Gut Training: Optimize Athlete Fueling for Endurance Performance

A 2025 joint position statement by Sports Dietitians Australia (SDA) and the Ultra Sports Science Foundation (USSF) highlights "gut training" as a fundamental, evidence-backed strategy for endurance athletes. This deliberate practice involves progressively consuming carbohydrates at race-target rates during training. It's now considered a Grade I intervention, meaning it has the strongest available evidence to support its efficacy Source.

This isn't a quick fix; measurable adaptations appear after a minimum of two weeks, but eight to twelve weeks are often needed for optimal race preparation. The expert consensus debunks popular but ineffective interventions such as probiotics, glutamine, and chronic low-carbohydrate high-fat (LCHF) diets for improving gut health and performance in this context.

What is Gut Training?

Gut training is a structured, multi-week protocol focused on increasing the gastrointestinal tract's ability to absorb carbohydrates during exercise. It's a progressive overload approach, similar to strength training, integrated into an athlete's overall preparation schedule. The goal is to adapt the gut to efficiently process fuel at rates required for sustained high-intensity efforts, particularly in races over 90 minutes. This practice is crucial and should not be left until the final taper phase leading up to a competition.

Why the Gut Adapts

The gut's ability to process carbohydrates relies on specific transporters: SGLT1 for glucose and maltodextrin, and GLUT5 for fructose. SGLT1 can become saturated at approximately 60g of glucose per hour, leading to digestive issues if intake exceeds this. By combining glucose and fructose, which use different pathways, athletes can increase total absorption to 90g/hr and even up to 120g/hr.

Repeated exposure to high carbohydrate intake during training, particularly with mixed glucose-fructose sources, upregulates the density and activity of these intestinal transporters. This means the gut literally becomes more efficient at absorbing fuel, directly reducing the risk of gastrointestinal distress and improving fuel delivery to working muscles.

Evidence-Based Interventions

The Costa et al. 2025 position paper graded various interventions based on evidence. Three emerged with Grade I status for both performance enhancement and gut protection:

• Carbohydrate during exercise (60–90 g/hr): This practice increases blood flow to the gut, mitigating exercise-induced gut injury.
• Two-week gut training: Structured gut training significantly reduces carbohydrate malabsorption and discomfort. A 2023 review, heavily cited in the 2025 statement, found a 47% reduction in gut discomfort and 54% less malabsorption after just two weeks of 30–90g/hr fueling during training.
• Low-FODMAP eating 24–48 hours pre-race: This has been shown to reduce GI symptom severity by about 50%.

Conversely, several popular approaches were graded as ineffective and not recommended:

• Probiotics: Showed no measurable effect on gut barrier function, inflammation, or symptoms.
• Glutamine: Findings were inconsistent and not clinically relevant for trained athletes.
• Chronic LCHF / ketogenic diets: These diets were linked to increased gut barrier injury markers and no symptom benefit in elite athletes.

A Sample 10-Week Gut Training Build

For endurance events, a phased gut training approach is recommended:

• Weeks 1–2 (Foundation): 30–45 g/hr. Establish consistent fueling, measure baseline tolerance.
• Weeks 3–4 (Build 1): 45–60 g/hr. Gradually increase, introduce dual-carb sources.
• Weeks 5–6 (Build 2): 60–75 g/hr. Target rates beyond SGLT1 saturation, using glucose + fructose.
• Weeks 7–8 (Race Simulation): 75–90 g/hr. Practice with race-day products, timing, and intensity.
• Weeks 9–10 (Refinement): 75–120 g/hr. Consolidate tolerance for higher rates with familiar products.

This progressive overload helps athletes adapt their digestive systems to handle the significant caloric demands of long-distance events without experiencing debilitating gastrointestinal issues.

Common Mistakes to Avoid

Athletes often make critical errors that undermine gut training efforts:

1. Late Start: Beginning too close to race day. Two weeks is a minimum; 8-12 weeks is ideal.
2. Glucose-Only Products: Relying solely on glucose-based fuels limits absorption to ~60 g/hr due to transporter saturation.
3. Lack of Race-Condition Training: Failing to train fueling in race-like heat, intensity, or stress conditions, which can exacerbate GI issues.
4. New Products on Race Day: Introducing unfamiliar products on race day is a recipe for disaster; stick to what has been practiced.
5. Confusing Tolerance with Ceiling: Being comfortable at 60 g/hr doesn't mean that's the limit; it's a signal to continue building tolerance.

By systematically training the gut, athletes can expect reduced bloating, greater carbohydrate intake, more stable energy, and a proven fueling protocol for race day.