What Is Gut Training? A Complete Guide for Endurance Athletes

The evolution of sports science over the past two decades has radically transformed our understanding of athletic performance. While cardiac output, lactate threshold, and running economy were long considered the sole determinants of endurance success, a fourth pillar has emerged as the ultimate limiting factor for many athletes: the gastrointestinal system. 

The concept of “Gut Training,” or intestinal training, is no longer merely a nutritional trick but a rigorous scientific discipline aimed at conditioning the digestive system so that it becomes an engine of performance rather than a source of failure.

In ultra-endurance events such as the Ultra-Trail du Mont-Blanc or Ironman-distance triathlons, the ability to absorb and oxidize carbohydrates often determines final speed and the ability to avoid the dreaded “wall.” However, intense physical exertion creates a physiological paradox: while the muscles require a constant supply of fuel, exercise diverts blood away from the intestine, reducing its functional capacity and increasing vulnerability to gastrointestinal disorders. 

This article explores in depth the mechanisms of digestive adaptation, programming protocols, and the crucial role of nutrient quality in the success of this preparation.

Physiology and pathophysiology of the digestive system during exercise

To understand the value of Gut Training, it is essential to analyze the biological stresses to which the digestive system is subjected during prolonged exertion. The digestive system is responsible for delivering carbohydrates and fluids to the bloodstream, a critical role in maintaining performance.

Blood redistribution and splanchnic ischemia

During exercise, the human body prioritizes immediate survival and mechanical performance by redirecting up to 80% of cardiac output to the muscles and skin for thermoregulation. This phenomenon, known as splanchnic ischemia, significantly reduces the supply of oxygen and nutrients to the cells lining the intestinal wall. This reduction in blood flow has several consequences:

• Impaired intestinal barrier: Lack of oxygen can damage the tight junctions between cells, increasing intestinal permeability and allowing endotoxins to enter the bloodstream. 
• Delayed gastric emptying: The stomach takes longer to release its contents into the small intestine, causing feelings of heaviness, bloating, and nausea.  
• Poor nutrient absorption: Sugar transporters become less effective, leaving unabsorbed carbohydrates in the colon, which causes osmotic water influx and diarrhea. 

The molecular mechanisms of absorption

The sugars you consume during exercise do not automatically pass into the bloodstream. They must use specific “transporters” present in the intestine. If these transporters are saturated, absorption slows down. Understanding these transporters is at the heart of modern nutrition strategy.

Research shows that a carbohydrate-rich diet can double the density and activity of SGLT1 transporters in just two weeks. This is one of the fundamental principles of Gut Training: by regularly “bombarding” the carbohydrate system, the body is forced to increase its transport capacity, enabling greater oxidation of the sugars from sports nutrition products and reducing fermentable residues. 

Gastrointestinal disorders in endurance sports: an old story

Statistics reveal that 30% to 50% of endurance athletes experience GI symptoms during competitions. These disorders are often the leading cause of DNF among trail runners and triathletes.

Symptom typology

Symptoms are generally classified into two categories: upper symptoms (stomach and esophagus) and lower symptoms (intestines and colon).

• Upper symptoms: Gastroesophageal reflux, nausea, vomiting, heartburn, gastric bloating.
• Lower symptoms: Abdominal cramps, urgent bowel movements, bloody diarrhea (in extreme cases), excessive gas.

Risk factors and triggers

Several variables increase the likelihood of developing these disorders:

• Mechanical: The repeated impacts of running cause intestinal microtrauma, unlike cycling, where the body is supported.
• Nutrition: Excessive consumption of fiber, fat, or protein immediately before exercise slows gastric emptying.
• Hydration: Dehydration further reduces intestinal blood flow, worsening splanchnic ischemia.
• Drink concentration: Hypertonic drinks (overly concentrated with sugar) delay water absorption and can irritate the intestinal wall.

The Science of Adaptation: Why Is the Gut a “Trainable” Muscle?

The concept of Gut Training is based on neuroplasticity and tissue adaptability. Just as muscles hypertrophy under stress, the digestive system adjusts its functional capacities in response to repeated nutritional stimuli.

