Runner's Trots: Why Running Upsets Your Stomach

Written and medically reviewed by Dr Zeeshan Afzal (MBBS), Medical Content Lead at Welzo. Last updated: July 2026. This article is for general information and is not a substitute for personalised medical advice.

Runner's trots is the informal name for the cramping, urgency, wind and diarrhoea that hit many runners during or straight after a run. It is not a character flaw, a sign of weak willpower, or something you simply have to accept as the price of endurance training. It is a recognised physiological phenomenon — researchers now group it under the umbrella term exercise-induced gastrointestinal syndrome (EIGS) — and most of the drivers behind it are modifiable. This guide explains exactly why running upsets your stomach, what the research says works, and how to build a race-day routine that keeps you out of the bushes.

Because runner's trots sit at the intersection of digestion, training load and nutrition, the fixes usually start with your baseline gut health. If you are new to this topic, our gut health hub is the best place to orient yourself, alongside our guides to gut health in the UK and gut barrier function. Runners who want targeted support most often look at probiotics, Akkermansia muciniphila, modified citrus pectin powder, Welzo Ultra Purity Berberine and Welzo Ultra Purity TUDCA — we cover where each of these does and does not have evidence further down.

Image: 2021 London Marathon, via Wikimedia Commons, CC BY-SA 4.0.

Table of contents

• What are runner's trots?
• How common are runner's trots?
• Why running upsets your stomach: the five mechanisms
• Dietary triggers that make runner's trots worse
• Who gets runner's trots most often?
• How to prevent runner's trots: an evidence-based plan
• Gut training: teaching your intestines to cope
• Supplements with evidence (and those without)
• What to do when it strikes mid-run
• Recovering after an episode
• When it is not just runner's trots
• Red flags: when to see a GP
• Your four-week runner's trots reset plan
• Frequently asked questions
• References

What are runner's trots?

Runner's trots — also called runner's diarrhoea, runner's stomach, runner's gut or, in older medical literature, runner's colitis — describes gastrointestinal symptoms that appear during or shortly after running. The defining feature is urgency: a sudden, hard-to-ignore need to open your bowels, often accompanied by cramping in the lower abdomen and loose or watery stools.

Clinicians usually divide the symptoms into upper and lower gastrointestinal complaints, because the two groups have partly different causes and partly different fixes.

Common symptoms of runner's trots

If you want a shared vocabulary for describing your stools to a clinician, the Bristol Stool Chart is the standard tool. Runner's trots typically produce type 6 or type 7 stools during or immediately after exercise, with a return to your normal type within a few hours.

Image: Bristol Stool Chart by Cabot Health / Wikimedia Commons, CC BY-SA 3.0.

How common are runner's trots?

Far more common than most runners realise. A widely cited review in Sports Medicine concluded that roughly 30–50% of athletes experience gastrointestinal complaints during exercise, with endurance athletes affected most.⁴ Later work in endurance-specific populations puts the figure higher still, with up to 70% of endurance athletes reporting symptoms such as abdominal pain, gurgling, or diarrhoea during or after intense exercise.⁸

In marathon-specific research from Liverpool John Moores University, around 27% of recreational marathon runners reported moderate or more severe gastrointestinal symptoms during a race, and in a subsequent randomised trial half of the participants reported at least one moderate symptom over the marathon distance.⁷

Two patterns matter for you as a runner. First, symptoms scale with intensity and duration — the harder and longer you go, the more likely you are to be affected. Second, running is worse than cycling or swimming for lower-gut symptoms, which is one of the strongest clues to the underlying mechanism.

Why running upsets your stomach: the five mechanisms

Modern sports gastroenterology frames runner's trots as the visible tip of exercise-induced gastrointestinal syndrome. A landmark systematic review in Alimentary Pharmacology & Therapeutics described two dominant pathways: a circulatory–gastrointestinal pathway (blood diverted away from the gut) and a neuroendocrine–gastrointestinal pathway (nervous and hormonal changes that slow the stomach and alter transit).¹ Layered on top are mechanical, nutritional and psychological factors.

