Where do marathon runners get their energy from? A nutrition deep dive

October 8, 2025 •

3 min read

For most runners, stored glycogen can only fuel about 90 minutes of exercise, which is why understanding exactly where do marathon runners get their energy from is critical for finishing a race strong. It's a precise metabolic dance between carbohydrates and fat.

Quick Summary

Marathon runners rely on carbohydrates (glycogen) and fat for fuel. Strategic carb-loading, consistent mid-race fueling, and training to improve fat oxidation are essential for sustaining energy over 26.2 miles.

Key Points

Carbohydrate-Loading: Maximise your glycogen stores in the days before a marathon by increasing carb intake and tapering your training.
Mid-Race Fueling: Consume 30-90g of carbohydrates per hour during the race to prevent glycogen depletion and sustain energy.
Fat Adaptation: Incorporate long, slower runs into your training to teach your body to burn fat more efficiently, sparing glycogen.
Hydration with Electrolytes: Sip water and electrolyte drinks regularly throughout the race, especially when consuming gels, to aid digestion and prevent cramping.
Train Your Gut: Practice your race-day fueling strategy during long training runs to help your digestive system adapt and avoid stomach issues on race day.
Macronutrient Balance: Focus on a diet rich in complex carbs, lean protein, and healthy fats throughout your training cycle, not just on race day.

The Primary Fuel Source: Carbohydrates

Carbohydrates are the most critical energy substrate for marathon running. Once consumed, they are broken down into glucose and stored as glycogen in the liver and muscles. This stored glycogen is the body's fastest and most accessible source of fuel for high-intensity exercise. However, these glycogen stores are limited, typically holding enough energy to last for about 90 to 120 minutes of moderate to high-intensity running.

To ensure maximum energy availability on race day, runners use a strategy called "carb-loading" in the days leading up to the marathon. This involves increasing carbohydrate intake while simultaneously reducing training volume, or tapering. This process helps maximize glycogen storage, allowing the runner to start the race with a full "fuel tank" and delaying the onset of fatigue. A typical carb-loading diet involves increasing carbohydrates to about 10-12 grams per kilogram of body weight for 36-48 hours before the event. Examples of good carb-loading foods include pasta, rice, potatoes, and other low-fiber, easily digestible options.

The Backup Fuel: Fats

While carbohydrates provide rapid, high-intensity fuel, fat serves as the body's vast, long-term energy reserve. Even the leanest athletes have enough stored fat to fuel multiple marathons. The energy derived from fat is not released as quickly as that from carbohydrates, making it a less efficient fuel for high-intensity efforts. However, during lower-intensity, longer-duration training runs, the body learns to utilize fat more efficiently as a fuel source. This metabolic adaptation helps spare the more limited glycogen stores for when they are most needed—for surges in pace or the final miles of the race.

Fat adaptation is achieved through consistent aerobic training, often involving long, slow runs in the optimal fat-burning heart rate zone (around 60-70% of maximum heart rate). Healthy fat intake, from sources like nuts, seeds, avocados, and fatty fish, is also a key component of a marathoner's diet.

The Race Day Strategy: Timing is Everything

During the race itself, runners must adopt a strategic fueling plan to supplement their stored glycogen and prevent an energy crash, known as "hitting the wall". Nutrition guidelines recommend consuming 30-90 grams of carbohydrates per hour, starting within the first 30-45 minutes of the race. The exact amount depends on the runner's speed and tolerance, with faster runners often benefiting from higher carbohydrate intake rates.

Common mid-race fuel sources include:

Energy gels
Chews and gummies
Sports drinks
Easily digestible foods like bananas or dates

These products are formulated for quick absorption and minimal digestive distress. In addition to carbohydrates, adequate hydration is crucial. Marathon runners lose significant amounts of fluids and electrolytes through sweat. Therefore, supplementing with electrolytes, typically found in sports drinks or salt capsules, is necessary to maintain fluid balance and prevent muscle cramps.

Training Your Gut to Tolerate Fuel

Just as athletes train their muscles and cardiovascular systems, they must also train their gastrointestinal (GI) tract to handle fuel intake during high-stress situations. Practicing your exact race-day nutrition strategy during long training runs helps your body adapt and learn to absorb carbohydrates and fluids efficiently while running. This minimizes the risk of GI issues, such as bloating, nausea, or cramping, on race day.

Comparison of Energy Sources


Feature	Carbohydrates (Glycogen)	Fats (Fatty Acids)
Energy Storage	Limited (approx. 90-120 mins of fuel)	Vast, virtually unlimited
Energy Release Rate	Fast, preferred for high-intensity exercise	Slow, preferred for low to moderate-intensity exercise
Primary Role	Main fuel for muscles, especially during high effort	Backup fuel for longer, slower efforts, helps spare glycogen
Enhancement Strategy	Increased through carb-loading pre-race	Increased through consistent aerobic training (fat adaptation)

Conclusion: A Balanced Approach to Fueling

Ultimately, marathon runners get their energy from a combination of carbohydrates and fats, with the specific ratio changing based on exercise intensity and duration. Success lies not in relying on a single source but in creating a robust and flexible nutritional strategy. By maximizing carbohydrate stores through proper carb-loading, training the body to utilize fat efficiently, and consistently refueling with fast-acting carbohydrates and electrolytes during the race, a runner can sustain their energy and performance throughout the demanding 26.2 miles. This approach, fine-tuned through practice and attention to the body's signals, is the key to mastering marathon fueling and avoiding the dreaded wall. TrainingPeaks offers extensive guidance on marathon fueling strategies to help runners develop their personalized plan.

Frequently Asked Questions

Hitting the wall is the phenomenon where a runner's muscle glycogen stores become severely depleted, leading to intense fatigue, a dramatic drop in pace, and sometimes mental fog. It is often avoidable with proper fueling and training.

The body naturally uses a mix of both. Carbs are the primary fuel for high-intensity efforts, but training your body to burn fat more efficiently helps spare limited glycogen stores for later in the race. Optimal performance comes from balancing both fuel sources.

Both are viable, but gels, chews, and sports drinks are formulated for quick digestion and absorption during intense exercise. Real foods like bananas can work but should be practiced during training to ensure they don't cause gastrointestinal distress on race day.

Hydration needs are highly individual and depend on your sweat rate, pace, and race conditions. A common recommendation is to aim for 400-800 mL of fluid per hour, ideally containing electrolytes. It is crucial to practice your hydration plan during training.

A modern carb-loading protocol typically takes place over 1 to 3 days leading up to the race. This involves increasing your carbohydrate intake significantly while tapering your training volume to top off your body's glycogen stores.

Your pre-race breakfast should be a high-carb, low-fiber, and low-fat meal eaten 2-4 hours before the race. Familiar foods you have tested in training, such as oatmeal, toast with jam, or a bagel, are excellent choices.

Electrolytes, particularly sodium, are lost in sweat. They are crucial for maintaining proper nerve and muscle function, fluid balance, and aiding the absorption of carbohydrates. Replenishing them is essential for preventing muscle cramps and performance decline.