New Gut Training Science: Fueling Endurance Athletes

A 2025 joint position statement by Sports Dietitians Australia and the Ultra Sports Science Foundation (Costa et al. 2025) has graded 'gut training' as a Grade I evidence-based intervention. This means that for endurance athletes, training the gut to process carbohydrates during exercise is a highly effective strategy for preventing gastrointestinal issues and enhancing performance Source.

Gut training involves the deliberate, progressive consumption of carbohydrates at race-target rates during training. The research indicates that while two weeks is the minimum effective dose for measurable adaptation, 8 to 12 weeks of consistent practice is necessary to achieve optimal race-day performance.

What is Gut Training?

Gut training is a systematic, multi-week protocol where athletes progressively increase their carbohydrate intake during exercise. It aims to adapt the gastrointestinal tract to absorb higher amounts of fuel, thereby preventing common issues like bloating, cramping, and malabsorption during long events. This training should be integrated into an athlete's overall preparation schedule, similar to intervals or strength work, rather than being a last-minute adjustment before a race Source. The benefits, such as a 47% reduction in gut discomfort and 54% less carbohydrate malabsorption, are not marginal gains but represent significant improvements.

How the Gut Adapts: SGLT1 and GLUT5

The gut adapts through increased activity and density of specific intestinal transporters: SGLT1 and GLUT5. SGLT1 absorbs glucose and maltodextrin, with a saturation point around 60g per hour. Exceeding this limit for glucose can lead to GI distress. GLUT5, conversely, absorbs fructose through a separate pathway. By combining glucose and fructose, athletes can bypass the SGLT1 saturation, allowing for total carbohydrate absorption rates to exceed 90g/hr, even reaching up to 120g/hr (Hearris et al. 2022). Repeated exposure to high-carbohydrate fueling upregulates SGLT1 activity, making the gut more efficient.

Key Findings from the 2025 Position Paper

Costa et al. (2025) graded various interventions based on evidence strength. Three practices received Grade I evidence, indicating strong support for their effectiveness:

• Carbohydrate consumption during exercise (60–90 g/hr): This increases blood flow to the gut, reducing ischemia that contributes to exercise-induced gastrointestinal syndrome (EIGS). Studies show a significant reduction in gut barrier injury markers when consuming 90 g/hr of carbohydrates in heat (Snipe et al. 2017).
• Two-week gut training: This practice can reduce carbohydrate malabsorption significantly, transitioning it from clinical levels to near zero (Costa et al. 2017; Miall et al. 2018; Martinez et al. 2023).
• Low-FODMAP eating 24–48 hours pre-race: This approach has been shown to reduce GI symptom severity by approximately 50% (Lis et al. 2018).

Conversely, several popular interventions were deemed ineffective:

• Probiotics: No effect was found on gut barrier integrity, inflammation, or symptoms.
• Glutamine: Its findings were inconsistent and not clinically relevant for trained athletes.
• Chronic Low-Carbohydrate/Ketogenic Diets: These diets were shown to increase gut barrier injury and endotoxin markers with no symptomatic benefits (McKay et al. 2023).

Building a Gut Training Protocol

A typical gut training program extends over 10 weeks for events longer than three hours, focusing on progressive carbohydrate intake. It starts with foundational weeks (1-2) at 30–45 g/hr, building gradually to 75–120 g/hr during race simulation and refinement phases (weeks 7-10). It is crucial to use dual-carb sources (glucose + fructose) to surpass the 60 g/hr limit imposed by glucose-only products Source.

Common Gut Training Missteps

Athletes and commercial entities developing products should be aware of common errors:

1. Starting too late: Gut training needs a minimum of two weeks, ideally 8-12 weeks, not just a few days before a race.
2. Glucose-only products: These limit carbohydrate absorption to about 60 g/hr. Products require fructose or sucrose to exceed this.
3. Not training in race conditions: GI tolerance decreases in heat, at high intensity, and under stress. Training should mimic race day conditions.
4. Introducing new products on race day: Stick to products already tested and tolerated during training.
5. Confusing tolerance for ceiling: Being comfortable at a certain intake (e.g., 60 g/hr) signals an opportunity to progress, not to stop.

Implementing structured gut training can lead to reduced bloating and nausea, increased carbohydrate absorption, more stable energy, and fewer 'bonk' moments, leading to significantly better race-day outcomes.