The Calorie Conundrum: Burn vs. Absorption
Unlike shorter endurance events, ultramarathons present a unique nutritional challenge. While the average runner can burn up to 1,000 calories every hour, the body is physically incapable of absorbing and processing calories at that same rate while under duress. The digestive system receives less blood flow during exercise, limiting its efficiency. The typical maximum absorption rate for carbohydrates—the body's preferred fuel—is around 240-360 calories per hour. This means that every ultramarathon involves a substantial caloric deficit, making proper fueling about minimizing the damage, not preventing it entirely. Your body will rely on stored glycogen and fat reserves, but consistent, strategic feeding is necessary to keep those systems topped off and prevent a complete energy collapse, or "bonking".
Calculating Your Hourly Intake: A Personalized Approach
There is no one-size-fits-all number for fueling an ultra. Your ideal hourly caloric intake depends on several factors, including body mass, pace, intensity, and environmental conditions. However, a recommended starting point for many ultrarunners is between 200 and 300 calories per hour. More experienced runners may train their gut to handle slightly more, sometimes reaching 400 calories per hour.
To begin determining your personalized plan, consider these steps:
- Start with a baseline: Aim for 200-250 calories per hour in your early long runs and monitor your energy levels.
- Evaluate your intake: Track the specific types and amounts of food you consume and how you feel. Use a training log to note what works and what causes gastrointestinal (GI) distress.
- Consider race distance and pace: A faster-paced 50k might tolerate different fuel sources than a slower-paced 100-miler where you can consume more solid food.
- Adjust for conditions: Hotter weather increases fluid and electrolyte needs, while colder conditions may require more calories for warmth.
- Listen to your body: Your appetite and cravings will shift over many hours on the trail. Be prepared to adapt and switch between sweet and savory options as needed.
Macronutrient Breakdown for the Long Haul
Carbohydrates: The Primary Fuel
Carbohydrates are your body's rocket fuel, quickly converted into energy for your muscles. Recommendations range from 30 to 90 grams of carbohydrates per hour, with more advanced athletes pushing the higher end. For higher intake, many athletes utilize a combination of glucose and fructose, often in a 2:1 ratio, as this uses different absorption pathways and can reduce GI issues.
Key carbohydrate sources include:
- Energy Gels and Chews: Compact, portable, and easily digestible. A staple for many runners.
- Sports Drinks: Replenish fluids, carbs, and electrolytes simultaneously. Convenient for sustained, consistent fueling.
- Refined Grains: Easily digestible, low-fiber options like white bread, rice cakes, and pretzels can provide a quick energy boost.
- Fruit Purees: Offer easily absorbed carbs and natural sugars.
Protein and Fats: The Supporting Cast
Protein and fats are less important for immediate energy but become valuable over longer distances (typically >6 hours) for satiety and muscle maintenance. Aim for a small amount of protein (5-10 grams per hour) and minimal fat during the race. Examples include nut butters, protein bars, or items from aid stations like soup.
Fueling Strategy Comparison
| Strategy | Target Calories/Hour | Primary Fuel Type | Best For | Considerations |
|---|---|---|---|---|
| Beginner | 150-250 kcal | Simple Carbs (Gels, Drinks) | 50K Race, First Ultra | Focus on easy digestion and hydration; practice variety. |
| Intermediate | 250-350 kcal | Gels + Real Food (Potatoes, Pretzels) | 50-100 Mile Race | Combats palate fatigue with solid food; requires gut training. |
| Advanced/Elite | 350-400+ kcal | High-Concentration Carb Drinks + Gels | 100 Mile Race, Fast Ultra | Requires significant gut training; relies heavily on specialized sports drinks. |
| Multi-Day | 2-3x BMR Daily Intake | Mixed Macronutrients (Carbs, Protein, Fat) | 24+ Hour, Stage Race | Focuses on minimizing the daily energy deficit over several days. |
The Art of Gut Training
Training your gut is just as vital as training your legs. Your digestive system is not accustomed to processing food and fluid while your body is under physical stress. You must teach it to do so effectively during your long training runs. This involves consistently practicing your fueling strategy in conditions that mimic race day, using the exact products you plan to consume.
Common gut training strategies:
- Replicate Race Conditions: If you plan to use aid station food, test it during a training run. Run at race pace for segments while fueling.
- Overload Strategically: Occasionally practice pushing your caloric intake slightly higher during a training run to increase your gut's tolerance.
- Consume Early and Often: Begin fueling within the first 45-60 minutes of your run and continue on a timer (e.g., every 15-30 minutes), even if you don't feel hungry.
Hydration and Electrolytes
Beyond calories, proper hydration is non-negotiable. Dehydration severely impacts digestion, so managing fluids is directly tied to your ability to absorb calories.
- Fluid Intake: A general guideline is to consume 400-800 ml (16-32 oz) of fluid per hour, depending on your individual sweat rate and environmental factors like temperature and humidity.
- Electrolyte Replacement: You lose electrolytes, particularly sodium, in your sweat. Replenishing these is critical to prevent cramping and a dangerous condition called hyponatremia (low blood sodium levels). Aim for 500-1000 mg of sodium per liter of fluid, adjusting for heat and sweat rate.
- Sweat Rate Test: Weigh yourself before and after a one-hour run without drinking. A pound of weight loss equals roughly one liter of fluid lost. Use this to personalize your hydration plan.
Conclusion
There is no perfect number for how many calories do I need for an ultra marathon, but successful fueling is a cornerstone of completing the distance. By understanding the physiological limitations of your body, consistently practicing your nutrition plan in training, and managing your intake of carbohydrates, protein, fats, and electrolytes, you can minimize the inevitable energy deficit and maximize your performance. The process requires a methodical approach of trial and error, but the result is a more resilient mind and body on race day. For further reading, consult the comprehensive Food for Your Sport – Ultra-Distance Running fact sheet from Sports Dietitians Australia.
