Endurance performance is often limited not by the legs or lungs, but by the digestive system. Many athletes experience stomach discomfort, nausea, or bloating when attempting to increase carbohydrate intake during long training sessions or races. Yet modern sports nutrition research consistently demonstrates that higher carbohydrate availability during exercise can improve endurance performance. For many athletes, the missing component is gut training.
Just as muscles adapt to training stress, the digestive system can adapt to process and absorb higher amounts of carbohydrate during exercise. With progressive exposure, the gastrointestinal system becomes more efficient at transporting nutrients and tolerating fueling strategies during prolonged endurance efforts.
Understanding how to train the gut allows athletes to fuel effectively during long training sessions and competition.
Why the gut limits endurance performance
During endurance exercise, the body relies heavily on carbohydrates as a primary fuel source. However, exercise places significant stress on the digestive system. Blood flow is redirected toward working muscles and away from the gastrointestinal tract. This can slow digestion and increase the likelihood of gastrointestinal distress.
Common symptoms athletes experience include:
- Stomach cramping
- Bloating
- Nausea
- Reflux
- Reduced appetite during exercise
Research led by sports nutrition scientist Asker Jeukendrup has shown that endurance athletes can absorb up to 90 grams of carbohydrate per hour when consuming combinations of carbohydrates that use different intestinal transport mechanisms. However, athletes who suddenly attempt this intake without preparation often experience digestive discomfort.
This is why gut training becomes essential. To understand more about how dual-source carbohydrate systems work at the absorption level, see our article on high-carbohydrate fueling strategy.
What is gut training?
Gut training refers to the intentional practice of consuming carbohydrates during training sessions in order to increase the body's ability to tolerate and absorb them during exercise. With consistent exposure, several adaptations occur:
- Increased intestinal transporter activity
- Improved gastric emptying
- Greater carbohydrate absorption capacity
- Reduced gastrointestinal discomfort
These adaptations allow athletes to tolerate higher carbohydrate intake during exercise without distress. Jeukendrup (2017) demonstrated that the gut is remarkably trainable, with measurable improvements in absorption and comfort occurring within as little as two weeks of structured practice.
Why higher carbohydrate intake matters
Carbohydrate intake during endurance exercise supports performance in several measurable ways. Maintaining carbohydrate availability helps sustain blood glucose levels, delay muscle glycogen depletion, support higher power output over longer durations, and reduce central fatigue that contributes to pacing decline in the later stages of events.
Research consistently shows improved endurance performance when carbohydrate intake increases from 30 to 60 grams per hour toward 60 to 90 grams per hour in longer events (Stellingwerff and Cox, 2014). However, these higher intakes require progressive adaptation. Without gut training, the digestive system simply cannot process the volume efficiently, leading to the familiar pattern of GI failure late in races.
Understanding why osmolality matters for performance is also critical here. The concentration of your fuel solution directly affects how well your gut can process it, even with training.
A practical gut training protocol
Athletes should increase carbohydrate intake gradually across multiple weeks of training. This is not a protocol that can be rushed. The adaptations are physiological, not psychological, and they require consistent stimulus.
Week 1 to 2: Introduce carbohydrates during training
Begin by consuming moderate carbohydrate amounts during longer training sessions. The goal at this stage is not to hit specific intake targets but to establish the habit of fueling during exercise and allow the gut to begin adapting.
- Start with one serving of carbohydrate fuel per hour during sessions longer than 60 minutes
- Dilute the mixture slightly with additional water if needed
- Consume gradually across the session rather than in large boluses
Products such as Hydrax Core can be introduced gradually to allow the digestive system to adapt. The dual-source carbohydrate formulation is designed with gut tolerance in mind, which makes it a practical starting point for structured gut training.
During this stage, comfort and tolerance are more important than hitting precise intake targets.
Week 3 to 4: Gradually increase intake
Once the digestive system tolerates carbohydrates comfortably during training, intake can be increased. The progression should be methodical: adding approximately 10 to 15 grams of carbohydrate per hour each week, while monitoring digestive comfort throughout.
