Boost Endurance Performance: New Gut Training Standards for Athletes

New research highlights critical insights into optimizing endurance nutrition, with a 2025 joint position statement by Sports Dietitians Australia and the Ultra Sports Science Foundation (Costa et al., 2025) emphasizing the importance of "gut training" for athletes. This practice, involving the deliberate and progressive consumption of carbohydrates during exercise, is now supported by Grade I evidence as a method to enhance performance and mitigate exercise-induced gastrointestinal syndrome (EIGS) Source.

Notably, the review graded popular supplements like probiotics and glutamine as ineffective for these purposes, and chronic low-carbohydrate high-fat (LCHF) diets were found to be detrimental.

What is Gut Training?

Gut training involves gradually increasing carbohydrate intake during exercise sessions to stimulate specific physiological adaptations within the gastrointestinal tract. Unlike a quick fix, it's a multi-week program, similar to interval training or strength work, and should be integrated into an athlete's preparatory periodization. The minimum effective dose for measurable adaptation is two weeks, but 8 to 12 weeks provides optimal results for race day.

This training aims to improve the gut's ability to absorb carbohydrates efficiently, preventing issues like bloating, cramping, and urgency that often sideline athletes during long events. For instance, a systematic review found a 47% reduction in gut discomfort and 54% less carbohydrate malabsorption after just two weeks of structured fueling at 30–90g/hr during training (Martinez et al., 2023).

How the Gut Adapts to Carbohydrate Intake

The effectiveness of gut training stems from how the body absorbs carbohydrates. Two key intestinal transporters, SGLT1 and GLUT5, play a crucial role:

• SGLT1: This transporter absorbs glucose and maltodextrin, but it saturates at approximately 60g of glucose per hour. Exceeding this limit with glucose-only sources can lead to digestive distress.
• GLUT5: This transporter handles fructose through a separate pathway. By combining glucose and fructose, total carbohydrate absorption can exceed 90g/hr, potentially reaching 120g/hr.

Repeated exposure to high-carbohydrate fueling upregulates the density and activity of SGLT1, allowing the gut to handle more fuel during exercise. Measurable changes can occur within days of consistent dietary stimulus.

Evidence-Based Interventions: What Works and What Doesn't

The 2025 position paper graded interventions based on their scientific evidence, from Grade I (strongest) to Grade IV (weakest). Three strategies received Grade I endorsement:

1. Carbohydrate consumption during exercise (60–90 g/hr): This practice increases blood flow to the gut, counteracting the splanchnic ischemia that contributes to EIGS.
2. Two-week gut training protocols: Shown to reduce carbohydrate malabsorption significantly and improve gut comfort.
3. Low-FODMAP eating 24–48 hours pre-race: Can reduce gastrointestinal symptom severity by about 50%.

Conversely, several popular approaches were deemed ineffective or even detrimental:

• Probiotics: No consistent effect on gut barrier function, inflammation, or symptoms across various studies.
• Glutamine: Findings were inconsistent and not clinically relevant for trained athletes.
• Chronic Low-Carbohydrate High-Fat (LCHF) diets: Shown to increase markers of gut barrier injury in elite athletes without providing symptom benefits.

A 10-Week Gut Training Protocol Example

For athletes targeting long endurance events, a progressive 10-week gut training build is recommended. This involves weekly increases in carbohydrate targets during training sessions lasting over 75 minutes:

• Weeks 1–2 (Foundation): 30–45 g/hr. Focus on establishing the habit and measuring baseline tolerance.
• Weeks 3–4 (Build 1): 45–60 g/hr. Gradually increase intake, introducing dual-carb sources.
• Weeks 5–6 (Build 2): 60–75 g/hr. Break the SGLT1 ceiling using glucose + fructose combinations.
• 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. Maintain high intake, but avoid new products.

This structured approach ensures the gut is prepared to handle the demands of race day, minimizing the risk of bonking or gastrointestinal distress.

Avoiding Common Gut Training Pitfalls

To maximize effectiveness, athletes should avoid several mistakes:

• Starting too late: Two weeks is a minimum, not a goal. Significant adaptation requires longer.
• Using glucose-only products: These limit carbohydrate absorption to about 60 g/hr.
• Not training in race conditions: GI tolerance decreases in heat, at high intensity, and under stress. Practice in similar environments.
• Introducing new products on race day: Stick to products the gut is already familiar with.
• Confusing tolerance with capacity: Being comfortable at 60 g/hr means it's time to progress, not to stop.

Consistent, progressive gut training helps athletes reduce bloating and nausea, absorb more carbohydrates, maintain stable energy, and establish a proven fueling protocol for competitive events.