Written by Dimitris Katsanos, Nutritionist at ATPLab Training
When we talk about preparing an athlete, we usually think about strength, endurance, speed, and technique. There is another system, however, that can and often should be trained: the gastrointestinal system.
This is exactly what gut training describes. It is the systematic practice of the nutrition strategy an athlete plans to use in competition, with the aim of consuming the necessary fluids and carbohydrates without being limited by bloating, nausea, cramping, or severe discomfort.
Put simply, you would not test your race pace for the first time on competition day. Your race nutrition should not be tested for the first time either.
What is gut training?
Gut training is the planned and repeated intake of food, fluids, and mainly carbohydrates during selected training sessions. The athlete uses the same products, similar quantities, and similar timing to those intended for competition.
The gastrointestinal system can adapt. Repeated carbohydrate intake during exercise may improve tolerance to food and fluids, reduce carbohydrate malabsorption, and decrease gastrointestinal symptoms.
A systematic review of interventions lasting from 4 to 28 days found that repeated carbohydrate feeding protocols may reduce gastrointestinal discomfort. Across two protocols lasting two weeks, average discomfort decreased by approximately 47%. Some studies also reported a reduction in estimated carbohydrate malabsorption. The results were not consistent across all studies, which is why the approach should always be individualised.
What exactly are we training?
We are not training the gut in the same way that we train a muscle. We are repeatedly exposing the athlete to the conditions in which food and fluid need to be consumed during intense movement. Over time, the body becomes more familiar with the demands of competition fueling.
This adaptation may involve four main areas.
1. Better tolerance of larger carbohydrate intakes
The athlete gradually becomes accustomed to consuming a sports drink, gel, or easily digested food while running, using an ergometer, or performing functional movements.
2. A possible improvement in intestinal absorption
Frequent carbohydrate exposure may influence the transport of glucose and fructose through the intestine. The SGLT1 and GLUT5 transporters are possible mechanisms behind this adaptation, although much of the evidence in humans remains indirect.
3. Less malabsorption
When a greater proportion of ingested carbohydrate is absorbed, less remains in the intestinal lumen. This may help reduce water retention, intestinal distension, and the urgent need to stop because of discomfort.
4. Greater tolerance of fullness
The athlete learns to keep running, rowing, performing burpees, or completing wall balls without feeling that fluid or food is sitting heavily in the stomach.
This does not mean that gut training necessarily improves every aspect of gastrointestinal function. Current evidence does not show that it increases gastric emptying in every athlete or that it improves every marker of intestinal integrity.
Why does it matter in HYROX?
HYROX combines 8 kilometres of running with eight functional workout stations. The first published study examining the physiological demands of the event reported an average completion time of approximately 86.5 minutes, with most of the race performed at high to very high intensity.
This creates a specific nutritional challenge. The athlete needs high carbohydrate availability while also having to run, push and pull a sled, complete burpees, row, lunge, and perform wall balls. Tolerance of competition nutrition therefore becomes part of competition preparation itself.
For high intensity efforts lasting longer than approximately one hour, an intake of around 30 to 60 grams of carbohydrate per hour may be appropriate, depending on the duration, intensity, pre competition nutrition, and the individual athlete.
This does not mean that every HYROX athlete needs 60 grams per hour. A fast athlete finishing close to one hour, with an effective carbohydrate strategy before the start, may need much less during the race. The value of gut training is that it allows the athlete to use the strategy that is actually required without unnecessary gastrointestinal risk.
How does it apply to CrossFit?
The application is different in CrossFit. A single 10 or 20 minute WOD clearly does not require 60 or 90 grams of carbohydrate per hour.
Gut training becomes valuable during long training days, double sessions, and competition days with several events. In those situations, the athlete must compete, refuel, digest, recover, and compete again within a few hours.
That is a skill, and skills need practice.
For a competitive CrossFit athlete, gut training does not mean taking a gel during every WOD. It means practicing the strategy that will be needed when three, four, or five events are scheduled on the same day. The athlete already knows what can be eaten between efforts, how much fluid is well tolerated, and how much time is needed before returning to high intensity work.
What can the athlete gain?
1. More carbohydrate with less discomfort
The most important benefit is the ability to gradually increase intake without severe bloating, nausea, cramping, reflux, diarrhoea, or a heavy feeling in the stomach.
2. Better use of competition fuel
The amount an athlete consumes is not the only factor that matters. What also matters is how much can be absorbed and used. Repeated exposure may reduce malabsorption and support blood glucose availability in some athletes.
3. Better support for performance
As exercise intensity rises, carbohydrate becomes increasingly important as a fuel source. Gut training enables the athlete to apply a nutrition strategy designed to support pace, power, and the ability to repeat intense efforts.
4. Greater consistency with the competition plan
A perfect plan on paper has little value if the athlete cannot tolerate it. Through practice, the athlete already knows what to drink, how much to drink, which gel works, which carbohydrate concentration is tolerated, and when food can be consumed between events.
5. Better preparation for a higher competitive level
As the level of competition rises, training volume, double sessions, event frequency, and recovery demands usually rise with it. A gastrointestinal system that can tolerate the required fuel can become an important factor supporting performance.
How is it done in practice?
Gut training should be progressive, specific, and suited to the demands of the event. An athlete should not immediately aim for 90 grams of carbohydrate per hour simply because an elite cyclist or marathon runner uses that amount.
The process can begin during selected longer sessions with 30 to 40 grams per hour. If there is a genuine competition need and tolerance is good, intake can gradually rise to 45, 60, 75, and, in specific cases, 90 grams per hour.
At the same time, the athlete should test fluid volume, intake frequency, gels, sports drinks, soft foods, and different carbohydrate combinations.
At higher intakes, combining glucose or maltodextrin with fructose may allow a greater absorption rate than using large amounts of glucose alone. Even then, very high carbohydrate intakes are not a universal target. Intakes of 90 to 120 grams per hour are mainly relevant to prolonged events and well trained athletes with specific needs. They are not necessary for a typical HYROX race or a short WOD.
Training sessions should resemble competition conditions as closely as possible. The athlete should test a similar intensity, movement pattern, time of day, available products, and recovery time between efforts. The goal is not simply to tolerate a set quantity of carbohydrate during an easy session. The goal is to know that the same strategy remains tolerable when the demands begin to resemble competition.
The most important point
Gut training is not a method for forcing every athlete to consume more carbohydrate. It is a process that allows each athlete to consume what is genuinely needed, when it is needed, without the gastrointestinal system becoming the limiting factor.
In HYROX and competitive CrossFit, the heart, muscles, and energy systems may all be prepared. If the nutrition plan cannot pass through the gut, however, the athlete cannot use it.
The gut is part of competition preparation.
Do not train only for your body to withstand the race. Train it to accept the fuel it will need to finish it.
References
1. Costa RJS, Gaskell SK, Henningsen K, and colleagues. Sports Dietitians Australia and Ultra Sports Science Foundation Joint Position Statement: A Practitioner Guide to the Prevention and Management of Exercise Associated Gastrointestinal Perturbations and Symptoms. 2025.
2. Martinez IG, Mika AS, Biesiekierski JR, and colleagues. The Effect of Gut Training and Feeding Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Medicine. 2023.
3. Jeukendrup AE. Training the Gut for Athletes. Sports Medicine. 2017.
4. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. 2016.
5. Kerksick CM, Arent S, Schoenfeld BJ, and colleagues. International Society of Sports Nutrition Position Stand: Nutrient Timing. 2017.
6. Brandt M, and colleagues. Acute Physiological Responses and Performance Determinants in HYROX: A New Running Focused High Intensity Functional Fitness Trend. 2025.
