The Caloric Math: Burn vs. Absorption
An ultra marathon is a physical feat that pits an athlete against an inevitable caloric deficit. While the body can burn upwards of 600–1000 calories per hour during a race, the digestive system can only process a maximum of roughly 240–280 calories per hour. This metabolic reality means that ultra runners cannot replace all the energy they expend in real-time. The core of any effective fueling strategy is to limit this energy gap by consistently consuming calories and relying on pre-stored energy (glycogen and fat) to cover the rest. Race finishers have been shown to consume more calories than those who fail to finish, underscoring the vital role of disciplined nutrition. The goal is to aim for a steady intake of 150-400 calories per hour, depending on intensity, race duration, and individual tolerance. High-performance athletes have even trained their gut to handle up to 90g of carbohydrates per hour, which equates to 360 calories.
Factors That Influence Calorie Needs
Individual calorie needs are far from a one-size-fits-all equation in ultra running. A multitude of factors, both internal and external, play a role in determining how much an athlete needs to consume to stay energized and avoid gastrointestinal (GI) distress.
Individual Factors
- Body Size: Larger athletes with more body mass will generally burn more calories and, therefore, require a higher caloric intake than smaller athletes.
- Intensity and Pace: The faster the pace and higher the intensity, the more calories are burned per hour, often increasing carbohydrate needs. Slower, longer efforts may allow for greater fat oxidation, but consistent carb intake remains critical.
- Gender: On average, men may consume slightly more calories per hour than women, although these are general guidelines, and individual variation is high.
- Experience and Metabolism: A seasoned ultra runner will have a more efficient metabolic system and a better-trained gut, allowing for more precise fueling and potentially higher absorption rates.
Environmental Factors
- Temperature and Humidity: Hot and humid conditions increase sweat rates, which affects hydration and electrolyte needs, and can also suppress appetite and alter gut function, requiring a different approach to fueling.
- Terrain and Altitude: Running on challenging terrain, such as mountainous inclines and descents, increases energy expenditure. Higher altitudes can also affect digestion and caloric requirements.
The Macronutrient Breakdown for Ultras
While the caloric number is the starting point, the specific macronutrient composition is equally important.
High-Carbohydrate Intake for Endurance
Carbohydrates are the primary and most efficient fuel source for the brain and muscles during endurance efforts. For ultrarunners, a strategy of high carbohydrate intake is recommended to preserve glycogen stores and maintain blood glucose levels. Many athletes aim for 60–90 grams of carbohydrates per hour. Some athletes achieve even higher absorption rates by using a mix of carbohydrate sources, such as a 2:1 ratio of glucose to fructose, which utilizes multiple transport pathways in the gut. This strategy helps avoid saturation of a single pathway and reduces the risk of GI issues.
The Role of Protein and Fat
Though carbohydrates dominate, protein and fat play supporting roles. For events lasting longer than 4-6 hours, consuming a small amount of protein (5-10 grams/hour) can help with muscle repair and delay protein breakdown. Similarly, incorporating some fat, particularly in the later stages of a long event, can help with satiety and provide a sense of comfort food, especially when sweet flavors become unappealing. This shift to savory foods is a common observation in multi-day or very long single-stage races.
Comparison of Fueling Sources
| Fuel Type | Pros | Cons |
|---|---|---|
| Energy Gels & Chews | Quick-digesting, easy to carry and consume, convenient for high intensity | High sugar content can lead to GI distress, flavor fatigue over long periods, can be difficult to get down without water |
| Liquid Nutrition (Mixes) | Provides carbs, electrolytes, and fluid in one, easily absorbed, gentle on the stomach for many | Can feel heavy or cause sloshing, taste can become an issue, limits variety in fueling |
| Real Food (Bars, Sandwiches, etc.) | Provides variety and satiety, includes fats and protein for longer efforts | Slower digestion, requires chewing, can cause GI upset if not tested and portioned properly |
The Importance of Gut Training
One of the most critical, yet often overlooked, aspects of ultra running nutrition is training your gut. Just as you train your muscles, you must train your digestive system to handle the unique demands of consuming calories and fluids while running. This involves practicing your race-day fueling plan repeatedly during long training runs to help your gut adapt to absorbing nutrients under stress. This practice improves gastric emptying and nutrient absorption, and reduces the likelihood of GI problems, which are a leading cause of ultra runners dropping out of a race.
Best practices for gut training:
- Start Early, Stay Consistent: Begin fuelling early in your runs, even if not hungry, and maintain a consistent schedule (e.g., every 30-45 minutes).
- Experiment with Diversity: Try a range of different foods and products—gels, chews, liquid mixes, and real foods—to build tolerance and combat flavour fatigue.
- Gradual Increase: If your target is 90g of carbs per hour, don't start there. Gradually increase your hourly intake during training sessions to allow your gut to adapt.
- Simulate Race Day: Practice with the exact foods, drinks, and quantities you plan to use on race day to ensure there are no surprises.
Common Fueling Mistakes to Avoid
Many ultra runners fall into predictable fueling traps that can derail their race. Awareness of these common errors is the first step toward a more successful outing.
Ignoring Training Nutrition
Failing to practice your nutrition plan during long training runs is a recipe for disaster on race day. The gut needs conditioning, and race day is not the time to introduce a new fuel source, food, or schedule.
Waiting Until You're Hungry
The sensation of hunger often lags significantly behind the body's actual energy needs during an ultra. Waiting until you feel hungry means you are already in a performance deficit. The best strategy is to eat proactively on a schedule.
Over-reliance on One Food Type
Palate or flavour fatigue is a real phenomenon in ultra running, where after hours of consuming sweet gels or chews, the thought of another one becomes unbearable. Having a variety of fuel options, including savory ones like potatoes or broth, is key for long races.
Poor Hydration and Electrolyte Balance
Attempting to fuel properly without adequate hydration and electrolyte intake is futile. Dehydration can compromise digestion, and imbalances can lead to cramping and nausea. It's crucial to balance fluid intake with electrolytes, especially sodium, which is lost in sweat.
Conclusion: The Personal Fueling Equation
Ultimately, figuring out how many calories do ultra runners eat is a personal equation. There is no magic number that applies to every athlete in every race. The process involves research, meticulous training, and self-awareness. By understanding the metabolic limitations, practicing your fuelling strategy, and listening to your body's unique signals, you can build a nutritional plan that becomes your secret weapon on race day. The disciplined approach to fueling can make the difference between a DNF and a triumphant finish, helping you conquer the mental and physical challenge of an ultra marathon.
It is always wise to consult an accredited sports dietitian for personalized advice, as they can help tailor a nutrition plan that is right for you, taking into account individual metabolism, race conditions, and training load. Source: International Society of Sports Nutrition Position Stand
