The Science Behind Your Fuel Needs
For a long run like 20 miles, carbohydrates are the body's primary and most efficient energy source. Your body stores carbohydrates as glycogen in the liver and muscles. However, these glycogen stores can only fuel about 90 minutes of high-intensity exercise. For anything longer, like a 20-mile run, you must consume additional carbohydrates to prevent hitting 'the wall'—a sudden feeling of extreme fatigue and energy depletion. Without proper fueling, your body is forced to rely on fat for energy, which is a slower, less efficient process and will drastically decrease your pace.
Carb-Loading: 2-3 Days Before Your Run
Effective fueling for a long run doesn't start the morning of the run—it begins days before. The practice of 'carb-loading' helps top off your glycogen stores and ensures your body's energy reserves are fully maxed out for the event. During your taper period, as your mileage decreases, you should simultaneously increase your carbohydrate intake. For most runners, aiming for 8-12 grams of carbohydrates per kilogram of body weight per day is recommended. For a 150-pound (68kg) runner, this translates to 544-816 grams of carbohydrates per day. During this period, it's wise to focus on low-fiber, easily digestible carbohydrate sources to avoid gastrointestinal issues, especially closer to the run.
Carb-Loading Food Examples
- White bread, bagels, or English muffins
- White pasta with a simple sauce
- Oatmeal
- White rice
- Bananas
- Low-fat sports drinks and juices
- Pancakes or waffles
The Pre-Run Meal: 1-4 Hours Out
On the morning of your 20-mile run, your final pre-run meal serves to top off your liver glycogen stores, which are depleted overnight. The timing and size of this meal depend on what your stomach can tolerate and how early you can wake up. The recommendation is to consume 1-4 grams of carbohydrates per kilogram of body weight in the 1-4 hours before starting.
- 4 hours before: For a more substantial meal, like oatmeal with a banana and nut butter, a 4-hour window allows plenty of time for digestion.
- 1-2 hours before: A smaller, easily digestible snack is a good option. Consider a bagel with jam or a simple sports drink.
- Less than 1 hour before: Stick to small, quick-burning options like a gel, chews, or a handful of pretzels for a rapid energy boost.
Mid-Run Fueling Strategy
During a long run, fueling is not optional; it’s mandatory. As you run, your body uses its limited glycogen stores. To maintain pace and prevent fatigue, you need to replenish these carbs regularly. Experts recommend consuming 30-60 grams of carbohydrates per hour for exercise lasting longer than 90 minutes. For seasoned runners or events longer than 3 hours, some athletes train their gut to tolerate up to 90 grams per hour by using a combination of glucose and fructose sources. Consistency is key—start fueling early, ideally within the first 30-45 minutes, and then continue with regular intervals (e.g., every 20-30 minutes).
Hydration and Electrolytes
Your fueling strategy is incomplete without considering hydration and electrolytes. For a long 20-mile run, you will lose significant fluids and essential electrolytes, especially sodium, through sweat. Dehydration can severely impact performance and lead to issues like cramping and nausea. Drink to thirst and consider using a sports drink or electrolyte tabs to replace sodium lost during the run. Adequate hydration is also essential for absorbing carbohydrates from your gut into your bloodstream.
Practice Makes Perfect: Training Your Gut
The most critical rule of race-day nutrition is to never try something new. Your 20-mile training runs are the perfect opportunity to practice your fueling strategy. This practice will help your stomach adapt to consuming food and fluids while running, minimizing the risk of gastrointestinal distress during your most important runs. Experiment with different types of fuel, such as gels, chews, and sports drinks, to see what your body tolerates best. Some runners prefer a mix of liquid and solid calories during longer efforts. By training your gut, you build confidence in your nutrition plan, leaving one less variable to worry about on race day.
Pre-Run Carb Intake Comparison for a 70kg Runner
| Time Before Run | Recommended Carb Intake | Example Meal | Rationale |
|---|---|---|---|
| 1 Hour | 70g (1g/kg) | Sports drink, gel, or plain bagel with jam | Easily digestible for a quick energy boost, avoids stomach upset. |
| 2 Hours | 140g (2g/kg) | Large bowl of oatmeal with banana and honey | More substantial energy without feeling too full. |
| 3 Hours | 210g (3g/kg) | Plate of pasta with low-fat sauce and chicken | Ample time for digestion, fuels muscle glycogen stores. |
Post-Run Recovery
Your nutritional needs don't end when you stop running. Proper post-run nutrition is vital for recovery and muscle repair, especially after a taxing 20-mile run. Aim to consume a mix of carbohydrates and protein within 30-60 minutes after your run to replenish depleted glycogen stores and begin muscle repair. A 3:1 or 4:1 carb-to-protein ratio is often recommended. Good options include chocolate milk, a smoothie with protein powder and fruit, or a turkey sandwich on whole-grain bread.
Conclusion
For a 20-mile run, the amount of carbohydrates you need depends on your body weight, metabolism, and personal tolerance. A general rule is to follow a carbohydrate-rich diet in the 2-3 days leading up to the run, consume a tailored pre-run meal 1-4 hours beforehand, and aim for 30-60 grams of carbs per hour during the run. However, the most important aspect of fueling is practice. By experimenting with and fine-tuning your nutrition strategy during training, you can step into your long run or race with confidence and ensure you have the energy needed to cross the finish line feeling strong.
Mayo Clinic Health System on fueling strategies for runners
What to Eat Before, During, and After a Run
- Before the Run: 1-4g carbs/kg bodyweight 1-4 hours prior
- During the Run: 30-60g carbs/hour for efforts >90min
- After the Run: 3:1 or 4:1 carb-to-protein ratio within 30-60min
Additional Fueling Tips
- Don't try new fuel on race day
- Consider electrolytes during long runs
- Hydrate adequately before and during
- Listen to your body
- Plan your nutrition in advance