Image: Human digestive system diagram by Mariana Ruiz (LadyofHats) / Wikimedia Commons, public domain.

1. Blood flow is diverted away from your gut

When you run, your body prioritises working muscles, skin and lungs. Blood supply to the splanchnic circulation — the vessels serving the stomach and intestines — falls sharply. In a controlled study published in PLOS ONE, healthy men who exercised for 60 minutes at 70% of maximum workload developed measurable splanchnic hypoperfusion, and their plasma levels of intestinal fatty acid binding protein (I-FABP, a marker of small intestinal cell injury) roughly doubled from around 309 to 615 pg/ml. The degree of hypoperfusion correlated with intestinal injury, and injury in turn correlated with a transient increase in intestinal permeability.²

Translated into plain English: intense running temporarily starves the gut lining of oxygen, causing short-lived damage to the intestinal barrier. The result is impaired absorption of water and nutrients, fluid left sitting in the bowel, and the cramping and urgency you recognise as runner's trots. This is also why the topic overlaps with gut barrier function and the debate over whether "leaky gut" is real — in athletes, transiently increased permeability is one of the best-documented findings in the field.

Why this reverses quickly

The reassuring part is that these changes are transient. Perfusion is restored within minutes of slowing down, and the barrier markers normalise over the hours that follow. That is why most episodes of runner's trots settle the same day.

2. Mechanical jostling of the bowel

Every stride sends an impact through your torso. Over a 10 km run that is roughly 8,000–10,000 repetitions of your abdominal contents being shaken, with the colon repeatedly jarred against surrounding structures. The strongest evidence for this mechanism is comparative: cyclists, who cover similar distances and reach similar intensities without the impact, report substantially fewer lower-gut symptoms than runners. Mechanical agitation is thought to speed colonic transit and reduce the time available for water reabsorption.

3. Nervous system activation and the gut–brain axis

Exercise is a sympathetic ("fight or flight") stimulus, and so is pre-race nerves. Both increase gut motility in the lower bowel while slowing gastric emptying above — a combination that leaves food sitting in the stomach and stool arriving at the rectum faster than usual. This is why many runners find their symptoms are worst at races rather than in training, even at a slower pace. If you notice a strong anxiety component, our guides to the gut–brain connection and the vagus nerve and the gut explain the physiology and the practical countermeasures.

4. Neuroendocrine and hormonal shifts

Prolonged exercise alters circulating levels of cortisol, catecholamines and several gut peptides. The systematic review evidence indicates these changes slow gastric emptying and delay orocaecal transit, which contributes to nausea, fullness and reflux in the upper gut while symptoms accumulate lower down.¹ Heat stress amplifies all of this, which is why symptoms are usually worse in a summer race than a winter one.

5. Carbohydrate malabsorption and osmotic load

If you take in more carbohydrate during a run than your intestinal transporters can absorb — or you drink a solution that is too concentrated — the unabsorbed sugars draw water into the bowel and are fermented by your gut bacteria. That produces gas, distension, cramping and osmotic diarrhoea. Studies using breath hydrogen testing show that malabsorption during endurance running is measurable and, importantly, trainable.⁵ The fermentation side of this is normal microbial behaviour: read more in our explainer on short-chain fatty acids.

Dietary triggers that make runner's trots worse

Physiology sets the stage; what you eat and drink determines whether the curtain goes up. These are the triggers with the most consistent evidence and the most straightforward fixes.

The FODMAP evidence, specifically

Fermentable oligosaccharides, disaccharides, monosaccharides and polyols — FODMAPs — are the same carbohydrates implicated in irritable bowel syndrome. In a randomised, single-blind crossover study of eleven recreationally competitive runners with a history of exercise-associated gut symptoms, six days on a low-FODMAP diet significantly reduced the daily symptom burden compared with a high-FODMAP diet, with the biggest reductions in flatulence, urge to defecate, loose stool and diarrhoea.⁶ A separate crossover trial in sixteen recreational runners published in the Journal of the International Society of Sports Nutrition reported similar improvements in exercise-related symptoms over a seven-day period.⁹

Two caveats matter. The trials were short and small, and a low-FODMAP diet is a diagnostic tool rather than a permanent way of eating — long-term restriction reduces the fermentable fibre your microbes depend on. Use it as a strategic 24–72 hour taper before key sessions and races, then reintroduce. Our guides to the low-FODMAP diet in the UK, the FODMAP food list and FODMAP reintroduction walk through the process properly.

