Gut Training: Why Your Digestive System Needs Training Too, and How to Handle Stomach Issues on Race Day

Why Endurance Athletes Need to “Train Their Gut”

Many cyclists and runners share a similar experience: during regular training, consuming energy gels and sports drinks is usually fine, but on race day—especially in long-distance, high-intensity events—sudden stomach cramps, nausea, or an urgent need to find a restroom can strike, instantly wiping out all the fitness they’ve worked so hard to build. This phenomenon is quite common in the endurance sports community, and cases of athletes being forced to slow down or even drop out of long-distance events due to gastrointestinal issues are far from rare.

Many people immediately blame “eating the wrong thing” or “unsuitable nutrition products,” but in reality, the more fundamental cause is often that the gut, like muscles and the cardiovascular system, also needs to be trained to improve its capacity. This concept is known as gut training in sports nutrition. This article will explain why the gut needs training, the underlying physiological mechanisms, how to progressively structure gut training in practice, and how to respond if gastrointestinal discomfort does occur during a race.

The Challenges the Gut Faces During Exercise: Physiological Mechanisms

Blood Flow Redistribution: The Gut’s “Resources Are Taken Away”

During exercise, the body prioritizes allocating its limited blood flow to the working muscles and the skin (for heat dissipation), which means the blood flow available to the digestive system decreases significantly. The higher the intensity, the more pronounced this redistribution becomes—which is why the gut’s ability to digest and absorb food during high-intensity exercise is markedly lower than at rest or during low-intensity exercise. This is a widely accepted physiological principle; the actual degree of reduction varies from person to person and changes with exercise intensity.

Mechanical Vibration: Running “Shakes Up” More Problems Than Cycling

Because every running stride involves vertical oscillation of the body, it creates continuous mechanical stimulation on the gastrointestinal tract. This is why runners generally report a higher rate of gastrointestinal discomfort than cyclists over the same duration of exercise. Although cycling involves less vibration, maintaining a forward-leaning position for extended periods compresses the abdomen, which can also place a certain burden on the digestive system—especially during climbing efforts or prolonged time in the drops.

The Challenge of High-Concentration Carbohydrates and Osmolality

To meet the energy demands of prolonged exercise, athletes often consume high-concentration energy gels or sports drinks. If the concentration or ingredient composition of these products is not appropriate, they can cause an osmotic imbalance in the gut, drawing water into the intestinal lumen and leading to bloating, diarrhea, and other discomforts. One of the purposes of gut training is to gradually acclimate the body to the osmolality and absorption patterns of such products, reducing the likelihood of discomfort during exercise.

Emotional Stress and Autonomic Nervous System Effects

Pre-race nervousness can influence gut motility and secretion through the autonomic nervous system. This is why many people “feel the urge to use the restroom from pre-race nerves” or “are especially prone to gastrointestinal discomfort during races,” even when their fueling strategy is identical to regular training. This factor is harder to resolve through gut training alone, but understanding that this mechanism exists helps athletes recognize that “not all gastrointestinal discomfort is the fault of the nutrition products.” Pre-race mental preparation matters just as much.

The Core Principles of Gut Training

The basic logic of gut training is similar to the overload and adaptation principles of muscle training: by repeatedly and progressively exposing the gastrointestinal system to the “stress stimulus” of eating during exercise and consuming carbohydrate-containing products, the body gradually adjusts the secretion efficiency of digestive enzymes, the gut’s absorption capacity for specific sugars, and its tolerance to mechanical vibration. Over time, this enhances the ability to ingest and absorb energy during exercise while reducing the likelihood of gastrointestinal discomfort.

This process cannot be achieved overnight; it requires weeks or even months of consistent practice to gradually build adaptation. Cramming at the last minute (for example, starting intensive eating practice only a week before a race) has limited effectiveness and may even backfire, placing excessive burden on a body that hasn’t yet adapted.

Practical Progressive Methods for Gut Training

Phase 1: Building Baseline Tolerance

For those who rarely or never eat during training, the first step is to start with small amounts and low frequency—for example, trying a small serving of energy gel or sports drink during a moderate-intensity, longer-duration workout and observing the body’s response. The focus of this phase is not to hit a target “quantity” of fueling, but rather to first confirm the body’s basic tolerance to the very concept of “eating during exercise.”

