Gut Training Boosts Athlete Performance, Reduces GI Issues

A new joint position statement from Sports Dietitians Australia (SDA) and the Ultra Sports Science Foundation (USSF), Costa et al. 2025, has clarified the science behind 'gut training' for endurance athletes. This deliberate, progressive fueling approach during training is now graded as Grade I evidence for enhancing performance and protecting gut health Source.

The statement emphasizes that gut training involves consistently consuming race-target carbohydrate rates during training. It also debunks several common but ineffective practices, such as chronic low-carbohydrate diets, probiotics, and glutamine supplementation.

What is Gut Training?

Gut training is a structured, multi-week program where athletes progressively increase carbohydrate intake during exercise. The goal is to induce physiological adaptations in the gastrointestinal (GI) tract, enabling it to absorb and utilize more carbohydrates efficiently during races. This is not a last-minute fix; it's a fundamental part of endurance preparation, similar to interval and strength training.

The minimum effective dose for measurable adaptation is two weeks. However, optimal results for competitive events require eight to twelve weeks. For races longer than three hours, a ten-week minimum is recommended, while two to three-hour races need six to eight weeks of preparation Source.

How the Gut Adapts

The adaptation process primarily involves two intestinal transporters: SGLT1 and GLUT5. SGLT1 absorbs glucose and maltodextrin, with a saturation point of approximately 60g per hour. Exceeding this limit with glucose-only products can lead to GI distress like bloating and cramping. GLUT5 absorbs fructose through a separate pathway.

By combining glucose and fructose, athletes can bypass the SGLT1 saturation, allowing carbohydrate absorption rates to climb past 90g/hr, and even up to 120g/hr. Repeated exposure to high carbohydrate fueling upregulates the density and activity of SGLT1 in the intestine, meaning the gut becomes more efficient at processing carbs over time Source.

Evidence-Based Interventions

The Costa et al. 2025 statement graded published interventions from Grade I (strongest evidence) to Grade IV (weakest). Three key practices received Grade I endorsement:

1. Carbohydrate consumption during exercise (60–90 g/hr): This practice increases blood flow to the gut, mitigating exercise-induced gastrointestinal syndrome (EIGS).
2. Two-week gut training protocols: Studies show that just two weeks of structured fueling (30–90 g/hr) can reduce gut discomfort by 47% and carbohydrate malabsorption by 54% Source.
3. Low-FODMAP eating 24–48 hours pre-race: This strategy can reduce GI symptom severity by about 50%.

Conversely, several popular interventions were found to be ineffective. Probiotics showed no impact on gut barrier injury, inflammation, or symptoms. Glutamine offered inconsistent and clinically irrelevant findings for trained athletes. Chronic low-carbohydrate/ketogenic diets were actually found to increase gut barrier injury and endotoxin markers Source.

Common Mistakes to Avoid

Athletes often make critical errors that hinder their gut training efforts:

• Starting too close to race day: Two weeks is a minimum, not a goal. Effective preparation requires months.
• Using glucose-only products: Without fructose, carbohydrate intake is capped at 60 g/hr, limiting potential gains.
• Not training in race conditions: GI tolerance can decrease in heat, at high intensity, and under stress. Training under these conditions is crucial.
• Introducing new products on race day: Race day nutrition should consist of products already tested and proven during training.
• Confusing tolerance with ceiling: Just because 60 g/hr feels comfortable doesn't mean it's the maximum effective intake; it's often a sign to progressively increase further Source.

Ultimately, consistent, progressive gut training fosters a resilient and efficient GI system, enabling athletes to fuel effectively and perform optimally during long events while minimizing discomfort.