absorption. Recommendations can range from 30 to 120 grams of carbohydrates per hour, depending on the exercise duration and intensity.

After Exercise (Recovery)

Post-exercise, the body is primed to replenish its glycogen stores. Consuming carbohydrates, ideally combined with protein, as soon as possible after a strenuous workout helps expedite this process, ensuring the athlete is ready for their next training session.

Conclusion

Ultimately, while fat provides a dense, long-lasting energy reserve, the answer to what is the major food fuel during exercise of long duration, especially at moderate-to-high intensity, is clear: carbohydrates. Through the dynamic interplay between carbohydrates and fats, athletes can manage their energy stores to maximize performance. Strategic carbohydrate intake before, during, and after exercise remains the cornerstone of endurance sports nutrition for delaying fatigue and ensuring a successful finish. For more in-depth nutritional guidance, consulting a sports dietitian is a valuable step for any serious athlete.

Frequently Asked Questions

Carbohydrates are crucial because they provide the most efficient and readily available source of fuel for moderate to high-intensity exercise. The body stores them as glycogen, which is used to power working muscles and maintain stable blood sugar levels during prolonged activity.

The body primarily uses fat for energy during rest and low-to-moderate intensity exercise. As the intensity of the workout increases, the reliance shifts towards carbohydrates because they can be metabolized more quickly.

Hitting the wall is the sudden and severe fatigue experienced by endurance athletes when their muscle glycogen stores become severely depleted. This leaves the body unable to sustain the current level of performance.

Carbohydrate loading is a nutritional strategy used before an endurance event to maximize the storage of muscle glycogen. It involves increasing carbohydrate intake in the days leading up to the race while decreasing training volume.

While the body uses fat for fuel, consuming fat during intense exercise is not ideal. Fat is slower to digest and can cause stomach upset. Ingesting carbohydrates is the priority for providing immediate, usable energy during sustained effort.

Protein is not a primary fuel source, but it is vital for muscle repair and recovery. If carbohydrate stores are too low, the body may break down muscle protein for energy, a process that hinders performance and leads to muscle damage.

Metabolic flexibility is the ability of the body to efficiently switch between using carbohydrates and fats for fuel. This helps trained endurance athletes spare their limited glycogen stores for high-intensity efforts by relying more on fat at lower intensities.