You’re 30 miles into your first 50-miler when your legs suddenly feel like concrete. Your pace drops, your mind fogs, and you wonder if you’ve hit the dreaded bonk. The culprit? Not consuming enough carbs per hour during your ultra running event. Understanding your carbohydrate needs isn’t just about performance—it’s the difference between finishing strong and struggling to the finish line.
The Science Behind Carbohydrate Needs in Ultra Running
Ultra running places unprecedented demands on your body’s energy systems. While your body can store approximately 2,000 calories as glycogen (roughly 500g of carbohydrates), this fuels only 90-120 minutes of intense exercise. For events lasting 4-30+ hours, strategic carbohydrate intake becomes non-negotiable.
Research published in Sports Medicine demonstrates that athletes can oxidize (burn) 60-90 grams of carbohydrates per hour when consuming multiple transportable carbohydrates. This absorption ceiling exists because your intestines can only process carbohydrates so quickly, regardless of how much you consume.
How Your Body Processes Carbohydrates During Endurance Events
Your intestinal lining uses specific transport proteins—SGLT1 for glucose and GLUT5 for fructose—to absorb carbohydrates. When you consume only glucose-based carbs, you hit a 60g/hour absorption limit. However, combining glucose and fructose activates both transport systems simultaneously, potentially increasing absorption to 90g/hour.
During ultra running, your body shifts between three primary fuel sources: muscle glycogen, liver glycogen, and blood glucose from consumed carbohydrates. As exercise duration extends beyond 2-3 hours, exogenous carbohydrate intake (what you eat and drink) becomes increasingly critical to maintaining blood glucose levels and performance.
Optimal Carbohydrate Intake: The 60-90 Gram Sweet Spot
The consensus among sports nutrition experts centers on 60-90 grams of carbohydrates per hour for ultra running events. However, this range requires nuance based on individual factors and race conditions.
Beginner Ultra Runners: Starting at 30-60g Per Hour
If you’re new to ultra running, start conservatively with 30-60 grams per hour. Your gut needs training just like your legs. Consuming too many carbs too quickly can lead to gastrointestinal distress—the second most common reason for DNFs (Did Not Finish) in ultra marathons after musculoskeletal issues.
Begin each training run with a fueling plan. A single energy gel (20-25g carbs) plus 500ml of sports drink (15-20g carbs) provides approximately 40g per hour. Gradually increase your intake over 8-12 weeks of training to allow your gut to adapt.
Practical Beginner Fueling Strategy
- Hours 1-2: 30-40g per hour (testing gut tolerance)
- Hours 3-6: 40-50g per hour (building capacity)
- Hours 6+: 50-60g per hour (maintenance phase)
Experienced Ultra Runners: Maximizing the 60-90g Range
Seasoned ultra runners with trained guts can target 60-90 grams of carbohydrates per hour. This upper range requires using multiple transportable carbohydrates—specifically a 2:1 ratio of glucose (or maltodextrin) to fructose.
Elite ultra runners like Jim Walmsley and Courtney Dauwalter often consume 80-100g per hour during competitive efforts, combining gels, drink mixes, and solid foods strategically. This aggressive fueling strategy helps maintain pace and delays fatigue during the latter stages of ultra marathons.
Adjusting Carbohydrate Intake Based on Intensity and Duration
Your carbohydrate needs fluctuate based on effort level and race distance. Here’s how to adjust:
50K to 50-Mile Events (4-10 hours):
- Target 60-75g per hour
- Higher intensity demands more carbs
- Simpler fuel sources (gels, drinks) work well
100K to 100-Mile Events (10-30+ hours):
- Start at 60-70g per hour
- Decrease to 40-50g per hour during night/low-intensity periods
- Incorporate solid foods for variety and satiety
Multi-Day Stage Races:
- Average 50-60g per hour during running
- Prioritize whole foods during rest periods
- Focus on carbohydrate loading between stages
Types of Carbohydrates: Not All Carbs Are Created Equal
The composition of your carbohydrate sources dramatically impacts absorption and performance during ultra running.
