Understanding Energy: Where Do Runners Get Their Energy From?

October 8, 2025 •

4 min read

The human body has three energy systems that create energy for running, with their contribution depending on the intensity and duration of the exercise. To understand where do runners get their energy from, it is essential to explore how macronutrients like carbohydrates, fats, and proteins are utilized by these different energy systems to power each step.

Quick Summary

Runners primarily derive energy from carbohydrates and fats, converted into ATP via complex metabolic pathways. The body's energy system adapts its fuel usage based on running intensity and duration, relying heavily on carbohydrates for high-intensity efforts and increasingly on fats for prolonged endurance activities.

Key Points

Carbohydrates are the primary fuel source: The body preferentially uses carbohydrates, stored as glycogen, for moderate-to-high intensity running to provide quick and readily available energy.
Fat fuels endurance: During lower-intensity, longer runs, fat becomes the main energy source, as the body has a large reserve of fat stores.
Three energy systems are at play: The body uses the phosphagen system for sprints, the glycolytic system for intense bursts, and the aerobic system for sustained endurance, using different fuel sources for each.
Timing your fuel intake is critical: Consuming carbohydrates before and during long runs, and a carb-protein mix after, is essential for maintaining energy and optimizing recovery.
Protein is for recovery, not primary fuel: While vital for muscle repair, protein is not an efficient primary fuel source during a run, especially if carbohydrate intake is sufficient.
Hydration is integral to energy: Maintaining fluid and electrolyte balance is crucial for transporting energy and preventing performance-zapping fatigue and cramping.

The Body's Primary Fuel Sources

At its core, a runner’s body is a highly efficient engine, and the fuel it uses comes from the macronutrients consumed through diet. The three main energy sources are carbohydrates, fats, and protein, with their usage changing based on the demands of the run.

The Carbohydrate Advantage

Carbohydrates are the body's preferred and most readily available source of energy, especially during moderate-to-high intensity exercise. When consumed, carbohydrates are broken down into glucose, which is then either used for immediate energy or stored in the muscles and liver as glycogen.

For runners, maintaining adequate glycogen stores is critical. When these stores are depleted—a point often referred to as "hitting the wall"—the body must rely more heavily on less-efficient fat stores, resulting in a significant decrease in pace and performance. This is why strategic carbohydrate intake is a cornerstone of a runner's diet.

Excellent sources of carbohydrates for runners include:

Complex Carbohydrates: Whole grains like oats, brown rice, and whole-wheat pasta provide sustained energy release.
Simple Carbohydrates: Foods like bananas, energy gels, and sports drinks offer a quick, easily digestible energy boost during longer runs.
Starchy Vegetables: Sweet potatoes, potatoes, and corn are excellent for building glycogen stores.

The Fat Reserve

While carbohydrates are crucial for high-intensity efforts, fat is the primary fuel for low-to-moderate intensity, long-duration exercise. The body has a nearly unlimited supply of fat stores, even in the leanest athletes, making it an essential fuel source for endurance. The fitter a runner is, the more efficiently their body can use fat for fuel, sparing limited glycogen reserves.

However, fat takes longer to convert into energy compared to carbohydrates, which is why a high-fat diet is not optimal for high-intensity or fast running. Healthy fats, rich in monounsaturated and polyunsaturated fats, are an important part of a runner's daily nutrition for overall health, reducing inflammation, and hormone regulation.

Healthy fat sources for runners include:

Avocado
Nuts and seeds
Fatty fish like salmon
Olive oil

The Role of Protein

Protein is primarily for repairing and rebuilding muscle tissue, not for fueling the run itself. While protein can be used as an energy source through gluconeogenesis, this typically only occurs when the body's carbohydrate intake is insufficient, a situation that is detrimental to muscle health. Regular protein intake, particularly within 30-60 minutes after a run, is crucial for recovery and adaptation.

The Energy Systems at Work

To understand how fuel is converted into energy, it's helpful to look at the body's three distinct energy systems:

The Phosphagen System (ATP-CP): This is the immediate, "power-on-demand" system used for explosive, short bursts of effort, such as a 5-6 second sprint. It relies on stored ATP and creatine phosphate in the muscles and does not require oxygen.
The Glycolytic System: This anaerobic system uses stored glucose (glycogen) to provide energy for more intense bursts lasting up to 90 seconds. It's less efficient than the aerobic system and produces lactic acid, which contributes to muscle fatigue.
The Aerobic System: For any running longer than a couple of minutes, the aerobic system is the primary driver. It requires oxygen and can use both carbohydrates and fats to produce a large, sustained amount of energy (ATP). This is the energy system that powers all endurance running.

