Carbs vs. Fats: What is the best energy source for long distance running?

October 8, 2025 •

5 min read

Did you know that for endurance athletes, carbohydrates provide most of the energy used during exercise? Understanding what is the best energy source for long distance running is key to preventing fatigue, optimizing performance, and achieving your goals.

Quick Summary

Carbohydrates are the primary fuel for long-distance running, stored as glycogen in muscles and the liver. Strategic consumption of complex carbs provides sustained energy, while simple carbs offer quick boosts during runs. Fat provides a dense energy source for lower-intensity efforts, and protein is vital for muscle repair.

Key Points

Carbohydrates are key: For long-distance and high-intensity running, carbohydrates are the body's main energy source and are stored as glycogen.
Strategic timing: Consume complex carbs daily for sustained energy, and use simple carbs (gels, chews) mid-run for quick replenishment.
Fat for endurance: Fat serves as an efficient fuel for lower-intensity, longer efforts, helping to spare limited glycogen stores.
Protein for recovery: After a run, consume protein to repair and rebuild muscle tissue damaged during exercise, ideally within 30-60 minutes.
Practice your plan: Always test your race-day nutrition strategy during long training runs to ensure your stomach tolerates the chosen fuels.
Stay hydrated: Replenish fluids and electrolytes consistently throughout training and races to prevent dehydration and muscle cramps.

Carbohydrates: The Runner's Primary Fuel

For any high-intensity activity, and especially for long-distance running, carbohydrates are the body's preferred and most efficient energy source. When you consume carbs, your body breaks them down into glucose, which is then stored in your muscles and liver as glycogen. This stored energy is the readily available fuel that your muscles rely on, and exhausting these stores leads to the dreaded feeling of "hitting the wall" or "bonking".

To power you through a race, a strategic approach to carbohydrate consumption is critical, focusing on both complex and simple carbohydrates. Complex carbohydrates, such as whole grains, potatoes, and legumes, are slow to digest and provide a steady, sustained release of energy. They should form the bulk of your daily diet during training. Simple carbohydrates, found in fruits, honey, energy gels, and sports drinks, are digested quickly, providing a rapid energy boost when you need it most, such as during a race.

Strategic Carbohydrate Intake

To maximize the benefits of carbohydrates, timing is everything. Here is how to structure your intake for a long event:

Carb-Loading (Pre-Race): In the 1-3 days leading up to a marathon or other long race, the goal is to maximize your glycogen reserves. This involves tapering your training while significantly increasing your carbohydrate intake, often to 8-12 grams per kilogram of body weight per day. Focus on easily digestible, high-carb, low-fiber foods like white rice, pasta, and potatoes to avoid gastrointestinal issues on race day.
During-Run Fueling: For any effort longer than 60-90 minutes, you need to replenish your glycogen stores mid-run. Aim for 30-90 grams of simple carbohydrates per hour, taking in small, frequent amounts every 15-30 minutes. This can be achieved through energy gels, sports drinks, chews, or simple foods like bananas or dried fruit.
Post-Run Recovery: The 30-60 minute window immediately after a long run is crucial for replenishing depleted glycogen stores. Consuming a meal or snack with a high carbohydrate-to-protein ratio (around 3:1 or 4:1) helps kickstart muscle repair and recovery.

The Role of Fat as an Energy Source

While carbohydrates are the primary fuel for higher intensity running, fat plays a vital role during lower-intensity, prolonged exercise, such as during long, slow training runs or the early stages of a marathon. The body has a much larger reserve of fat stores compared to glycogen, making it a valuable, long-lasting energy source that spares your limited carbohydrate reserves.

Endurance training improves the body's ability to burn fat for fuel, a process known as fat oxidation. Including a variety of healthy fats—such as avocados, nuts, seeds, and fatty fish—in your daily diet is essential for overall health, hormone production, and the absorption of fat-soluble vitamins. However, avoid high-fat meals right before a run, as fat takes longer to digest and can cause stomach discomfort.

The "Train Low" Concept

Some endurance athletes use a strategy called "train low, compete high," where they intentionally perform some training sessions with low glycogen stores. The goal is to enhance the body's fat-burning efficiency, potentially sparing glycogen for high-intensity efforts. For example, an athlete might do a low-intensity run first thing in the morning without eating. This is an advanced strategy with potential risks, as it can be taxing on the body and potentially impair performance if not managed correctly. It is crucial to practice this approach sparingly during training and never before a major race.

Protein's Role in Muscle Repair

Protein is not a significant energy source during a long run, typically contributing less than 10% of total energy expenditure. Its importance lies in muscle repair and recovery post-run. Long-distance running causes microscopic damage to muscle fibers, and consuming adequate protein is essential for rebuilding and strengthening this tissue.

