Sudden bowel urgency, diarrhoea and, in severe cases, faecal incontinence can occur during hard endurance exercise. The phenomenon commonly called “runner’s gut” is not caused by a single mechanism: intense exercise can redirect blood away from the digestive tract, alter intestinal movement, increase mechanical stress from running, and change how the gut absorbs and handles food and fluid. Heat, dehydration, exercise intensity, diet and psychological stress can add to the problem.
The symptoms are common enough to be a recognised exercise-physiology problem, but they are not inevitable and they should not automatically be dismissed as harmless. Research reviews report gastrointestinal symptoms in roughly 30–50% of endurance athletes, while more serious problems such as gastrointestinal bleeding or ischaemic injury require medical assessment. Running appears particularly likely to provoke lower-GI symptoms because it combines high physiological demand with repeated mechanical movement of the abdominal organs.
What is “runner’s gut”?
“Runner’s gut” is an informal term rather than a single medical diagnosis. It describes the collection of gastrointestinal symptoms that can appear during or shortly after strenuous exercise.
These can include:
- abdominal cramps;
- bloating;
- nausea;
- reflux;
- an urgent need to defecate;
- loose stools or diarrhoea;
- abdominal pain; and
- in extreme cases, loss of bowel control.
The lower-GI symptoms are particularly relevant to runners because exercise can affect intestinal motility and colonic activity. Older research identified urgency and diarrhoea among the most common complaints in runners, while later reviews have continued to identify gastrointestinal distress as a frequent problem in endurance exercise.
Importantly, having gastrointestinal symptoms during exercise does not necessarily mean that an athlete has an underlying gastrointestinal disease. Exercise itself can temporarily change how the digestive system functions.
Why does intense exercise affect the gut?
The digestive system requires substantial blood flow to absorb nutrients, regulate fluid movement and maintain the intestinal lining. During vigorous exercise, however, the cardiovascular system prioritises working muscles, the heart and skin.
The sympathetic nervous system becomes more active, contributing to constriction of blood vessels supplying the digestive organs. Reviews have reported substantial reductions in splanchnic blood flow during prolonged or intense exercise, particularly when dehydration and heat are also present.
This creates a physiological trade-off:
Working muscles need more blood → blood flow to the digestive system can fall → gastrointestinal function becomes more vulnerable to disruption.
At moderate exercise intensity, the body can generally manage this redistribution. As intensity, duration, environmental heat or dehydration increases, the strain on gastrointestinal function can become greater.
Researchers refer to this broader phenomenon as exercise-induced gastrointestinal syndrome (EIGS). Current research suggests that reduced gut blood flow, disruption of the intestinal barrier and changes in gastrointestinal function can interact rather than operating independently.
Why running can trigger diarrhoea and urgency
Running introduces another factor that cycling or swimming largely do not: repeated mechanical movement.
Every foot strike produces movement of the torso and abdominal contents. Over a long or intense run, that repeated mechanical stress can contribute to gastrointestinal symptoms. Research comparing endurance activities has historically found GI complaints to be more frequent during running than during cycling or swimming.
That does not mean the physical bouncing alone causes diarrhoea. Rather, mechanical stress appears to interact with physiological changes in blood flow, intestinal motility and digestion.
This helps explain why an athlete can feel perfectly normal before a race but develop an urgent need to use the toilet once running intensity increases.
What happens to the intestines?
Exercise can alter several aspects of gastrointestinal function simultaneously.
Research has identified changes in:
- intestinal blood flow;
- stomach emptying;
- intestinal transit;
- absorption of nutrients and water;
- intestinal permeability;
- secretion of gastrointestinal substances; and
- intestinal motility.
The direction and severity of these changes vary between athletes and depend partly on exercise intensity, duration, environmental conditions and what has been consumed before and during exercise.
For lower-GI symptoms, increased colonic activity and altered motility can contribute to the sensation that an athlete urgently needs to defecate. If the urgency becomes sufficiently strong and the athlete cannot reach a toilet, accidental bowel leakage can occur.
That is different from saying that intense exercise simply “shakes the bowel empty.” The underlying physiology is considerably more complicated.
How dehydration makes the problem worse
Hydration is one of the most important variables because dehydration reduces circulating blood volume and can compound the cardiovascular strain already created by strenuous exercise.
Research reviews consistently identify dehydration or hypohydration as a contributor to exercise-associated gastrointestinal symptoms. Reduced circulating volume can make it harder to maintain blood flow to the digestive tract while also increasing cardiovascular and thermoregulatory stress.
Heat can compound this effect.
When body temperature rises, blood is also directed toward the skin to support heat loss. At the same time, working muscles require increased blood supply. The digestive system can consequently become a lower priority in the body’s distribution of blood flow.