Painkillers deserve a special warning

Many runners take ibuprofen prophylactically before long races. In a controlled crossover study in trained men, taking 400 mg of ibuprofen twice before cycling significantly aggravated exercise-induced small intestinal injury and produced measurable gut barrier dysfunction. The authors concluded that NSAID use by athletes is not harmless and should be discouraged.³ If you regularly rely on anti-inflammatories, read our guide to ibuprofen and stomach damage and discuss alternatives with your GP or physiotherapist.

Who gets runner's trots most often?

• Distance and endurance runners. Symptoms rise sharply beyond about 60–90 minutes of continuous effort.
• Runners doing hard intervals or races. Higher intensity means greater blood flow diversion.
• People with existing IBS or functional gut disorders. Exercise acts as an additional stressor on an already sensitive gut — see our overview of IBS types.
• Younger runners and, in several surveys, women more than men. Hormonal fluctuations across the menstrual cycle appear to modulate gut sensitivity.
• Runners training in heat. Exertional heat stress compounds splanchnic hypoperfusion.
• Newer runners increasing volume quickly. The gut adapts more slowly than the cardiovascular system.

If you train seriously, our dedicated article on gut health for athletes covers the wider picture, including immune function and recovery.

How to prevent runner's trots: an evidence-based plan

Fix your pre-run timing first

This single change resolves symptoms for a large proportion of runners. Aim to finish your last substantial meal 2–4 hours before you set off, and keep it low in fat, low in fibre and moderate in size.

Build a reliable pre-run bowel routine

Consistency beats improvisation. Wake at a similar time before key runs, have your usual warm drink, and allow 20–30 minutes for a bowel movement before leaving the house. A gentle 5–10 minute walk or easy jog often triggers the gastrocolic reflex earlier and on your terms rather than at mile eight.

Hydrate properly, but sensibly

Dehydration measurably worsens the fall in gut blood flow, so starting a run already fluid-depleted is a reliable way to provoke symptoms. Equally, drinking a litre of concentrated sports drink in one go creates its own osmotic problem. Sip to a schedule you have practised, and consider a lower-carbohydrate drink with added electrolytes for hot days.

Rethink fibre timing rather than fibre amount

Fibre is not the enemy: it is fundamental to a diverse microbiome, and long-term restriction is counterproductive. What matters is when you eat it. Keep intake high on rest and easy days — see high-fibre foods in the UK, how to increase fibre and the 30 plants a week target — and reduce it in the 12–24 hours before a hard session.

Do not test anything new on race day

Every gel, drink, breakfast and caffeine dose you intend to use in a race should have been rehearsed at race pace at least twice in training. This is the most common preventable cause of race-day gut disasters.

Gut training: teaching your intestines to cope

The gastrointestinal tract is adaptable. Repeatedly exposing it to carbohydrate during exercise upregulates intestinal sugar transporters and improves tolerance — an approach sports scientists call "gut training".

In a two-week repetitive gut-challenge study published in the Scandinavian Journal of Medicine & Science in Sports, runners who consumed carbohydrate during exercise sessions across a fortnight showed reduced gastrointestinal symptoms and reduced carbohydrate malabsorption compared with placebo, alongside improved performance in an effort bout.⁵ A systematic review of gut-training and feeding-challenge protocols found that gut discomfort fell by an average of around 47% with two-week repetitive carbohydrate feeding protocols, and carbohydrate malabsorption fell by 45–54% in the studies that measured it.¹⁰ Not every trial has found benefit — in already elite athletes the effects appear smaller — but the direction of travel is consistent.¹¹

A simple gut-training progression

1. Weeks 1–2: take 30 g of carbohydrate per hour on your long run, with 400–500 ml of water per hour.
2. Weeks 3–4: increase to 45 g per hour, using a glucose plus fructose blend rather than glucose alone.
3. Weeks 5–6: progress towards 60–70 g per hour if you are racing beyond two hours, always at race pace.
4. Throughout: record symptoms on a simple 0–10 scale after each session so you can see the trend rather than relying on memory.

