Gut Training for Endurance Athletes: The 2025 Science Update

A 2025 joint position statement from prominent sports science organizations has solidified gut training as a highly effective strategy for endurance athletes, according to Nutritional Revolution Source. This practice involves consuming increasing amounts of carbohydrates during training to enhance gastrointestinal adaptation and absorption, directly impacting race-day performance.

The publication, Costa et al. 2025, which represents the SDA/USSF Joint Position Statement on exercise-induced gastrointestinal syndrome (EIGS), assigns Grade I evidence to gut training. This indicates it is a primary intervention with strong supporting research. The statement also debunks practices like using probiotics, glutamine, or chronic low-carbohydrate, high-fat (LCHF) diets, grading them as ineffective or even detrimental.

Understanding Gut Training

Gut training is a planned, multi-week process of progressively increasing carbohydrate intake during exercise. It's not a quick fix for race week but a fundamental part of an athlete's preparation, alongside physical training. Minimum effective dose is two weeks, but eight to twelve weeks is recommended for optimal race performance.

How the Gut Adapts to Carbohydrate Intake

The effectiveness of gut training lies in its ability to upregulate intestinal transporters—specifically SGLT1 and GLUT5—which are responsible for moving carbohydrates from the gut into the bloodstream. SGLT1 handles glucose and maltodextrin, saturating at approximately 60g per hour. Beyond this, excess glucose can cause gastrointestinal distress.

GLUT5, on the other hand, absorbs fructose. By combining glucose and fructose, absorption rates can surpass 90g/hr, reaching up to 120g/hr or more in adapted individuals. Repeated exposure to high-carbohydrate fueling increases the density and activity of SGLT1, allowing the gut to handle more fuel efficiently without discomfort.

Key Findings from the 2025 Position Paper

Costa et al. 2025 graded various interventions based on evidence, revealing what truly impacts athlete gut health and performance:

Grade I Interventions (Strongest Evidence)

1. Carbohydrate consumption during exercise (60–90 g/hr): This practice enhances blood flow to the gut, mitigating exercise-induced gastrointestinal syndrome (EIGS). Studies show a significant reduction in gut barrier injury markers with 90g/hr intake.
2. Two-week gut training: Systematic reviews indicate a 47% reduction in gut discomfort and 54% less carbohydrate malabsorption after just two weeks of structured fueling at 30–90g/hr.
3. Low-FODMAP diet 24–48 hours pre-race: This strategy can reduce GI symptom severity by about 50%.

Interventions to Avoid

The position paper also clearly states that certain popular interventions are ineffective:

• Probiotics: No measurable benefits for gut barrier integrity, inflammation, or symptoms in endurance athletes.
• Glutamine: Inconsistent and clinically irrelevant findings for trained athletes.
• Chronic LCHF/ketogenic diets: These diets can actually increase gut barrier injury and endotoxin levels, offering no symptom benefit for athletes.

Building a Gut Training Protocol

For managers overseeing product development or athlete programs, understanding a structured gut training build is essential. A typical 10-week plan progresses carb targets:

• Weeks 1–2 (Foundation): 30–45 g/hr to establish habit.
• Weeks 3–4 (Build 1): 45–60 g/hr, introducing dual-carb sources.
• Weeks 5–6 (Build 2): 60–75 g/hr, utilizing glucose + fructose to exceed the SGLT1 ceiling.
• Weeks 7–8 (Race Sim): 75–90 g/hr, using race-day products and timing.
• Weeks 9–10 (Refinement): 75–120 g/hr without introducing new products.

This progressive approach helps the body adapt without aggressive overload. Athletes aiming for races longer than three hours typically require a minimum of ten weeks of gut training.

Common Pitfalls to Avoid

Businesses should advise against these common gut training errors:

• Starting too late: Training should begin well before the taper phase. Two weeks is the minimum for adaptation, not the ideal.
• Glucose-only products: These limit carbohydrate absorption to about 60 g/hr. Dual-carb (glucose + fructose) formulations are necessary for higher rates.
• Not training in race conditions: GI tolerance can decrease in heat or during high-intensity efforts. Training should mimic race environments.
• Introducing new products on race day: Athletes should use only familiar products during competition.
• Confusing tolerance with the ceiling: Comfort at a certain intake level means the gut is ready for further progression, not that the maximum has been reached.

By prioritizing evidence-based gut training strategies, businesses can support athletes more effectively and develop products that genuinely enhance performance and recovery.