Introduction: When Digestion Limits Performance
Bloating and nausea halfway through, cramps at the 15th kilometer, DNF (did not finish) during an ultra-trail: digestive issues ruin many endurance races every year, even with impeccable physical preparation.
Fortunately, it is possible to limit these risks with a nutritional approach used by top-level athletes: gut training.
What exactly is it and how can you implement it to perform without being hampered by your digestion? We explain everything in this guide.
What is gut training?
Gut training refers to a nutritional strategy aimed at improving digestive tolerance during exercise, in order to reduce the risk and/or severity of gastrointestinal issues during prolonged physical activity. This approach is primarily used in endurance sports to accustom the digestive tract to the conditions in which it will be challenged during a competition.
Why gut training is essential for endurance athletes
During prolonged exertion, your body undergoes a redistribution of blood flow: it primarily sends blood to active muscles to support muscle contraction, at the expense of digestive organs.
This decrease in intestinal perfusion is associated with an increased risk of gastrointestinal issues.
At the same time, your glycogen reserves, i.e., the carbohydrate reserves stored in muscles and the liver, decrease after about 1 hour 30 to 2 hours of sustained effort. To maintain the pace, you need a carbohydrate intake.
You then find yourself facing a paradox: your body needs energy, but your digestive system is slow and no longer tolerates your energy drink.
This is precisely where gut training becomes a strategic factor.
By gradually training your digestive system to tolerate nutritional intake during exercise, you improve your chances of maintaining optimal performance over long distances.
How gut training works
The principle of repeated exposure
As Asker Jeukendrup emphasizes in his review Training the Gut for Athletes published in Sports Medicine, the gastrointestinal system is highly adaptable. It is capable of adapting to repeated nutritional strategies, with potential effects on digestive tolerance and performance.
Just as you train your muscles to bear more load, you can accustom your digestive system to several parameters:
- the type of nutrients (mainly carbohydrates, sometimes proteins, electrolytes)
- the quantity ingested (higher intake)
- the texture (liquids, gels, solid foods or meals)
- the timing (before and during exercise)
In practice, you start with small quantities, then gradually increase over several weeks, depending on your tolerance and the event's objectives.
The adaptive role of the digestive system
Several adaptation mechanisms associated with gut training are described in scientific literature.
Increased intestinal transporters
A carbohydrate-rich diet can increase the number and activity of sodium-glucose cotransporter 1 (SGLT1) (among others), a protein located in the intestinal wall that facilitates the entry of glucose into intestinal cells. This adaptation could help improve carbohydrate absorption capacity during exercise, especially when high intakes are targeted.
Improved gastric tolerance
Several studies analyzed in the review by Martinez et al. (2023) report a decrease in certain gastrointestinal symptoms (nausea, heaviness, discomfort) after repeated exposure to carbohydrates during several training sessions. However, results vary among individuals and protocols.
Improved gastric emptying
Repeated exposure to carbohydrate intake during exercise could promote more effective gastric emptying in some athletes. Effects seem to depend on the type of nutrient consumed and the protocol used.
Better utilization of exogenous carbohydrates (carbohydrates from food)
By progressively improving digestive tolerance, gut training can help some athletes maintain higher carbohydrate intake during prolonged efforts (marathon, triathlon, running long distance, ultra-trail).
How to implement a gut training program
Step 1: choose the right products
The first phase involves selecting products that are easy to digest and suitable for prolonged exertion: isotonic drinks, energy bars or gels, compotes, purées, or broths.
In endurance nutrition, it is often advised to combine different carbohydrate sources, aiming for a ratio close to 2:1, meaning two parts glucose or maltodextrin (complex carbohydrate derived from starch) to one part fructose. This strategy becomes particularly useful when intake exceeds approximately 60 g/h (Jeukendrup, 2014).
As a reminder, glucose and fructose do not use the same absorption pathways.