This is one reason gastrointestinal problems can become particularly troublesome during long, hard events in hot conditions.
Food can either help or trigger symptoms
What an athlete eats before and during exercise can substantially affect gastrointestinal tolerance.
Large meals, high-fat foods, high-fibre foods and certain concentrated drinks can be difficult to tolerate close to intense exercise. Earlier physiological research found that pre-exercise intake of foods rich in fibre, fat and protein, as well as strongly hypertonic drinks, could contribute to upper-GI symptoms.
During exercise, the concentration and type of carbohydrate also matter.
A drink or gel containing more carbohydrate than the intestine can comfortably absorb may leave additional solute in the intestinal tract, potentially contributing to gastrointestinal discomfort. The individual response varies, which is why athletes often need to practise their race-fuelling strategy rather than introducing unfamiliar products on competition day.
Recent systematic-review evidence suggests that appropriate carbohydrate strategies and gradual “gut training” can reduce gastrointestinal symptoms for some endurance athletes, although the evidence does not support one universal nutritional solution.
What does “training the gut” actually mean?
“Training the gut” sounds unusual, but it describes a real area of sports-nutrition research.
The basic idea is to repeatedly expose the gastrointestinal system to the types and quantities of food and fluid an athlete expects to consume during competition. Over time, this can improve tolerance to race fuelling and may reduce symptoms for some athletes.
A 2025 systematic review identified gut-training protocols as promising and found evidence that optimising carbohydrate intake during exercise can reduce the incidence or severity of gastrointestinal symptoms. However, the researchers also emphasised the considerable individual variation between athletes.
The goal is therefore not to make the digestive system immune to exercise. It is to make the athlete’s planned combination of food, fluid and exercise intensity more familiar and tolerable.
Does drinking more water prevent runner’s gut?
Not necessarily.
There is a difference between avoiding dehydration and drinking as much fluid as possible.
Insufficient fluid replacement can increase the risk of gastrointestinal problems, but excessive intake can also cause problems, depending on the circumstances. The concentration of carbohydrates and electrolytes in a drink, the amount consumed, environmental conditions and individual sweat losses all matter.
Sports-nutrition guidance therefore increasingly emphasises individualised hydration rather than a universal volume that every athlete should drink.
The objective is to support fluid balance without creating another gastrointestinal burden.
Can nerves cause bowel urgency?
Yes, psychological stress can interact with gastrointestinal function.
Competition creates a strong sympathetic response, and research into the psychobiology of exercise-related gastrointestinal distress suggests that stress and anxiety can influence gut motility, stomach emptying, blood flow and intestinal sensitivity.
That helps explain why some athletes experience an urgent bowel movement shortly before a race even before the physical exertion begins.
However, the evidence does not support the simplistic idea that race-day bowel problems are merely psychological. Stress is one potential contributor within a much larger physiological system.
Why some athletes are more susceptible
There is no single “runner’s gut” profile.
Risk can be influenced by:
| Factor | How it can contribute to GI distress |
|---|---|
| High exercise intensity | Greater cardiovascular and sympathetic demand can reduce digestive blood flow |
| Long duration | Extends the period during which the gut is exposed to physiological stress |
| Running | Adds repeated mechanical movement to physiological stress |
| Dehydration | Can reduce circulating volume and worsen gut perfusion |
| Heat | Increases thermoregulatory demand and can compound blood-flow competition |
| Food and drink | Fibre, fat, concentrated carbohydrate and unfamiliar fuelling can affect tolerance |
| Stress | Can alter gastrointestinal motility and sensitivity |
| Individual physiology | Athletes differ substantially in susceptibility and nutritional tolerance |
| Existing GI problems | Underlying conditions can make exercise-related symptoms more significant |
These factors can interact. An athlete who tolerates a particular gel during a cool training session may react differently after several hours of racing in heat while dehydrated.
Why bowel control can actually be lost
Bowel urgency and bowel incontinence are not the same thing.
An athlete may experience increased intestinal or colonic activity and suddenly feel an intense need to defecate. Usually, the person can respond by stopping and reaching a toilet.
Incontinence occurs when the urge or bowel movement becomes impossible to control in the circumstances.
Exercise can create a combination of urgency, loose stool, abdominal contractions and mechanical stress. If the athlete is simultaneously running at high intensity and unable or unwilling to stop, the risk of an accident increases.
This is one reason elite athletes can experience the same basic physiological problem as recreational runners despite having exceptionally high fitness levels. Exercise-related GI symptoms are not simply evidence of poor conditioning. Reviews have found them across athletic populations, with strenuous exercise and dehydration among important contributors.