Image: Marathon runners by Chris Brown / Wikimedia Commons, CC BY 2.0.

Supplements with evidence (and those without)

No supplement replaces the timing, hydration and gut-training work above. Used sensibly, however, a few have supportive data in athletes. Always check interactions with any medication you take, and speak to a pharmacist or GP if you are unsure.

Probiotics

This is the most studied supplement category in runners. In a randomised controlled trial of 24 recreational marathon runners, 28 days of a multi-strain probiotic (25 billion CFU of Lactobacillus acidophilus CUL60 and CUL21, Bifidobacterium bifidum CUL20 and Bifidobacterium animalis subsp. lactis CUL34) reduced the severity of gastrointestinal symptoms in the final third of the race compared with placebo. The probiotic group also slowed less: their average speed dropped by 7.9% from the first to the last third of the race versus 14.2% in the placebo group.⁷ A three-month trial in long-distance runners reported reduced symptom incidence and better self-rated health, though systematic reviews caution that trials remain small and heterogeneous.⁸

Practical points: start at least four weeks before a target race, not the week of it; choose a product that states the specific strains and CFU count at end of shelf life. Our guides to the best probiotics in the UK, how to choose a probiotic, when to take probiotics and how long probiotics take to work cover the detail, and you can browse the full Welzo probiotics range. Runners travelling to races abroad may also want to read about probiotics for traveller's diarrhoea and Saccharomyces boulardii, and about probiotic safety before starting.

Akkermansia muciniphila

Akkermansia muciniphila lives in the mucus layer that protects the intestinal lining, and interest in it has grown because of its relationship with barrier integrity. Evidence in runners specifically is still limited, so treat it as a barrier-support option rather than an acute anti-diarrhoeal. See Welzo Akkermansia and our comparison of Akkermansia versus conventional probiotics.

Soluble fibre and pectin

Soluble fibres form a gel that can firm stool consistency, which is why some runners with persistently loose stools find them useful on rest days. Citrus pectin powder is one such option; introduce it slowly and away from key sessions, since any fermentable fibre can increase wind initially. Our comparison of psyllium versus inulin explains which fibre types tend to be better tolerated.

Peppermint oil and gut-directed options

Enteric-coated peppermint oil has reasonable evidence for abdominal pain and cramping in IBS, and some runners with a cramp-dominant pattern find it helpful between sessions. Read our review of peppermint oil capsules for IBS and our roundup of the best supplements for IBS in the UK.

Mucosal support: zinc carnosine, glutamine and colostrum

These are studied in the context of intestinal barrier integrity rather than runner's trots as such. Evidence in athletes is preliminary and effect sizes are modest. If you want to explore them, our guides to zinc carnosine, glutamine versus collagen for the gut and colostrum set out what is and is not established.

Berberine and TUDCA: a note on appropriate use

Berberine is primarily studied for metabolic outcomes such as blood glucose and lipids, and it has meaningful drug interactions — see berberine interactions. It is not a treatment for runner's trots and can itself cause gastrointestinal upset. TUDCA is a bile acid derivative studied mainly for liver and bile-flow support; runners whose symptoms centre on pale, urgent, post-meal diarrhoea should read about bile acid malabsorption and see a GP for testing rather than self-treating, and should also review TUDCA side effects.

What to do when it strikes mid-run

1. Slow down immediately. Dropping the pace restores splanchnic blood flow within minutes and frequently settles cramping without a toilet stop.
2. Walk for two to three minutes. Deep, slow nasal breathing shifts the balance towards parasympathetic activity and relaxes the abdominal wall.
3. Stop taking on carbohydrate temporarily. Adding more sugar to an already struggling gut worsens the osmotic load. Take plain water instead.
4. Use the toilet if one is available. Trying to hold on usually means running slower and more uncomfortably for the rest of the session.
5. Do not take an anti-diarrhoeal to push through a race. Masking urgency while continuing to run hard risks dehydration and, rarely, more serious ischaemic complications.
6. Rehydrate with an oral rehydration solution afterwards rather than plain water alone if you have had significant diarrhoea.