Simple vs. Complex Carbohydrates for Ultra Running
Simple Carbohydrates (glucose, fructose, sucrose) digest rapidly and provide quick energy. Energy gels, sports drinks, and honey exemplify simple carbs ideal for sustained efforts.
Complex Carbohydrates (starches in potatoes, rice, bread) digest slower, providing sustained energy but requiring more digestive capacity. Save complex carbs for aid stations during longer events when you’re moving slower.
The Multiple Transportable Carbohydrates Advantage
Studies from Asker Jeukendrup’s research group at Loughborough University demonstrate that combining glucose and fructose increases carbohydrate oxidation rates by 20-50% compared to glucose alone. This translates to 75-90 grams per hour absorption versus the 60g ceiling with single-source carbs.
Look for products listing “multiple transportable carbohydrates,” “dual-source carbs,” or specific glucose:fructose ratios. Many premium sports nutrition brands like Maurten, SIS Beta Fuel, and Precision Hydration use this technology.
Best Carbohydrate Sources for Ultra Running
Liquid Carbohydrates (Fastest Absorption):
- Sports drinks with 6-8% carbohydrate concentration
- Maurten gels and drink mixes (hydrogel technology)
- Tailwind Nutrition (complete fuel source)
Gel-Based Carbohydrates (Moderate Absorption):
- Traditional energy gels (20-25g per packet)
- Honey packets (15-20g, natural option)
- Maple syrup packets (natural, easy on stomach)
Solid Food Carbohydrates (Slower Absorption):
- Boiled potatoes with salt (aid station favorite)
- Rice balls (onigiri) with pickle (electrolytes)
- Bananas (15-20g, easy to digest)
- Fig bars or date bars (whole food option)
Avoiding the Bonk: Signs You Need More Carbs Per Hour
Recognizing early warning signs of insufficient carbohydrate intake allows you to adjust before performance catastrophically declines.
Early Bonk Warning Signals
- Sudden drop in pace despite unchanged perceived effort
- Mental fog or difficulty concentrating on navigation
- Irritability or mood changes
- Loss of appetite (paradoxically indicates you need fuel)
- Craving savory or fatty foods (body seeking quick energy)
The Glycogen Depletion Cascade
When muscle glycogen stores deplete below critical levels (approximately 25% of capacity), your body initiates protective mechanisms. Pace involuntarily slows, technique deteriorates, and mental performance suffers. By this point, recovery requires 60-90 minutes of aggressive carbohydrate intake—time you can’t afford in a race.
Prevention beats treatment. Consistent carbohydrate intake from hour one prevents glycogen depletion rather than attempting to reverse it mid-race.
Training Your Gut to Handle Carbohydrates
Your gastrointestinal system adapts to regular carbohydrate intake during exercise, increasing its absorption capacity over 6-12 weeks of consistent training.
Progressive Gut Training Protocol
Weeks 1-4: Foundation Building
- Start with 30-40g per hour during long runs
- Use simple, well-tolerated products (single gel type)
- Practice on runs over 2 hours
- Note any GI discomfort and adjust accordingly
Weeks 5-8: Capacity Expansion
- Increase to 50-60g per hour
- Introduce variety (gels, drinks, solid foods)
- Test different brands and flavors
- Practice race-day fueling timing
Weeks 9-12: Race Simulation
- Target race-day carbohydrate intake (60-90g per hour)
- Use exact products planned for race
- Simulate aid station transitions
- Dial in personal preferences and timing
Common Gut Training Mistakes to Avoid
- Increasing intake too quickly (leads to GI distress and setbacks)
- Only training gut with water (doesn’t build carb tolerance)
- Skipping fueling practice in training (race day disaster)
- Using different products in training vs. racing (untested variables)
- Ignoring individual tolerance signals (forcing uncomfortable fueling)
Calculating Your Personal Carbohydrate Needs
While general guidelines provide starting points, individual variation requires personal experimentation. Factors affecting your specific needs include:
Individual Variables Affecting Carb Requirements
Body Size and Composition:
- Larger athletes typically require more absolute carbohydrates
- However, carbs per hour per kg body weight varies less than total needs
