The Science Behind Marathon Fueling
During endurance exercise like a marathon, your body relies on two primary fuel sources: stored glycogen and fat. While fat stores are nearly inexhaustible, they can only be broken down for energy at a slower rate, insufficient for high-intensity running. Glycogen, the stored form of carbohydrate in your muscles and liver, provides quick, readily available energy. However, these glycogen stores are limited and can be depleted in just 60 to 90 minutes of high-intensity running, a phenomenon often called "hitting the wall". To prevent this, you must consume exogenous (external) carbohydrates during the race to spare your glycogen and maintain performance. By continuously replenishing your fuel, you can sustain your pace and delay fatigue throughout the entire race.
Establishing Your Hourly Carbohydrate Goal
Your hourly carbohydrate intake depends on several factors, including your pace, body weight, race duration, and digestive tolerance. Most sports nutrition guidelines provide a tiered approach to fueling based on race intensity and length.
For most recreational and moderate-pace marathoners, a target of 30-60 grams of carbohydrate per hour is the standard recommendation. This can be a great starting point for those running at moderate intensity or for a longer duration. For experienced or faster runners, especially those aiming for a sub-3-hour finish, a higher intake of 60-90 grams per hour is often beneficial. This increased intake is possible due to the use of dual-source carbohydrates, which include both glucose and fructose, allowing the body to absorb more energy per hour. For very high-performing or "ultra" endurance athletes, intakes exceeding 90 grams per hour (even up to 120g) may be possible, but this requires significant gut training.
Practical Fueling Strategies
- Start Early: Begin your fueling strategy within the first 30 minutes of the race. Don't wait until you feel tired or hungry, as this means your energy stores are already dipping. Consistent, small intakes are better than large, infrequent ones.
- Practice in Training: The golden rule of race-day nutrition is "nothing new on race day". Practice your exact fueling plan during your long training runs to test products, determine timing, and train your gut to absorb carbohydrates efficiently.
- Mix and Match: Use a variety of fuel sources like gels, chews, and sports drinks to meet your target. Combining sources can also help with flavor fatigue over a long race.
- Combine with Hydration: Always take fluids when you consume a carbohydrate source, as it aids in digestion and absorption. Many sports drinks offer both carbs and electrolytes.
Comparing Marathon Fueling Strategies
| Strategy | Target Carb Intake per Hour | Ideal For... | Common Fuel Sources | Potential Considerations |
|---|---|---|---|---|
| Standard | 30-60 grams | Most recreational runners, moderate intensity, events up to 3 hours. | Standard energy gels, sports drinks, chews, bananas. | Easy to tolerate; provides consistent energy without overwhelming the gut. |
| High-Performance | 60-90+ grams | Faster runners, elite athletes, longer events (>2.5 hours). | Dual-source gels (glucose + fructose), high-carb sports drinks. | Requires significant gut training; not suitable for all runners due to potential GI distress. |
| Real Food | Variable | Athletes who prefer whole foods, longer/slower efforts. | Bananas (~25g), rice cakes, salted potatoes, sports bars. | Can be harder to digest; requires more practice to prevent stomach issues. |
Preventing Gastrointestinal Distress
Digestive issues are a common and frustrating problem for marathoners. During intense exercise, blood is shunted away from the digestive tract to working muscles, slowing digestion and nutrient absorption. Several strategies can help minimize this risk:
- Gradual Gut Training: Over several long training runs, gradually increase the carbohydrate intake you practice with. This helps your digestive system adapt to processing more fuel under stress.
- Choose the Right Carbs: Simple, high-glycemic carbs like those found in gels and sports drinks are often easier to digest during a run than high-fiber or fatty foods. Dual-source carbs can also improve absorption.
- Don't Overdo It: Consuming too much carbohydrate at once can cause stomach upset. Small, frequent doses are generally more effective than large quantities all at once.
- Stay Hydrated: Dehydration can worsen GI symptoms by thickening the contents of the gut. Ensure you are consistently drinking fluids, including electrolytes, throughout the race.
Conclusion
Determining how many carbs to eat per hour during a marathon is a personalized process. While general recommendations range from 30-60g for most runners to 60-90g for more experienced athletes, the ideal amount depends on individual pace, body composition, and digestive tolerance. Consistency, early fueling, and practicing your specific strategy during long training runs are more critical than any single magic number. By understanding the science and implementing a well-tested plan, you can effectively fuel your body to cross the finish line feeling strong.
For more in-depth information on marathon fueling, consider consulting a sports dietitian or exploring resources like the Sports Dietitians Australia factsheet.