Is runner’s gut dangerous?
Most exercise-related gastrointestinal symptoms are uncomfortable rather than medically dangerous. But that is not true of every episode.
Severe abdominal pain, persistent vomiting, significant dehydration, fainting, blood in the stool, black stools, bloody diarrhoea or symptoms that are unusually severe or persistent warrant medical attention.
Exercise-associated gastrointestinal injury can, in uncommon circumstances, become serious. Reviews have identified conditions including gastrointestinal bleeding and intestinal ischaemia among the potentially serious manifestations of exercise-related GI problems.
An athlete should therefore avoid assuming that every episode of abdominal pain or diarrhoea is simply “runner’s gut.”
Recurrent symptoms outside exercise, unexplained weight loss, persistent changes in bowel habits or repeated episodes of incontinence also deserve medical evaluation rather than repeated self-treatment.
What athletes can do to reduce the risk
There is no guaranteed prevention strategy, but evidence and sports-nutrition guidance point toward several practical principles.
Practise race nutrition
Do not make competition day the first time an athlete consumes a particular gel, drink or large carbohydrate load.
Training provides an opportunity to determine what the individual’s gut tolerates.
Avoid unnecessary dietary experiments before racing
A familiar pre-race meal is generally easier to assess than a new high-fibre, high-fat or unusually large meal immediately before intense exercise.
Build fuelling gradually
Gut-training research suggests that repeated exposure to planned carbohydrate intake can improve tolerance in some athletes.
Manage hydration rather than chasing a universal number
Athletes should consider exercise duration, sweat losses, environmental conditions and the composition of their drinks rather than assuming that more fluid is always better.
Consider intensity
If gastrointestinal symptoms repeatedly appear when training intensity rises, reducing the intensity and investigating the pattern can be useful. Older clinical literature specifically recommends reducing exercise intensity when runners develop significant GI symptoms, followed by a gradual return once symptoms resolve.
Pay attention to patterns
A training diary can reveal relationships between symptoms and:
- particular foods;
- caffeine;
- carbohydrate products;
- hydration;
- heat;
- running intensity;
- race anxiety; or
- training duration.
Individual responses matter because research has not identified one intervention that works for everyone. A 2025 practitioner position statement similarly emphasises individual assessment and tailored strategies.
What about low-FODMAP diets and supplements?
Low-FODMAP diets have attracted attention because some athletes with exercise-related GI symptoms may benefit from reducing certain fermentable carbohydrates.
Recent systematic-review evidence describes low-FODMAP approaches as potentially useful for some athletes, but also notes their restrictive nature. They should not automatically be adopted as a long-term diet simply because an athlete experiences gastrointestinal distress.
Supplements are similarly complicated.
Probiotics, specialised carbohydrate products and other interventions have been studied, but the evidence is mixed. The 2025 systematic review found inconsistent evidence for probiotics and no demonstrated advantage for hydrogel carbohydrate products over standard carbohydrate products.
That makes individual testing and professional dietary guidance more useful than assuming an expensive product will solve the problem.
Why HYROX-style races can be challenging for the gut
HYROX combines running with repeated high-intensity functional exercises. That means athletes are not simply maintaining a steady endurance pace.
They repeatedly accelerate cardiovascular demand, perform heavy movements and return to running.
The general science of exercise-induced gastrointestinal distress applies to this type of competition, although the specific prevalence and mechanisms in HYROX athletes have not been established well enough to claim that HYROX itself produces a particular rate of bowel problems.
What can reasonably be inferred is that a competition combining sustained running with intense functional exercise creates several of the conditions known to challenge gastrointestinal function: high intensity, repeated mechanical movement, substantial cardiovascular demand and, depending on the event environment, heat and dehydration.
That distinction matters. The science supports the underlying physiology, but it would be misleading to turn evidence from endurance running into a precise prediction about an individual HYROX competitor.
The gut is part of athletic performance
“Runner’s gut” is sometimes treated as an embarrassing side effect of racing, but the physiology is more consequential than the nickname suggests.
During hard exercise, the digestive system has to function while competing with muscles, skin and the cardiovascular system for blood flow. At the same time, the gut must process whatever the athlete has eaten and drunk, tolerate mechanical movement and maintain its intestinal barrier.
For most athletes, thoughtful training, appropriate nutrition, hydration and individual experimentation can reduce the problem. The strongest current evidence favours personalised preparation rather than a single miracle food, supplement or hydration formula.
And when symptoms become severe, recurrent or involve bleeding or significant illness, “runner’s gut” should not be used as a diagnosis that ends the investigation. The same exercise that challenges a healthy digestive system can, under certain conditions, reveal a medical problem that requires proper assessment.