Endurance athletes are advised to consume 1.2-1.4 grams of protein per kilogram of body weight per day. A recovery snack combining both carbohydrates and protein, like chocolate milk or Greek yogurt with fruit, is highly effective for jumpstarting the recovery process. Including lean protein sources like fish, poultry, eggs, and legumes throughout your daily meals also ensures steady muscle support.

Fueling Strategy Comparison Table

Here is a comparison of common fuel sources used during a long run or race.


Fuel Type	Main Benefit	Timing	Considerations
Energy Gels/Chews	Quick, concentrated source of simple carbs; easy to carry.	Every 30-45 minutes, with water.	Highly effective, but flavors should be tested in training to avoid GI distress.
Sports Drinks	Replaces carbohydrates and electrolytes simultaneously; hydrates effectively.	Sip every 15-20 minutes, alternating with water, especially in the heat.	Provides both fuel and hydration, but can feel heavy or cause stomach issues for some.
Simple Foods (e.g., Bananas, Dried Fruit, Pretzels)	Whole food source; some fiber and potassium; often gentler on the stomach.	Every 45-60 minutes, with water.	Can be harder to digest for some runners; bulkier to carry than gels.

Conclusion: Personalize Your Fueling Strategy

There is no single best energy source for long-distance running, as both carbohydrates and fats are essential components of an endurance athlete's diet. Carbohydrates are the primary, most readily available fuel for performance, especially during high-intensity efforts, while fats provide sustained energy during lower-intensity exercise. Protein, though not a main fuel, is critical for muscle repair and recovery. The key is to strategically incorporate all macronutrients and practice your fueling plan during training to understand what works best for your body. By doing so, you can prevent fatigue, maximize your energy stores, and optimize your performance on race day.

Pre-run meal ideas

Oatmeal with fruit and nuts: A mix of complex carbs, simple sugars, and a little protein/fat.
Bagel with peanut butter and banana: Excellent blend of carbs, protein, and potassium.
Toast with nut butter and honey: Provides a mix of quick-digesting carbs and slower-releasing energy.
Smoothie with yogurt, berries, and a little protein powder: Hydrating and easy to digest.
White rice with eggs and vegetables: For more time to digest, this offers a balanced meal.

Post-run recovery options

Chocolate milk: The classic post-run recovery drink with an optimal carb-to-protein ratio.
Greek yogurt with berries and granola: Combines quick carbs, protein, and antioxidants.
Turkey sandwich on whole grain bread with avocado: A balanced meal with complex carbs, lean protein, and healthy fats.
Pasta with lean meat or plant-based protein: Replenishes glycogen and provides protein for muscle repair.
Smoothie with protein powder and fruit: Convenient and customizable for rapid recovery.

Gainful offers customizable protein supplements and nutrition advice to help runners meet their specific needs, supporting recovery and performance during marathon training.

Frequently Asked Questions

For most runners, especially before an important long run or race, carbohydrates are the most important energy source. Complex carbs like oatmeal 2-3 hours before provide sustained energy, while a small, simple carb snack like a banana closer to the start provides a quick boost.

'Hitting the wall' refers to the sudden and overwhelming fatigue that occurs when your muscle and liver glycogen stores are depleted during prolonged exercise. To avoid it, ensure adequate carbohydrate intake before and throughout your long run, typically 30-90 grams of simple carbs per hour for runs over 90 minutes.

Energy gels and chews are concentrated sources of simple carbohydrates designed for easy, rapid digestion during exercise, which can be very effective. Whole foods like bananas or dried fruit also work but may be harder to digest for some. It is best to experiment in training to find what your stomach tolerates best.

Within 30-60 minutes of finishing, consume a snack or meal containing both carbohydrates and protein, such as chocolate milk, a smoothie, or Greek yogurt with fruit. This helps to rapidly replenish glycogen stores and kickstart muscle repair.

Yes, especially during runs over an hour or in hot weather. Your body loses electrolytes like sodium through sweat, and replenishing them helps with hydration, fluid balance, and muscle function. Sports drinks or electrolyte tablets are effective options.

While such diets can increase fat oxidation at lower intensities, they can impair your body's ability to use carbohydrates for high-intensity efforts, which is crucial for competitive running. Most competitive endurance athletes perform best on a diet with sufficient carbohydrates.

To minimize gastrointestinal issues, avoid high-fat, high-fiber, and overly spicy foods in the day or two leading up to a race. Stick to foods you have practiced with during training and know your body tolerates well.