Phase 2: Gradually Increasing Intake Frequency and Total Amount

Once the body shows no significant discomfort with small amounts of fuel, you can begin to gradually extend the duration of fueling, increase the amount per serving, or increase the frequency of intake, giving the body the opportunity to adapt to a fueling rhythm closer to what an actual race requires. It is recommended to keep a training log during this phase, recording the type of fuel, amount, and physical response for each attempt, making it easier to track progress and identify problem areas.

Phase 3: Simulating Race Conditions

During the training block leading up to an important race, schedule several long-distance sessions with a “race simulation” nature, replicating as closely as possible the types of fuel, amounts, timing, and intensity rhythm planned for race day. This allows the body to fully adapt to the entire fueling strategy before the actual race, while also letting the athlete confirm whether the strategy is viable—rather than trying it for the first time on race day.

Reference Timeline for Progressive Gut Training

The timeline in the table above is only a conceptual reference. Actual scheduling should be adjusted based on individual gut sensitivity, race distance, and available training time. Athletes with particularly sensitive guts may need a longer adaptation period.

Core Principle: Test It in Training First, Use It in Racing

The most important practical rule of gut training is: “Never try a new nutrition product or fueling strategy for the first time on race day.” This principle sounds simple, yet it is a major reason many athletes experience gastrointestinal problems during races—they’re drawn to novel products at expo booths, or they follow a friend’s recommendation and try something they’ve never used mid-race, only to have their body fail to adapt in time. Any product—whether energy gel, sports drink, or solid food—should be tested repeatedly in training and confirmed to be well tolerated before being incorporated into a race-day fueling plan.

How to Handle Gastrointestinal Discomfort During a Race

Even with thorough gut training, gastrointestinal discomfort can still occur during a race due to factors such as weather, nervousness, or a disrupted fueling rhythm. Knowing how to respond is equally worth preparing for in advance.

Mental Preparation for Contingencies

Beyond the physiological responses, mental preparation is equally important. Many athletes become anxious the moment they experience gastrointestinal discomfort during a race, worrying that “today is ruined.” This anxiety can, through the autonomic nervous system, further aggravate the gut symptoms, creating a vicious cycle. If athletes can establish the mental framework before the race that “gastrointestinal discomfort is a relatively common occurrence in endurance events and can usually be alleviated by adjusting the pace,” they are more likely to respond calmly when it happens—rather than making worse decisions in a panic, such as forcing down large amounts of water or cramming in more fuel.

Gut Training Priorities for Different Sports

Cycling: Prolonged Posture Compression and Fueling Rhythm

The gastrointestinal challenges in cycling differ from running—mechanical impact is relatively smaller, but maintaining a forward-leaning, low-handlebar position for extended periods creates sustained compression on the abdomen. This is especially relevant when preparing for long-duration rides such as Wuling Challenge or round-island tours, where maintaining posture for hours can have a significant effect on the gut. Additionally, fueling on the bike is relatively easy to execute (you can reach into jersey pockets or frame bags while riding), which also makes cyclists more prone to the trap of “taking in too much at once,” increasing gastrointestinal burden. It is recommended to adopt a strategy of small, frequent intakes during rides, and to use rest stops or downhill sections—when climbing effort is reduced—for major fueling, allowing more blood flow to be available for digestion.

Road Running: Mechanical Impact and Breathing Coordination

Because of the vertical impact of every footstrike, running places more mechanical stress on the gut than cycling. This is why runners, in addition to practicing intake volume and frequency, need to pay special attention to the timing of fueling—for example, avoiding eating during sudden changes in pace (starting sprint, uphill surges), and instead fueling during relatively steady-paced segments while maintaining a regular breathing rhythm to reduce discomfort from intense impact immediately after eating. Runners preparing for full marathons such as the Taipei Marathon or Wan Jin Shi Marathon are advised to repeatedly practice the motions and rhythm of grabbing water and energy gels at aid stations during long training runs, so that on race day these actions become instinctive reactions that require no extra thought.

