Many regular runners know the feeling: your stomach suddenly turns mid-run, cramps appear without warning, or you urgently need to find the nearest toilet. Runner’s stomach, sometimes referred to as runner’s trots, is one of the most common gastrointestinal complaints among both recreational and elite runners. It is rarely dangerous, but it can be very disruptive, both during a run and for hours afterwards.
What is runner’s stomach?
Runner’s stomach is not a medical diagnosis in itself. Rather, it is an umbrella term for gastrointestinal symptoms that occur in connection with running or other forms of endurance exercise.
It is therefore not a single condition, but a group of symptoms, including cramping, nausea, gas and, in some cases, diarrhoea, that often share a common underlying mechanism: the body’s redistribution of blood flow during physical exertion.
What happens in the body when we run?
During intense exercise, the body redirects blood flow away from the gastrointestinal tract towards the working muscles, heart and skin. As a result, the gut may receive considerably less oxygen and nutrients than it does at rest.
At the same time, the digestive system is exposed to repeated mechanical movement with every stride, which may stimulate intestinal motility. Studies have linked this combination of reduced blood flow and mechanical stress to many of the gastrointestinal symptoms commonly experienced by runners.
Symptoms of runner’s stomach and why they can continue after your run
Symptoms vary from person to person, but some of the most common include:
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Abdominal cramps: often in the lower abdomen and may come and go during exercise
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Nausea: particularly during high-intensity exercise or in hot conditions
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Gas and bloating: sometimes appearing relatively early in a run
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An urgent need to use the toilet: one of the more inconvenient symptoms
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Diarrhoea: particularly during or after long-distance running
Many runners also find that their symptoms do not disappear immediately after finishing. They may continue for several hours or, occasionally, into the following day, sometimes described as having “runner’s stomach the next day”.
One possible explanation is that the gastrointestinal tract needs time to restore normal blood flow and motility after strenuous exercise.
Blood in the stool, however, should not be considered a normal part of runner’s stomach and should always be assessed by a healthcare professional.
What causes a runner's stomach?
There is rarely one single cause. Instead, several factors may contribute:
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Reduced blood flow to the gut: one of the key mechanisms described above
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Food before running: meals high in fibre, fat or strong spices may increase symptoms in some people
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Hydration and caffeine: both dehydration and large amounts of caffeine or highly concentrated sugary sports drinks may contribute
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Duration and intensity of exercise: longer and more intense sessions generally increase the risk
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Individual differences: some people simply appear to have a gastrointestinal tract that is more sensitive to physical exertion
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Stress and nervousness: particularly before races, when the body’s stress response can also influence gut function
The role of gut health in runner’s stomach
One area that has received increasing scientific attention in recent years is the gut microbiota.
The intestine is home to trillions of microorganisms that, among other things, help break down components of our diet and contribute to maintaining a functioning intestinal barrier.
Studies have shown that endurance athletes may have a somewhat different gut microbiota compared with sedentary individuals, potentially reflecting differences in diet, lifestyle and repeated exposure to physical training.
However, it is not yet clear whether particular characteristics of the gut microbiota contribute to runner’s stomach, or whether repeated exercise gradually shapes the microbiota over time.
In other words, the relationship is unlikely to be a simple matter of cause and effect. Instead, it appears to involve a complex interaction that researchers are still working to understand.
Runner’s stomach and other gut conditions, what is the connection?
Several of the mechanisms involved in runner’s stomach are also discussed in relation to other gastrointestinal conditions. This does not mean that runner’s stomach is the same as these conditions, but there may be some overlap in symptoms or underlying biological processes.
IBS (irritable bowel syndrome):
Sports medicine research uses the term exercise-induced gastrointestinal syndrome to describe gastrointestinal disturbances associated with strenuous exercise, and there are several parallels with IBS.
Runners with IBS may experience more pronounced gastrointestinal symptoms during exercise than runners without IBS. At the same time, some studies suggest that moderate physical activity, including running, may improve IBS symptoms in certain individuals rather than worsen them. The relationship is therefore more complex than it may initially appear.
Increased intestinal permeability (“leaky gut”):
This is perhaps one of the clearest biological links to strenuous endurance exercise, as it is associated with the same reduction in intestinal blood flow described above.
Research has shown that prolonged or intense endurance exercise can produce measurable changes in intestinal barrier function and permeability.
The term “leaky gut” is commonly used outside scientific literature, but it is not a formal medical diagnosis. Researchers generally refer instead to increased intestinal permeability or changes in intestinal barrier function.
SIBO (small intestinal bacterial overgrowth):
Because symptoms such as gas, bloating and diarrhoea overlap, it is understandable to wonder whether SIBO may contribute to recurrent runner’s stomach.
However, the limited research directly investigating SIBO in runners has not provided evidence that it is a common explanation for exercise-induced gastrointestinal symptoms.
That does not mean that an individual runner cannot have SIBO, but there is currently insufficient scientific evidence to consider it a common underlying cause of runner’s stomach.
Histamine intolerance:
Exercise is associated with increased histamine activity in the body. Histamine is involved in the normal physiological response to exercise, and this should not be confused with an allergic reaction.
In people who believe they are sensitive to histamine, or who have a reduced capacity to metabolise histamine, it has been suggested that exercise-related histamine activity could potentially contribute to symptoms.
However, a direct relationship between histamine intolerance and runner’s stomach has not been established. At present, this remains more of a biological hypothesis than a confirmed explanation for exercise-induced gastrointestinal symptoms.
Overall, runner’s stomach is not always an entirely isolated phenomenon. In some people, the way the gut responds during exercise may also reflect how sensitive the gastrointestinal system is more generally.
Anyone experiencing frequent or severe symptoms may therefore benefit from discussing them with a healthcare professional to rule out other underlying conditions rather than assuming that running itself is always the sole explanation.
Can diet and probiotics help with runner’s stomach?
There is no supplement that can guarantee prevention of runner’s stomach, and claims suggesting otherwise should be treated with caution.
A limited number of studies have investigated whether specific probiotic strains may influence gastrointestinal function in endurance athletes, for example by supporting intestinal barrier function or reducing gastrointestinal symptoms during strenuous exercise.
So far, however, the results are mixed and many studies involve relatively small numbers of participants. It is therefore too early to draw firm conclusions about the effects of individual probiotic strains specifically on runner’s stomach.
For most runners, the strongest practical evidence still relates to basic nutritional strategies. These may include avoiding very large amounts of fibre, fat or caffeine immediately before running, testing new foods, sports drinks and supplements well before race day rather than trying them for the first time during a competition, and maintaining appropriate fluid and electrolyte intake.
Summary
Runner’s stomach is a common but usually harmless problem associated with running and other endurance exercise. Reduced blood flow to the gastrointestinal tract, mechanical movement during running and individual factors such as diet, hydration, exercise intensity and stress can all play a role.
Gut health and intestinal barrier function are increasingly being studied in this context, but research into the specific effects of probiotics on runner’s stomach is still at an early stage.
For people who regularly experience gastrointestinal symptoms during running, reviewing food and fluid intake around training is often a sensible first step. Persistent, severe or unusual symptoms should always be discussed with a healthcare professional.