Recovering after an episode

Most episodes resolve within a few hours to two days. Replace fluid and electrolytes, eat simple low-fibre foods for the first 24 hours if your gut still feels raw, then return to your normal varied diet as soon as you comfortably can — prolonged bland eating slows microbial recovery. Our guide to recovering after a stomach upset covers reintroduction sensibly, and the best foods for gut health plus fermented foods support the rebuild.

Use the episode as data. Write down what you ate, when, the temperature, your pace and your symptoms. Patterns emerge within three or four episodes far more reliably than through recall.

When it is not just runner's trots

Exercise can unmask a condition that was already there. If your symptoms occur off the run as well as on it, runner's trots is unlikely to be the whole story.

Red flags: when to see a GP

Runner's trots is usually benign. The following, however, should always be assessed by a doctor rather than managed with training tweaks:

• Blood in your stool, or black, tarry stools
• Diarrhoea persisting more than seven days, or vomiting for more than two days
• Unintentional weight loss
• Severe or persistent abdominal pain, or pain that wakes you at night
• Fever alongside gut symptoms
• Symptoms that persist on non-running days for three weeks or more
• Signs of dehydration such as very dark urine, dizziness or passing little urine
• New bowel symptoms if you are over 50, or at any age with a family history of bowel cancer

The NHS advises contacting a GP if you notice a change in bowel habit or blood in your poo lasting three weeks or more.¹³ Blood in the stool during or after running is occasionally caused by transient ischaemic colitis, and NSAID use increases that risk — it always warrants assessment rather than self-management. Our guides on when to see a GP about stomach symptoms and bowel cancer screening in the UK explain the pathways.

Your four-week runner's trots reset plan

If symptoms persist at the end of four structured weeks, that is the point to involve your GP or a sports dietitian rather than to escalate supplements. A gut microbiome test or a stool test may occasionally add information, but neither replaces a clinical assessment.

Image: Runner, via Wikimedia Commons, CC BY-SA 2.0.

Frequently asked questions about runner's trots

What are runner's trots?

Runner's trots are gastrointestinal symptoms — most often cramping, urgency, wind and diarrhoea — that occur during or shortly after running. They are part of what researchers call exercise-induced gastrointestinal syndrome, driven mainly by reduced blood flow to the gut, mechanical jostling of the bowel, and nervous system activation during exercise.

Why does running make me need to poo?

Three things happen at once: blood is diverted away from your intestines to your muscles, every stride physically agitates your colon, and sympathetic nervous system activity speeds transit in the lower bowel. Together these reduce water reabsorption and increase colonic activity, producing the sudden urge to open your bowels.

How do I stop runner's trots?

Finish your last substantial meal 2–4 hours before running, keep that meal low in fat and fibre, reduce high-FODMAP foods for 24–48 hours before hard sessions, start well hydrated, dilute concentrated sports drinks, avoid NSAIDs, and practise gut training with in-run carbohydrate. Most runners who apply all of these consistently see a substantial improvement within four to six weeks.

Should I run on an empty stomach to avoid runner's trots?

Running fasted helps some people with upper-gut symptoms such as nausea and reflux, but it does not reliably prevent lower-gut urgency, and it limits the fuelling you need for longer or harder sessions. Adjusting the timing and composition of your pre-run meal is usually a better strategy than skipping it.

Do probiotics help runner's trots?

There is supportive but not conclusive evidence. A randomised trial in marathon runners found that 28 days of a multi-strain probiotic reduced symptom severity in the final third of the race and was associated with better pace maintenance. Start at least four weeks before a target event, and choose a product that names its strains and CFU count.

Is runner's trots the same as IBS?

No, though the symptoms overlap and the two often coexist. Runner's trots is triggered by exercise and settles afterwards. IBS produces symptoms independent of exercise, on rest days as well as training days. If your gut is unsettled when you are not running, see your GP for assessment.

Can dehydration cause runner's trots?

