Building Gut Tolerance for High Carb Fueling Strategies
Carbohydrate (carb) fueling is essential to the success of endurance sports. Yet many athletes fail to prioritize fueling or underfuel out of fear of gastrointestinal (GI) distress—the leading cause of underperformance in endurance races. If you’ve experienced nausea, vomiting, and diarrhea, you know firsthand how a poorly planned fuel strategy can sabotage race day. The good news is, just as you train your body, your digestive system is highly adaptable and can be trained to handle higher carb and fluid volumes during exercise.
While there are no scientifically established gut-training protocols, insights from top sports nutrition experts like Asker Jeukendrup, Trent Stellingwerff, and Aitor Viribay —who have extensive lab and field experience — provide practical guidance. Here, I’ve compiled the data into a framework to help you implement gut training leading up to your key race.
Gut training involves repeated exposure to progressive carb load during training and daily nutrition to boost gut tolerance, gastric emptying, the efficiency of intestinal carb transporters, and stomach comfort. These strategies help your digestive system absorb and metabolize carbs more efficiently, reducing GI issues and enhancing performance and recovery.
Gut training starts with your daily diet
Before diving into the specific guidelines, it’s important to highlight a broader point: anyone aiming to improve performance through better carb absorption will benefit from a diet rich in carbs. To process carbs effectively during training, you should first consume adequate amounts in your diet—especially around longer sessions and those including intensity. By doing this, you’ve begun training your gut.
Training the gut guidelines
For sessions over 2 hours, current guidelines recommend 60 to 90 grams of carbs per hour, using multiple transportable carbs—glucose and fructose. Combining both maximizes absorption and minimizes GI distress. Glucose, taken up via SGLT1 transporter, peaks at around 60g per hour, while fructose, absorbed through GLUT5, enables carb oxidation beyond that limit. To fuel effectively above 60g per hour without GI issues, you need both glucose and fructose, given the SGLT1 transporter’s limit of 1g glucose per minute.
Who benefits: Carb fueling and gut training are particularly advantageous for competitive and elite triathletes training for long-distance events like IRONMAN and Half IRONMAN, where energy demands are highest. For beginners or those focusing on shorter events, a less structured approach may suffice.
When to start: Begin gut training during the base or build phase, ideally 8-12+ weeks before a key race, so your gut has time to adapt without interfering with peak training.
Gradual progression: Start your baseline carb dose that you can tolerate without GI issues. Progressively increase by 10-15g per hour every one to two weeks until reaching your optimal dose.
Daily Nutrition: Prioritize high-carb meals and snacks around key sessions. On low-volume days, aim for 3-5g/kg body weight; on heavier days, >6g/kg. Immediately before and after training, choose low-fiber, low-fat foods like rice, pasta, sourdough bread, oatmeal, potatoes, crackers, applesauce, and bananas.
Duration and Frequency: Practice gut training in at least three sessions per week. During each gut training session, fuel early (within the first 15-20 minutes) to maximize intake. These sessions should mimic race conditions—intensity, duration, and environmental factors—and during “B” and “C” races leading up to your key event.
Carbohydrate types: Use sport products containing glucose and fructose to maximize absorption and minimize GI issues. Liquid calories, such as sports drinks, are ideal because they are rapidly absorbed. Supplement with gels and chews to reach your target dose. Establish your preferred products beforehand to avoid surprises on race day.
Hydration: Dehydration impairs gut emptying and carb absorption. Hydrate alongside fueling and ensure your products contain sufficient sodium to support hydration.
Personalization: Adjust your intake based on your individual tolerances. Adopt a conservative approach, gradually increasing carbs to ensure the gut adapts without undue stress.
Monitor progress: Track each session’s details: products used, carbs/hr, timing, GI symptoms (if applicable), performance, perceived effort, and environmental conditions.
Signs you’re overdoing it: Symptoms of excessive intake or rapid ramping include GI distress (bloating, gas, cramping, diarrhea), feeling overly full, stomach discomfort, and sluggishness. While frustrating, these signs serve two purposes: identifying your current gut capacity and applying the training overload principle to encourage gut adaptation. In training, we apply progressive overload by pushing the body beyond its comfort zone or capacity, triggering physiological improvement. And the same stimulus for adaptation applies to the gut.
Progressive carb gut training guide
| Step | Task | Carb intake | Frequency | Duration |
| Preparation | Increase carbs around training | Carb-rich foods/liquids – low fiber/fat. | 3x/week | 2+ weeks |
| Step 1 | Start at tolerable/baseline dose | +10-15g/hour | 3x/week | 7-14 days. |
| Step 2 | Progress if tolerated | +10-15g/hour | 3x/week | 7-14 days |
| Step 3 | Identify max tolerable dose | +10-15g/hour | 3x/week | 7-14 days |
| *Step 4 | Increase max dose by 10% (i.e., if 90g is your max, increase to 100g) | +10% of race day dose/hour | 2-3x/week | 7-10 days |
| *Step 5 | Increase max dose by 20% (i.e., if 90g is your max, increase to 110g) | +20% of race day dose/hour | 2-3x/week | 7-10 days |
| **Step 6, into race taper |
Reduce dose to your max tolerable dose from Step 3. | 70-90g/hour | 2-3x/week |
*Once you’ve established your max tolerable dose, you can opt to push it slightly beyond (by 10-20%), then bring it back down, applying the similar progressive overload principles used in physical training.
** 70-90g per hour is usually sufficient for most age group athletes.
Ideal carb blend when exceeding 60g/hr.
- Glucose + fructose
- Maltodextrin + fructose
- Glucose + sucrose + fructose
Carb types used in sports nutrition products
- Dextrose = glucose
- Fructose = fructose
- HFCS = 50:50 glucose/fructose
- Maltodextrin = glucose
- Sucrose = 50:50 glucose/fructose
- Maltose = 50:50 glucose/glucose
Carb ratios based on intake per hour.
- 70g per hour – 1:0.16 ratio: 60g glucose + 10g fructose = 70g/hr.
- 80g per hour – 1:0.3 ratio: 60g glucose + 20g fructose = 80g/hr.
- 90g per hour – 2:1 ratio: 60g glucose + 30g fructose = 90g/hr.
- 100g per hour – 1:0.6 ratio: 60g glucose + 40g fructose = 100g/hr.
- 110g per hour – 1:0.8 ratio: 60g glucose + 50g fructose = 110g/hr. .
Practical Tips
- Use easily digestible carb sources as outlined.
- Maintain adequate hydration based on your sweat rate, as fluids support digestion and carb absorption.
- Practice gut training during training sessions that simulate race intensity and conditions, including long rides and runs
- In hot, humid climates, fueling at or above the upper limits may often trigger GI symptoms due to dehydration—adjust accordingly.
- Individual tolerance varies, and exceeding 90g per hour generally offers no additional benefit and can increase the risk of GI distress.
Final thoughts
Structured gut training can be highly effective for endurance athletes, particularly those targeting long-distance races. The key is consistent, gradual adaptation, listening to your body, and adjusting as needed. Although carb fueling is essential for peak performance, gut tolerance determines how much you can practically utilize. The most important takeaway is to find a carb dose your body can handle without GI side effects. Remember, more is not always better; gradual, personalized adjustments are the path to success, not the pursuit of maximum intake.
For a customized sports fueling plan, check out Race Day Fueling nutrition service.
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