What is Gut Training? A complete guide for endurance athletes

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

The concept of " Gut Training " is no longer a simple nutritional trick but a rigorous scientific discipline aimed at conditioning the digestive system to become an engine of performance rather than a source of failure.

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

This article explores in depth the mechanisms of digestive adaptation, the 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 benefits of Gut Training, it is essential to analyze the biological stresses the digestive system undergoes during prolonged exertion. The digestive system is responsible for delivering carbohydrates and fluids to the bloodstream, a critical role for 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, called splanchnic ischemia, significantly reduces the supply of oxygen and nutrients to the cells of the intestinal wall.  This reduction in blood flow has several consequences:

• Impairment of the intestinal barrier: Lack of oxygen can damage the tight junctions between cells, increasing intestinal permeability and allowing endotoxins to pass into the bloodstream.  
• Slowing of 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 efficient, leaving unabsorbed carbohydrates in the colon, which causes osmotic water draws and diarrhea.  

The molecular mechanisms of absorption

The sugars you consume during exercise don't automatically pass into the bloodstream. They must use specific transporters in the intestine. If these transporters are saturated, absorption slows down. Understanding these transporters is central to modern nutritional strategy.

Research shows that a high-carbohydrate 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, we force the body to increase its transport capacities, allowing for increased oxidation of sugars from sports nutrition products and a reduction of fermentable residues.  

Gastrointestinal problems in endurance sports: an old story

Statistics reveal that 30% to 50% of endurance athletes suffer from IBS during competitions.  These disorders are often the primary cause of DNF in trail runners and triathletes.

Typology of symptoms

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

• Superior Symptoms: Gastroesophageal reflux, nausea, vomiting, heartburn, gastric bloating.
• Lower Symptoms: Abdominal cramps, urgent urges to defecate, bloody diarrhea (in extreme cases), excessive gas.

Risk factors and triggers

Several variables increase the likelihood of developing these disorders:

• Mechanics: The repeated impacts of running cause micro-trauma to the intestines, unlike cycling where the body is supported.
• Nutritional: Excessive consumption of fiber, fat, or protein just before exercise slows gastric emptying.
• Hydration: Dehydration further reduces intestinal blood flow, worsening splanchnic ischemia.
• Concentration of beverages: Hypertonic drinks (too concentrated in sugar) delay water absorption and can irritate the intestinal lining.

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 competitive athletes show that the stomach can be trained to hold massive volumes with minimal discomfort.  For the athlete, this means that it is possible to reduce the feeling of "overloading" by practicing running with large volumes of fluids or food.  This adaptation allows for optimal hydration and 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 stomach half-emptying time 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

The implementation of a Gut Training protocol must be gradual to avoid inducing the very disorders that one seeks to prevent.  It is recommended to start these strategies approximately 8 to 12 weeks before the main objective.

Strategy 1: The "Full Stomach" workout

This method involves performing training sessions (usually at low intensity) immediately after consuming a meal rich in carbohydrates.

• Objective : Desensitize the mechanoreceptors of the stomach wall.
• Implementation: Start with short runs of 30 to 45 minutes after a light meal, then 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 runs or specific intensity sessions.

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

Strategy 3: Volumetric hydration

Practicing drinking volumes of fluids close to the actual rate of sweating, even if it exceeds the sensation of thirst, helps to improve gastric comfort.  Studies have shown that by practicing regular holds (every 15-20 min), athletes report a significant decrease in discomfort after 4 to 5 sessions.

Gut Training Program over 12 Weeks

A structured approach allows for sustainable adaptation of intestinal tissues and optimization of enzymatic processes.

Initiation phase (weeks 1-4)

During this period, the focus is on volume tolerance. The athlete incorporates a weekly session after a meal rich in complex carbohydrates (pasta, rice, sweet potato).  He also begins testing energy bars, gels or compotes to identify his favorite flavors and validate the basic digestibility of natural ingredients.

Development phase (weeks 5-8)

The goal is to increase calorie density. During long outings, carbohydrate intake is set at 60g/h.  This is the perfect time to test alternating textures: energy bars for slower sections and energy compotes/gels for higher intensities.  The athlete must accurately 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 (e.g., 80-90g/h) during specific training blocks.  Mathieu Blanchard suggests performing three consecutive days of loading with total nutritional simulation on the last day to mimic the digestive fatigue at the end of a race.

Nutritional excellence: why natural ingredients are a game changer

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 synthetic flavors, colors and preservatives.  This approach is not only ethical or gustatory; it is functional.

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

The impact of taste and the prevention of exercise-induced anorexia

In ultra-endurance events, many athletes stop eating because they are disgusted by the sweet taste of classic gels.  The variety of Mulebar flavors (including original options like Lemon-Ginger or Salted Caramel) helps maintain appetite and therefore calorie intake throughout the race.

Environmental contexts and adjustments to the nutrition plan

Gut training must be modulated according to external conditions that directly impact digestive physiology.

The effect of heat

During the summer months, the body needs to 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 solid carbohydrate load in favor of liquid or semi-liquid textures (compotes) and increase sodium intake (300-700mg/h) to facilitate glucose transport.

Altitude and hypoxia

Altitude decreases 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 large meals just before climbing mountain passes.

Gut Training: A holistic 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 microbiota, essential factors for immunity and nerve recovery.

Impact on the microbiota

Regular exercise positively alters the composition of the microbiota, favoring bacterial species that produce short-chain fatty acids, which are beneficial for energy and reducing inflammation.  A well-conducted Gut Training, with natural foods, supports this fragile ecosystem by avoiding the chemical aggression of industrial additives.

The metabolic window and recovery

Digestive training continues after the race. Getting your body used to absorbing nutrients (proteins and carbohydrates) within 30 to 60 minutes after exercise is crucial for muscle repair.  Mulebar's plant-based protein bars are designed to be easily assimilated 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 an integral component of training, just like interval training or strength training.

By adopting these strategies and choosing products that respect human physiology, athletes transform their digestive system into a powerful ally. Gut training not only allows them to cross the finish line successfully, but also to do so with pleasure, without the unnecessary suffering of an upset stomach. Science confirms that the gut is a remarkably adaptable organ; it is up to each runner to give it the means to shine.

 

Frequently Asked Questions:

Why does digestion become more difficult during running?

The difficulty stems primarily from the redistribution of blood flow. Muscles and skin absorb the majority of oxygen, depriving the digestive system of its functional energy. Physical exertion and hormonal changes slow down intestinal transit.

How exactly does Gut Training improve performance?

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

How long does it take for your gut to adapt?

The first enzymatic and transport adaptations occur in 10 to 14 days.  However, mechanical adaptation (volume tolerance) and strategy refinement generally require an 8- to 10-week cycle of regular practice.

What nutrients should be included in one's digestive training?

It is essential to incorporate the same carbohydrate sources as those planned for the race. Using a glucose and fructose mix is recommended to maximize total absorption.  Mulebar products, thanks to their base of agave syrup and brown rice, naturally offer this beneficial blend.

Should Gut Training be done at every session?

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

What to do in case of intestinal cramps during a race?

If problems occur, the first step is to slow down to allow better blood flow to the intestines.  You must stop eating solid food and limit yourself to small sips of plain water until the symptoms disappear.  Deep breathing can also help to release abdominal tension.