Triathlon: The Compound Challenge Across Disciplines

Triathletes face more complex gut-training challenges because a single race involves three different body positions and impact patterns—swimming, cycling, and running—meaning the gut must adapt to a wider variety of situations. Common issues include the gut not yet adjusting from the horizontal swimming position when fueling must begin immediately in the cycling leg, or over-fueling during the bike leg leading to discomfort from impact once the run begins. For such athletes, gut training should include simulation sessions that incorporate transitions (such as “brick” workouts that combine cycling followed by running), practicing fueling strategies during the transition phase within the same session, rather than separately practicing bike fueling and run fueling without ever training the body’s response to the connection between the two.

Fuel Product Selection and Its Alignment with Gut Training

Gut training is not simply about “practicing eating”—choosing the right type of fuel products is equally important. There are notable differences in tolerance among athletes to products with different sugar compositions. Some people are more prone to bloating with single-source sugar products (e.g., pure glucose), and switching to products with multiple sugar sources (e.g., glucose and fructose combined) may improve absorption efficiency and gastrointestinal tolerance. This is because the gut uses different absorption pathways for different sugars, and mixed sugar sources can theoretically increase overall absorption efficiency and reduce the likelihood of saturating a single pathway. However, the actual effectiveness of this mechanism also varies individually, and ultimately, repeated testing during training is needed to identify the product type and concentration your body can reliably tolerate, rather than simply applying theory directly on race day.

Beyond sugar composition, the physical form of fuel products (gel-like energy gels, liquid sports drinks, solid foods such as bananas or rice balls) also affects gastrointestinal burden and absorption rate. Liquid fuels are typically absorbed faster and are relatively gentler on the gut, but relying solely on liquid sources during long races may make it difficult to reach sufficient total calorie intake. Solid foods, on the other hand, offer higher caloric density but place a greater digestive burden and require a longer adaptation period. Most experienced endurance athletes adopt a mixed strategy of liquid and solid fuels, and this mixed approach also requires gradual fine-tuning of the right ratio and timing through gut training.

Common Misconceptions and Cautions

Misconception 1: Choosing “Natural” or “Light” Fuel Products Will Prevent Gut Issues

The causes of gastrointestinal discomfort are multifactorial, including concentration, intake rate, exercise intensity, mechanical impact, and emotional stress. Simply switching to products labeled “natural” will not necessarily prevent issues—the key remains whether sufficient gut-training adaptation has been achieved.

Misconception 2: Not Eating at All During a Race Will Avoid Gut Problems

While consuming no fuel at all does avoid gut discomfort caused by fuel products, in long-distance endurance events, insufficient carbohydrate intake can lead to early glycogen depletion and “hitting the wall,” which can significantly degrade performance and even increase the risk of injury. The purpose of gut training is precisely to enable athletes to fuel with confidence and avoid hitting the wall—not to avoid fueling altogether.

Misconception 3: Gut Training Is Only Relevant for Ultra-Distance Athletes

Even in relatively shorter races (such as half marathons or shorter road time trials), as long as the race duration is long enough and intensity high enough, gastrointestinal discomfort can still occur. Gut training has practical value for endurance athletes across all distances—it is not only needed by ultramarathoners or long-distance triathletes.

Special Populations and Medical Warning Signs

The following groups should exercise heightened caution and consult medical professionals before undertaking gut training or addressing exercise-related gastrointestinal symptoms:

• Those with a history of irritable bowel syndrome, inflammatory bowel disease, or other chronic gastrointestinal conditions: Exercise-induced gut symptoms may overlap with symptoms of underlying disease and require a physician’s assessment—general gut-training protocols should not be self-applied.
• Those with known intolerance to specific sugars (such as fructose): Extra attention is needed when selecting fuel product ingredients, and suitable alternatives should be discussed with a nutritionist.
• Pregnant women: Gastrointestinal tolerance during exercise differs from normal conditions, and fueling strategies should be adjusted in consultation with an obstetrician.
• Those with a history of eating disorders: Gut training involves deliberate manipulation of eating behavior; this population should seek professional advice before attempting it to avoid triggering or worsening eating-disorder-related behaviors.