Adaptation of Gastric Capacity

The stomach is an organ with a great capacity for distension. Studies conducted on competitors show that the stomach can be trained to hold massive volumes with minimal discomfort. For athletes, this means it is possible to reduce the sensation of being “too full” by practicing running with large volumes of fluids or food. This adaptation makes it possible to maintain optimal hydration and a high caloric intake without the psychological stress associated with gastric discomfort.

Accelerated Gastric Emptying

If an athlete regularly consumes glucose-rich solutions, their stomach develops mechanisms to empty these solutions more quickly. Research indicates that a daily intake of 400g of glucose for just 3 days can reduce the half-emptying time of the stomach for this substance. This suggests that last-minute nutritional preparation (carb-loading) has a dual effect: replenishing glycogen stores and “priming” the gastric pump.

Gut Training Practice Protocols

A Gut Training protocol should be implemented gradually to avoid inducing the very symptoms it is intended to prevent. It is recommended to begin these strategies approximately 8 to 12 weeks before the main goal.

Strategy 1: “Full Stomach” Training

This method consists of training sessions, generally at low intensity, immediately after consuming a carbohydrate-rich meal.

• Objective: Desensitize the mechanoreceptors in the stomach wall.
• Implementation: Start with short 30- to 45-minute runs after a light meal, then gradually increase the duration of the effort and the size of the meal.

Strategy 2: Increasing hourly carbohydrate intake

The athlete must train to consume increasing amounts of carbohydrates during long outings or specific-intensity sessions.

Using glucose/fructose ratios (ideally 1:0.8 or 2:1) is essential here to saturate both transport pathways (SGLT1 and GLUT5) without overloading them.

Strategy 3: Volume hydration

Training to drink fluid volumes close to the actual sweat rate, even if this exceeds the sensation of thirst, helps improve gastric comfort. Studies have shown that with regular intake (every 15–20 min), athletes report a significant reduction in discomfort after 4 to 5 sessions.

12-Week Gut Training Program

A structured approach enables lasting adaptation of intestinal tissues and optimization of enzymatic processes.

Initiation phase (weeks 1–4)

During this period, the focus is on tolerance to volume. The athlete incorporates one weekly session after a meal rich in complex carbohydrates (pasta, rice, sweet potato). They also begin testing bars, gels, or energy fruit purées to identify their preferred flavors and confirm the basic digestibility of natural ingredients.

Development phase (weeks 5–8)

The goal is to increase caloric density. During long outings, carbohydrate intake is set at 60g/h. This is the ideal time to test alternating textures: bars for slower sections and fruit purées/gels for higher intensities. The athlete must carefully record their digestive reactions in their training log.

Specificity phase (weeks 9–12)

This is the race simulation phase. The athlete aims for their final target (for example, 80–90g/h) during specific training blocks. Mathieu Blanchard suggests completing three consecutive loading days, with a full nutrition simulation on the final day to mimic the digestive fatigue experienced at the end of a race.

Nutritional excellence: why natural ingredients make all the difference

Gut Training is more effective when the “fuel” used is of good quality. Highly processed ingredients, artificial sweeteners, and preservatives can irritate an intestinal lining already weakened by exercise.

The Mulebar philosophy: naturalness and digestibility

Since its creation in 2008, Mulebar has banned artificial flavors, colorants, and preservatives. This approach is not merely ethical or a matter of taste; it is functional.

• Agave syrup vs. maltodextrin: Mulebar uses agave syrup as the main source of sugar in its gels. Rich in natural fructose, it has a low glycemic index (15) and provides a more stable energy release than corn maltodextrin, which is often responsible for insulin spikes and gastric disorders.
• Brown rice syrup: This is the other syrup used in most products in the range. Brown rice syrup has a high glycemic index, resulting in a rapid release of energy.
  