- Slightly increase carbohydrate concentration or volume per hour
- Maintain adequate fluid intake alongside increased carbohydrate load
- Monitor comfort during and after training sessions, noting any symptoms
Progressive exposure allows the gastrointestinal system to upregulate transporter proteins (specifically SGLT1 for glucose and GLUT5 for fructose) and improve the coordination between gastric emptying and intestinal absorption.
Week 5 and beyond: Approach race-level intake
Many endurance athletes ultimately aim for 60 to 90 grams of carbohydrates per hour during long events. By this stage of the gut training protocol, the digestive system has adapted to handle larger carbohydrate loads during exercise.
These higher intake levels support sustained energy availability and improved endurance performance. The key insight is that race-day nutrition should never be the first time you test these intake levels. Every race nutrition plan should have been validated repeatedly in training.
Hydration and gut training
Hydration plays a critical role in carbohydrate tolerance. Insufficient fluid intake can slow gastric emptying and increase gastrointestinal discomfort. When increasing carbohydrate intake, athletes should ensure adequate hydration throughout the session.
Some athletes find that slightly increasing water content early in the gut training process improves tolerance. This dilution reduces the osmolality of the solution reaching the intestine, facilitating absorption and reducing the osmotic stress that can trigger GI symptoms.
Hydration strategies can also be refined using individual data from a sweat rate assessment, which can be estimated using the Hydrax Sweat Loss Calculator. Understanding how much sodium you actually need during exercise further supports the development of a complete fueling and hydration strategy.
Common gut training mistakes
Increasing intake too quickly
Suddenly attempting very high carbohydrate intake can overwhelm the digestive system. The intestinal transporters have finite capacity that increases with training, but cannot be forced beyond their current limits without consequence. Gradual progression allows physiological adaptation to occur without creating negative associations between fueling and discomfort.
Only practicing fueling on race day
Gut training must occur during training sessions, not just during races. The adaptations require repeated exposure over weeks. An athlete who only tests their nutrition plan during competition is essentially conducting an uncontrolled experiment under maximum physiological stress. The outcome is predictable.
Ignoring individual differences
Athletes vary significantly in carbohydrate tolerance due to differences in gut microbiome composition, intestinal transporter density, gastric emptying rates, and exercise intensity thresholds that trigger splanchnic vasoconstriction. Training sessions provide the opportunity to identify individual limits and develop personalised strategies that work reliably under race conditions.
The role of carbohydrate source selection
Not all carbohydrate sources are equally well tolerated during exercise. Research strongly supports the use of multiple transportable carbohydrates, specifically combinations of glucose (or glucose polymers such as maltodextrin) and fructose. These carbohydrate types use different intestinal transporters, allowing total absorption to exceed the limits of either transporter alone.
A ratio of approximately 2:1 maltodextrin to fructose has been shown to optimise absorption rates while minimising GI distress (Jeukendrup, 2010). Products formulated with this ratio in mind offer a practical advantage during gut training, as the reduced osmotic load per gram of carbohydrate supports both absorption and comfort.
Final thoughts
Gut training is one of the most overlooked components of endurance nutrition. By progressively practicing carbohydrate intake during training, athletes can improve digestive tolerance and support sustained fueling during long endurance efforts.
With consistent practice, the digestive system becomes another trainable component of endurance performance. The evidence is clear: athletes who systematically train their gut alongside their cardiovascular and musculoskeletal systems are better prepared for the nutritional demands of competition.
This article is for educational purposes and does not constitute medical advice. Consult a sports nutritionist or physician before beginning any new training or fueling protocol. Individual tolerance to high-carbohydrate intake may vary.
References
- Jeukendrup AE. Training the Gut for Athletes. Sports Medicine. 2017;47(Suppl 1):101-110.
- Jeukendrup AE. Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care. 2010;13(4):452-457.
- Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Applied Physiology, Nutrition, and Metabolism. 2014;39(9):998-1011.
- Costa RJS, Miall A, Khoo A, et al. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism. 2017;42(5):547-557.
- de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine. 2014;44(Suppl 1):79-85.