The following situations should be treated as medical warning signs and should not be dismissed merely as “insufficient training”:

• Severe abdominal pain or diarrhea persisting for several days after exercise
• Blood in the stool or noticeably abnormal stool color
• Recurrent severe gastrointestinal symptoms that cannot be improved by adjusting fueling strategies
• Accompanying fever, severe vomiting, or obvious signs of dehydration (such as significantly reduced urine output, extreme thirst, confusion)
• Unexplained weight loss combined with long-term gastrointestinal discomfort

This article provides general knowledge about gut training for endurance sports and cannot replace medical professionals’ diagnosis and evaluation of individual gastrointestinal symptoms. If any of the above warning signs are present, please seek medical attention promptly.

Practical Recommendations for Taiwanese Endurance Athletes

1. Treat the gut as an organ that needs training, just like muscles and the cardiorespiratory system—it requires regular, progressive exposure to improve adaptability.
2. Never try a new fuel product or fueling strategy for the first time on race day—any attempt should first be repeatedly tested during training.
3. Gut training must be progressive: start with small amounts and gradually increase intake volume and frequency, and schedule long training sessions that simulate race conditions before race day.
4. If gastrointestinal discomfort occurs during a race, first try slowing down and reducing fuel concentration, rather than making more aggressive adjustments in a panic.
5. Develop the mental awareness that “gut discomfort is a relatively common occurrence in endurance events” to prevent anxiety from worsening physical symptoms.
6. Do not completely avoid fueling out of fear of gut discomfort—this can lead to more severe glycogen depletion. The purpose of gut training is precisely to make fueling go more smoothly.
7. Athletes with a history of chronic gastrointestinal disease, eating disorders, or those who are pregnant should consult medical professionals before planning gut training and should not self-apply general guidelines.
8. Seek immediate medical attention when warning signs such as bloody stool, severe abdominal pain, fever, or signs of dehydration appear—these should not be attributed solely to training or fueling issues.

Gut training is a frequently overlooked component of endurance sports, yet it can directly determine race success or failure. Rather than treating gastrointestinal discomfort as “bad luck,” it is better to view it as an ability that can be improved through training. Investing time and effort into practicing it will, in the long run, be a preparation task with a very high return on investment.

Additional Considerations for Gut Training in Taiwan’s Climate

Taiwan’s hot and humid summer environment poses an additional challenge to the gastrointestinal system. When exercising in high temperatures, the body diverts more blood flow to the skin for heat dissipation, which means the blood resources available for digestion are more constrained than in cooler weather, potentially increasing the likelihood of gastrointestinal discomfort. For enthusiasts preparing for summer events or training in the scorching heat, it is recommended to pay special attention to the osmolality of nutrition products and intake pacing in gut training arrangements, along with adequate fluid replenishment, to avoid the double burden of overly concentrated nutrition combined with high-temperature conditions.

Although the wet and cold weather brought by the plum rain season and the northeast monsoon places less stress on heat dissipation, the humidity itself may affect some athletes’ gastrointestinal comfort and appetite. If pre-race training happens to coincide with this type of weather pattern, it is also advisable to factor the current environmental conditions into nutrition strategy adjustments, rather than completely replicating the nutrition pacing validated under dry, cool weather.

In addition, the spacing of aid stations and the types of nutrition provided at most long-distance road races and marathons in Taiwan are usually announced in advance in the race brochure. It is recommended to confirm these details before the race and, as much as possible, prepare similar types of nutrition for testing during training. This reduces the risk of having to try unfamiliar products on race day because the items offered at aid stations are completely different from those used in training. If the items at race aid stations differ significantly from what you are accustomed to, consider carrying your own familiar nutrition to ensure you can maintain the training-adapted, body-recognized nutrition pacing throughout the entire race.

Related Reading

• Gut Training: How to Get Your Digestive System Up to Race Intensity
• Gut Training for Running: How to Get Your Gut Used to Digesting Nutrition While Running
• Gut Training for Cycling: Reducing Gastrointestinal Issues in Long-Distance Rides
• Gastrointestinal Issues for Endurance Athletes: Solutions for Runner’s Diarrhea and Cyclist’s Bloating