Yes. Dehydration worsens the exercise-induced reduction in blood flow to the gut, which increases intestinal stress and symptoms. It also creates a vicious circle, because diarrhoea causes further fluid loss. Start every run well hydrated and drink to a rehearsed schedule in longer sessions.

Why do I get diarrhoea after running but not during?

Reperfusion is the likely explanation. When you stop, blood floods back into the intestinal circulation and normal motility resumes, often with a rush. Fluid that accumulated in the bowel during the run is then moved along quickly. This post-run pattern is common and usually settles within a few hours.

Should I take Imodium before a race?

Routinely using loperamide to push through a race is not advisable. It masks urgency while you continue to stress an already compromised gut, and it does not address the cause. If gut symptoms are severely affecting your racing, speak to a GP or sports physician rather than self-medicating before every event.

When should I worry about runner's trots?

See a GP if you notice blood in your stool, black or tarry stools, unintentional weight loss, fever, severe or night-time abdominal pain, symptoms lasting three weeks or more on non-running days, or diarrhoea persisting beyond a week. These features are not typical of runner's trots and need proper assessment.

References

1. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome — implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics. 2017;46(3):246–265. PubMed
2. van Wijck K, Lenaerts K, van Loon LJC, et al. Exercise-induced splanchnic hypoperfusion results in gut dysfunction in healthy men. PLOS ONE. 2011;6(7):e22366. Full text
3. van Wijck K, Lenaerts K, van Bijnen AA, et al. Aggravation of exercise-induced intestinal injury by ibuprofen in athletes. Medicine & Science in Sports & Exercise. 2012;44(12):2257–2262. PubMed
4. de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine. 2014;44(Suppl 1):79–85. Full text
5. Miall A, Khoo A, Rauch C, et al. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports. 2018;28(2):630–640. Full text
6. Lis DM, Stellingwerff T, Kitic CM, Fell JW, Ahuja KDK. Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Medicine & Science in Sports & Exercise. 2018;50(1):116–123. DOI
7. Pugh JN, Sparks AS, Doran DA, et al. Four weeks of probiotic supplementation reduces GI symptoms during a marathon race. European Journal of Applied Physiology. 2019;119:1491–1501. Full text
8. Smarkusz-Zarzecka J, Ostrowska L, Leszczyńska J, Cwalina U. Effect of a multi-strain probiotic supplement on gastrointestinal symptoms and serum biochemical parameters of long-distance runners: a randomized controlled trial. International Journal of Environmental Research and Public Health. 2022;19(15):9363. Full text
9. Effect of a short-term low fermentable oligosaccharide, disaccharide, monosaccharide and polyol (FODMAP) diet on exercise-related gastrointestinal symptoms. Journal of the International Society of Sports Nutrition. 2019;16:1. Full text
10. 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;53(6):1175–1200. Full text
11. Gaskell SK, Rauch CE, Costa RJS. Gastrointestinal assessment and therapeutic intervention for the management of exercise-associated gastrointestinal symptoms: a case series translational and professional practice approach. Frontiers in Physiology. 2021;12:719142. Full text
12. NHS. Diarrhoea and vomiting. National Health Service, UK. nhs.uk
13. NHS. Bowel cancer — symptoms. National Health Service, UK. nhs.uk
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16. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. Journal of the International Society of Sports Nutrition. 2025. Full text

About the author

Dr Zeeshan Afzal (MBBS) is a practising physician and Medical Content Lead at Welzo, with a special interest in digestive health, preventive medicine and evidence-based supplementation. He writes and clinically reviews Welzo's gut health content to ensure it reflects current UK guidance and peer-reviewed research.

Medical disclaimer

This article is intended for general information only and does not constitute medical advice, diagnosis or treatment. Supplements are not a substitute for a varied diet, appropriate training or prescribed medication. Speak to your GP, pharmacist or a registered sports dietitian before starting a new supplement, particularly if you are pregnant or breastfeeding, take prescription medicines, or have an existing gastrointestinal condition. If you experience blood in your stool, unexplained weight loss, persistent pain or symptoms lasting three weeks or more, contact your GP or call NHS 111.