Glucose passes through a sodium-glucose cotransporter SGLT1 (Sodium-Glucose Linked Transporter 1).
Fructose, on the other hand, mainly depends on GLUT5 (GLucose Transporter 5), a specialized intestinal protein for fructose absorption (Ferraris & Diamond, 1997).
In an exercise context, it is this mobilization of several intestinal absorption pathways that explains why the glucose-fructose combination is associated with a higher oxidation of exogenous carbohydrates, meaning greater utilization of ingested carbohydrates as an energy source, compared to glucose alone.
Furthermore, when choosing your sports drink, prefer a moderate carbohydrate concentration (around 6 to 8%) and the presence of sodium, to support hydration during exercise.
Step 2: train in real conditions
Gut training is only useful if it reproduces real race conditions. This involves:
- the same type of product or combination (gel, drink, bar, food)
- the same consumption time
- similar conditions (heat, elevation, duration)
- comparable intensity
It is recommended to adapt carbohydrate intake based on the duration and intensity of the effort:
- 30 to 60 g/h for efforts of 1 to 2 h 30
- 60 to 90 g/h for efforts exceeding 2 h 30
- sometimes more, up to 120 g/h for some highly trained athletes, after gradual adaptation
Indicative example: 6-week progression (to be adapted according to your tolerance)
- Week 1–2: 30 to 40 g/h
- Week 3–4: 45 to 60 g/h
- Week 5: 60 to 75 g/h
- Week 6: 75 to 90 g/h (or more depending on goal and tolerance)
Step 3: adjust based on digestive reactions
Gut training is not a uniform protocol. Digestive tolerance varies greatly from person to person, depending on several factors:
- Exercise conditions: intensity, duration, heat, and hydration status alter the perfusion and function of the stomach and intestine.
- Individual factors: sensitivity to certain types of carbohydrates (especially fructose) or the presence of pre-existing digestive disorders can influence tolerance.
In practice, keep a detailed nutrition journal (amount consumed, type of product, symptoms experienced, timing, weather conditions) to identify what works or triggers your issues.
In case of persistent symptoms, several adjustments are possible:
- temporarily decrease doses
- reduce the drink's concentration
- test another carbohydrate source
- further divide intakes
- adjust beverage temperature
Daily diet for a high-performing gut
Digestive training is not limited to intake during exercise. Your daily diet influences overall digestive tolerance.
Maintain a daily carbohydrate intake consistent with training
During moderate training phases, aim for around 5 to 7 g/kg/day, and up to 7 to 10 g/kg/day during intensive training periods, depending on your goals (Burke, 2001).
Preserve gut health and microbiota
Prolonged exercise can temporarily disrupt your intestinal permeability. This phenomenon is part of a set of digestive disturbances described by Costa et al. (2017).
Furthermore, some data suggest that endurance could also modify the composition of the gut microbiota, with potential effects on certain physiological adaptations (Przewłócka et al., 2020).
In this context, prioritize fiber-rich foods and beverages or naturally fermented ones (fermented yogurts and prebiotics) to support your microbiota.
Mistakes to avoid during gut training
Certain practices greatly increase the risk of failure.
Going too fast: abruptly increasing carbohydrate intake (e.g., from 30 to 90 g/h at once) leads to significant digestive problems. Progressivity is the most reliable rule. An effective strategy should be spread over several weeks, with incremental dose increases.
Changing strategy on race day: new gel, new brand of drink, different dosage: modifying your protocol at the last minute is a classic mistake. Your strategy must be tested in training several weeks before the competition. Ideally, consume the same products, at the same doses, and at the same times, at least three to five times under similar conditions before an important race.
Focusing only on carbohydrates without testing the rest: gut training primarily concerns carbohydrate tolerance, but it also involves the ability to manage fluid and sodium intake during exercise.
Not keeping a nutrition training journal: without written tracking (amount consumed, timing, symptoms experienced, weather conditions), you won't be able to adjust and standardize your protocol. Systematically record your intake and all this useful data.