Comparison of Fuel Sources for Runners


Feature	Carbohydrates	Fats	Protein
Primary Function	Quick/Sustained Energy	Endurance Fuel	Muscle Repair/Recovery
Primary Use during Run	High-Intensity Exercise	Low-Intensity Exercise	Not a primary fuel source
Energy Efficiency	High	Lower (slower conversion)	Not an efficient fuel source
Storage Capacity	Limited (Glycogen)	Very Large	Limited (Muscle Tissue)
Key Food Examples	Pasta, Oats, Bananas, Gels	Avocados, Nuts, Olive Oil	Lean Meats, Eggs, Beans
Meal Timing	Before, During, and After	Daily Intake	Primarily Post-Run

Optimizing Your Fueling Strategy

A runner's performance is not only dependent on training but on a well-timed and balanced nutrition plan. By strategically adjusting your intake of macronutrients, you can optimize your energy levels for any distance or intensity.

Fueling Before, During, and After a Run

Before Your Run: A high-carbohydrate, low-fat, and low-fiber meal or snack 1-3 hours before a run ensures optimal glycogen stores. Examples include a banana, oatmeal, or toast with a small amount of peanut butter.
During Your Run: For runs over 60-90 minutes, consuming 30-60 grams of carbohydrates per hour is recommended. Energy gels, chews, or sports drinks are excellent options for easily digestible fuel during exercise. Be sure to practice your fueling strategy during training to avoid any GI distress on race day.
After Your Run: A combination of carbohydrates and protein is key for recovery. Aim for a 3:1 or 4:1 carb-to-protein ratio within 30-60 minutes post-run to replenish glycogen stores and repair muscle tissue. Chocolate milk, a turkey sandwich, or Greek yogurt with fruit are great choices.

Hydration's Crucial Role

Hydration is integral to energy delivery and regulation. Water helps transport nutrients and regulates body temperature, while electrolytes (like sodium and potassium) are lost through sweat and are essential for proper muscle function and nerve signaling. Dehydration can lead to fatigue and cramping, severely impairing performance.

Conclusion

In conclusion, a runner’s energy is a complex interplay between the body's three energy systems and the fuel sources provided by nutrition. While carbohydrates serve as the primary fuel for higher-intensity efforts, fats provide the steady energy needed for long-distance endurance. Protein is not a key fuel but is indispensable for muscle repair and recovery, enabling the body to adapt and grow stronger. By understanding these mechanisms and strategically managing the intake of carbohydrates, fats, and proteins—along with maintaining proper hydration—runners can significantly enhance their performance, overcome the dreaded "wall," and achieve their running goals.

For more in-depth nutritional guidance, consider consulting with a qualified sports dietitian, who can help you tailor a plan to your specific training needs.

Frequently Asked Questions

Hitting the wall, or 'bonking', is when a runner's muscle glycogen stores become depleted, forcing the body to switch to less-efficient fat burning. To avoid it, runners should practice carbohydrate loading before a long race and consume 30-60 grams of easily digestible carbohydrates per hour during the event.

Energy gels offer a convenient, concentrated, and quickly absorbed source of carbohydrates, making them a popular choice for long runs and races. While whole foods like bananas can work, gels are specifically formulated for easy digestion during exercise, though runners should experiment during training to avoid stomach issues.

Hydration is extremely important, as it helps regulate body temperature, transport nutrients, and maintain electrolyte balance. Dehydration can lead to fatigue, cramping, and decreased performance. For runs over an hour, consuming water and electrolytes is crucial.

For most distance runners, a very low-carbohydrate diet like keto is not recommended, as carbohydrates are the body's preferred fuel for endurance and high-intensity running. A balanced diet with adequate carbs is essential for optimal performance and recovery.

Protein should be a consistent part of your daily diet, but a particularly important time to consume it is within 30-60 minutes after a run. A carb-protein snack post-run helps replenish glycogen and supports muscle repair.

Runners should focus on complex carbohydrates like whole grains, oats, and vegetables for sustained energy as part of their regular diet. Before and during runs, quick-digesting simple carbohydrates from fruits, energy gels, or sports drinks are more effective for immediate fuel.

For runs shorter than 60 minutes, pre-run hydration and adequate daily fueling are typically sufficient. Your body’s stored glycogen will likely provide enough energy, and you can focus on refueling after your run.