How, and why, to train your gut
Gut Training in endurance running: optimising fuel absorption for performance
What is Gut Training?
Gut training refers to the process of conditioning the digestive system to handle and efficiently absorb fuel during endurance exercise. It is an essential component of race preparation for endurance runners, particularly those competing in marathons and ultras. The ability to ingest and absorb carbohydrates effectively can be the difference between sustaining energy levels or suffering from bonking, gastrointestinal distress, or underperformance.
Why is Gut Training important?
Many endurance athletes experience stomach issues during long races, including nausea, bloating, and vomiting. While these issues are often attributed to food choices, the ability to tolerate and absorb carbohydrates is trainable. Research shows that trained endurance athletes can absorb and utilise more ingested carbohydrate than untrained individuals.
Additionally, gut training is crucial for optimising energy intake. The body’s stored glycogen is limited (around 90 minutes’ worth), and an endurance event lasting multiple hours requires continuous fuel replenishment. Runners who can efficiently absorb carbohydrates during exercise have a competitive advantage in maintaining pace and delaying fatigue.
How does Gut Training work?
Beyond the stomach: the role of the intestines
Many runners think of gut training as merely preparing the stomach to tolerate food intake without discomfort. However, much of the adaptation occurs in the intestines. Carbohydrate absorption takes place in the small intestine via transport proteins such as SGLT1 (for glucose) and GLUT5 (for fructose). Regularly consuming carbohydrates during training can upregulate these transporters, increasing the gut’s ability to absorb and utilise fuel.
Changes in the microbiome
Recent research suggests that endurance training and diet influence gut microbiota composition. Specific bacterial strains associated with enhanced carbohydrate metabolism may increase with regular carbohydrate ingestion during training. This adaptation could improve gut health and fuel efficiency, reducing gastrointestinal distress while enhancing energy availability.
Does Gut Training affect energy yield?
While the caloric content of carbohydrates remains the same, gut training can improve the efficiency with which energy is absorbed and utilised. Untrained athletes may experience a bottleneck in digestion, where carbohydrates sit in the stomach or intestines without being effectively absorbed. Training the gut to process higher carbohydrate loads can reduce this limitation, allowing more consistent energy delivery to working muscles.
Practical approaches to Gut Training
Gradual carbohydrate exposure: start with lower amounts (e.g., 30g per hour) and increase gradually to 60-90g per hour in training sessions.
Train with planned race nutrition: use the same carbohydrate sources (e.g., gels, drinks, solid foods) planned for race day to familiarise the gut. Also do some training with the on-course nutrition if it’s a long race to minimise issues if you have to use it.
Approriate fluid and sodium intake: proper hydration aids digestion and reduces gastrointestinal issues.
Practice fueling at race intensity: the gut responds differently at higher effort levels, so fuelling should be practiced during tempo and long runs. Fast finish long runs, or mid-long aerobic threshold interval runs are great for this.
Use multiple transportable carbs: combining glucose and fructose improves absorption by using different transport pathways.
Gut training is an often overlooked yet crucial component of endurance performance. By adapting the stomach, intestines, and microbiome, runners can improve their carbohydrate absorption and energy availability while minimising gastrointestinal issues. As with any aspect of training, consistency and gradual progression are key to optimising race-day fuelling strategies.
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