FAQ: everything you need to know about gut training
Is gut training only for professionals?
No. If you engage in prolonged sports activity (beyond 90 minutes), gut training can be useful, especially if you suffer from recurring digestive issues during your outings.
How long does it take to see results?
Initial subjective improvements can appear within a few days for some athletes. However, to achieve more lasting digestive adaptations, it generally takes several weeks of regular practice, often around 4 to 6 weeks depending on the individual.
Can you do gut training without specific products?
Yes. Fruit purees, dates, raisins, bananas, potatoes, rice, or honey can be perfectly suitable. The key is to regularly consume carbohydrates during exercise and gradually increase quantities.
Can you do gut training without sugar?
No, not really. Gut training specifically involves training your gut to better tolerate and absorb carbohydrates during exercise. Without carbohydrate intake, the desired digestive adaptations (improved intestinal transporters, better oxidation) cannot occur.
What are the signs of good adaptation?
You know it's working when:
- your digestive problems gradually decrease
- you tolerate carbohydrate intake better during exercise
- you feel less fatigue on long outings
Conclusion: training your gut, the key to sustainable performance
In conclusion, gut training allows you to train your digestive system, just as you would train your muscles or cardiovascular system.
The objective is to expose your gut and stomach to carbohydrate and fluid intake during training sessions to optimize gastric emptying, nutrient absorption, and reduce the risk of nausea, abdominal cramps, and gastric discomfort during an event.
Although these adaptations are less visible than muscle development, they play a crucial role in your ability to maintain optimal performance during prolonged efforts.
To benefit from it, apply regular progression, test your strategy in each training session, and adjust it according to your individual tolerance.
Bibliography
Costa, R. J. S., Snipe, R. M. J., Kitic, C. M., & Gibson, P. R. (2017). Systematic review: Exercise-induced gastrointestinal syndrome—implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246–265. https://doi.org/10.1111/apt.14157
Jeukendrup, A. E. (2017). Training the Gut for Athletes. Sports Medicine, 47(Suppl 1), 101–110. https://doi.org/10.1007/s40279-017-0690-6
King, A. J., Etxebarria, N., Ross, M. L., Garvican-Lewis, L. A., Heikura, I. A., McKay, A. K. A., Tee, N., Forbes, S. F., Beard, N. A., Saunders, P. U., Sharma, A. P., Gaskell, S. K., Costa, R. J. S., & Burke, L. M. (2022). Short-term very high carbohydrate diet and gut-training have minor effects on gastrointestinal status and performance in highly trained endurance athletes. Nutrients, 14(9), 1929. https://doi.org/10.3390/nu14091929
Jentjens, R. L. P. G., Moseley, L., Waring, R. H., Harding, L. K., & Jeukendrup, A. E. (2004). Oxidation of combined ingestion of glucose and fructose during exercise. Journal of Applied Physiology, 96(4), 1277–1284. https://doi.org/10.1152/japplphysiol.00974.2003
Martinez, I. G., Mika, A. S., Biesiekierski, J. R., & Costa, R. J. S. (2023). The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Medicine, 53(6), 1175–1200. https://doi.org/10.1007/s40279-023-01841-0
Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25–S33. https://doi.org/10.1007/s40279-014-0148-z
Ferraris, R. P., & Diamond, J. (1997). Regulation of intestinal sugar transport. Physiological Reviews, 77(1), 257–302. https://doi.org/10.1152/physrev.1997.77.1.257
Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(Suppl 1), S17–S27. https://doi.org/10.1080/02640414.2011.585473
Przewłócka, K., Folwarski, M., Kaźmierczak-Siedlecka, K., Skonieczna-Żydecka, K., & Kaczor, J. J. (2020). Gut-muscle axis exists and may affect skeletal muscle adaptation to training. Nutrients, 12(5), 1451. https://doi.org/10.3390/nu12051451
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