- General range: 0.8-1.2g per kg body weight per hour
Training Status:
- Highly trained athletes exhibit greater fat oxidation capacity
- May sustain performance on slightly less carbohydrate
- Still require minimum 50-60g per hour for events over 4 hours
Environmental Conditions:
- Heat increases carbohydrate oxidation rates
- Cold may decrease appetite but not requirements
- Altitude (above 2,500m) increases carbohydrate dependency
Creating Your Personalized Fueling Calculator
Use this formula to estimate your baseline carbohydrate needs:
Base Formula: Body Weight (kg) × 1.0g × Hours = Total Carbs Needed
Example: 70kg runner, 8-hour 50-miler: 70kg × 1.0g × 8 hours = 560g total carbohydrates 560g ÷ 8 hours = 70g per hour target
Adjust this baseline based on:
- Reduce by 10-20g per hour if temperatures exceed 25°C (75°F)
- Reduce by 10-15g per hour during overnight sections (lower intensity)
- Increase by 10-20g per hour for mountain courses above 10% average grade
Race Day Carbohydrate Strategy: Putting It All Together
Successful ultra running nutrition requires a detailed plan, practiced in training and executed with discipline on race day.
Hour-by-Hour Fueling Template
Hours 1-2: Front-Loading Strategy
- Consume 60-70g per hour even if not hungry
- Build carbohydrate reserves before appetite suppression
- Establish consistent fueling rhythm and habit
Hours 3-8: Maintenance Phase
- Target 60-80g per hour based on individual tolerance
- Alternate between liquid and solid sources
- Every 30 minutes: gel/chew + sip sports drink
- Every 60 minutes: brief aid station stop for variety
Hours 8-16: Mental Game
- Maintain minimum 50-60g per hour despite fatigue
- Rely on systems over motivation
- Set phone/watch alarms for fueling reminders
- Accept that eating becomes a job, not pleasure
Hours 16+: Finish Strong
- Increase to 70-80g per hour if stomach tolerates
- Simple carbs (gels, coke, cookies) often work best
- Focus on liquid calories if solid food becomes unappealing
Aid Station Navigation for Optimal Carb Intake
Efficient aid station execution preserves time while ensuring adequate nutrition:
- Approach with plan (know exactly what you need)
- Grab fluids first (hydration priority)
- Take 2-3 portable items (gels, bars for next section)
- Consume quick calories on-site (cola, broth, fruit)
- Exit within 2-3 minutes (unless crew support/longer stop needed)
Calculate carbohydrates between aid stations. If stations are 5 miles (60-90 minutes) apart and you need 70g per hour, consume approximately 100g at each station (combination of on-site eating and portable fuel for the trail).
Key Takeaways
- Target 60-90 grams of carbohydrates per hour for ultra running events, with beginners starting at 30-60g per hour and building tolerance
- Use multiple transportable carbohydrates (glucose + fructose) to maximize absorption and achieve the upper range of 80-90g per hour
- Train your gut progressively over 8-12 weeks to handle race-day carbohydrate intake without gastrointestinal distress
- Front-load carbohydrate intake during the first 2 hours of racing before appetite suppression and fatigue compromise fueling discipline
- Adjust intake based on intensity, temperature, time of day, and individual tolerance rather than following rigid guidelines
Fuel Smart, Finish Strong
Determining how many carbs per hour you need for ultra running isn’t about following a universal formula—it’s about understanding the science, respecting your individual physiology, and practicing relentlessly. The 60-90 gram range provides a scientifically-backed framework, but your personal sweet spot emerges through consistent training, careful documentation, and willingness to adjust based on feedback.
Start your next long training run with a concrete fueling plan. Set a timer for every 20-30 minutes. Track your intake, note how you feel, and adjust accordingly. Your legs will thank you at mile 40 when runners around you are walking while you’re still moving strong. Remember: in ultra running, the best fueling strategy is the one you’ve practiced so many times it becomes automatic—even when your brain is too tired to think.
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