• Plant proteins (pea and soy): In its recovery products and long-distance bars, Mulebar favors pea protein over soy protein and whey. The latter is a frequent source of intestinal discomfort for athletes sensitive to lactose or bovine proteins under physiological stress.
• Fruit: Using candied fruit or fruit purées (mango, apple, cherry, apricot...) provides essential micronutrients and antioxidants to help limit exercise-related inflammation.

Taste impact and prevention of exercise-induced anorexia

During ultra-endurance events, many athletes stop eating because they are put off by the sweet taste of classic gels. Mulebar's variety of flavors, including original options such as Lemon-Ginger and salted caramel, helps maintain appetite and therefore caloric intake throughout the race.

Environmental contexts and nutritional plan adjustments

Gut Training must be adapted according to external conditions that directly affect digestive physiology.

The effect of heat

In summer, the body must send more blood to the skin to dissipate heat through sweating, which worsens intestinal ischemia. Absorption becomes slower and the risks of fermentation increase. 

Recommendation: Slightly reduce the amount of solid carbohydrates in favor of liquid or semi-liquid textures (fruit purées) and increase sodium intake (300–700mg/h) to facilitate glucose transport.

Altitude and hypoxia

Altitude reduces blood oxygen saturation, which also affects the digestive lining. Many athletes report a loss of appetite and more difficult digestion above 2500m.

Recommendation: Choose highly digestible foods and avoid overly large meals just before climbing mountain passes.

Gut Training: a comprehensive approach to athlete health

Beyond pure performance on race day, gut training contributes to better overall health. Optimized digestion allows for better absorption of micronutrients and a more stable balance of the gut microbiome, both essential factors for immunity and nervous system recovery.

Impact on the microbiome

Regular exercise positively alters the composition of the microbiome, promoting bacterial species that produce short-chain fatty acids, which benefit energy production and reduce inflammation. Well-conducted Gut Training, using natural foods, supports this fragile ecosystem by avoiding the chemical assault of industrial additives.

The metabolic window and recovery

Digestive training continues after the race. Teaching the body to absorb nutrients (protein and carbohydrates) within 30 to 60 minutes after exertion is crucial for muscle rebuilding. Mulebar's plant-based protein bars are designed to be easily absorbed during this "metabolic window", even when the stomach is still tired.

Final summary for the endurance athlete

Gut Training should no longer be considered a last-minute experiment, but rather an integral part of training, just like interval training or strength training.

By adopting these strategies and choosing products that respect human physiology, athletes turn their digestive system into a powerful ally. Gut Training not only helps them cross the finish line successfully, but also lets them do so with enjoyment, without the unnecessary suffering of a distressed stomach. Science confirms that the gut is remarkably adaptable; it is up to each runner to give it the means to thrive.

 

Frequently Asked Questions:

Why does digestion become more difficult during a race?

The main difficulty comes from the redistribution of blood flow. The muscles and skin capture most of the oxygen, depriving the digestive system of the energy it needs to function. Body jolts and hormonal changes slow intestinal transit.

How does Gut Training concretely improve performance?

It acts on three levels: it increases the number of sugar transporters in the intestine (more energy available), speeds up the passage of food from the stomach to the intestine (less bloating), and reduces nerve sensitivity to gastric fullness.

How long does it take to adapt the gut?

The first enzymatic and transport adaptations occur within 10 to 14 days. However, mechanical adaptation (volume tolerance) and fine-tuning the strategy generally require a cycle of 8 to 10 weeks of regular practice.

Which nutrients should be included in digestive training?

It is essential to include the same carbohydrate sources planned for the race. Using a glucose and fructose blend is recommended to maximize total absorption. Thanks to their agave syrup and brown rice base, Mulebar products naturally provide this beneficial combination.

Should Gut Training be practiced during every session?

No. It is advisable to dedicate 1 to 2 sessions per week to this practice, ideally during long runs or sessions at a pace specific to the competition. The rest of the time, a balanced, easily digestible diet is sufficient.

What should you do if you experience intestinal cramps during a race?

If problems occur, the first step is to slow down to allow better blood flow to the intestine. Stop consuming all solid food and limit yourself to small sips of still water until the symptoms disappear. Deep breathing can also help relax the abdominal muscles.