Sport & digestion · Evidence-based guide
Intense exercise and your digestive system: when healthy turns harmful
Runner's diarrhea, intestinal ischemia, bleeding and the 2026 finding on polyps in endurance runners. With the real numbers, and without the scaremongering.
Moderate exercise is excellent for your gut. But when the effort becomes very intense and very long, your body takes blood away from the intestine to give it to the muscles. That temporarily damages the intestinal barrier and explains the nausea, vomiting and runner's diarrhea. The threshold sits at around 2 hours of sustained effort, and it happens even if you are extremely fit.
It is nearly always reversible and without consequences. What should not be normalised is bleeding, or pain that does not settle when you stop: that can mean ischemic colitis. And one practical warning almost nobody gives: ibuprofen before running multiplies the damage instead of preventing it.
~2 h
of sustained effort: the threshold from which the gut starts to suffer
1 in 4
runners has digestive symptoms during the race
39 °C
core temperature: above this, the intestinal barrier is always altered
Informational guide. This article is not an argument for giving up training: exercise remains one of the best health decisions there is. It is information to help you train better and know when to seek advice.
I wrote a while ago about the benefits of physical exercise for the digestive system, and I stand by every one of them. But in clinic I see more and more runners, triathletes and trail enthusiasts with digestive symptoms nobody has explained to them, and some with findings that frighten them. Because the curve is not a straight line: there is a point at which exercise stops adding and starts attacking the gut. Let's look at where that point is and what to do about it.
1Part 1 of 6
What happens to your gut when you push hard
It has a name of its own: exercise-induced gastrointestinal syndrome. The systematic review that coined the term sums it up like this: as intensity and duration increase, evidence accumulates of intestinal injury, increased permeability and bacterial products passing into the blood, together with slower gastric emptying, slowed transit through the small intestine and malabsorption. 1
The most useful figure in that review is the threshold: around 2 hours of exercise at 60% of your maximum capacity is the point beyond which the disturbances become significant, and — this is the important part — it happens regardless of your fitness level. Being extremely well trained makes you faster; it does not make you immune. 119
Why does it happen? Through three mechanisms that stack up:
- Blood flow redistribution. The muscle demands blood and the digestive territory gives it up. That hypoperfusion is the main cause of damage to the lining.2
- Heat. A systematic review of 16 studies found a strong correlation between rising core temperature and increased intestinal permeability (r = 0.79), and that going above 39 °C was always associated with a more permeable barrier.320
- Mechanical jolting. Running shakes the viscera in a way that swimming or cycling does not; that is why symptoms are more frequent in running.1
Key idea: it isn't that you have a "bad stomach". It's that your gut is temporarily left without blood supply so your legs can keep going. That is physiology, not weakness.
2Part 2 of 6
Nausea, vomiting and runner's diarrhea
Let's deflate the alarm a little first, because exaggerated figures circulate. In a study of almost 2,000 recreational runners, 26% had digestive symptoms during the race: mainly bloating (18.6%), urge to defecate (17.8%) and stomach pain (16.5%). Symptoms peaked in the middle of the event, and were somewhat more frequent in men. 4 Another study in marathons gave similar numbers: 27% during the race, with nausea in 8%. 5
In other words: it is common, but it does not happen to everybody, and in most people it is a nuisance, not a disease. If your nausea is constant outside sport too, that is something else, and I cover it here: constant nausea.
A surprising nuance: it isn't only what you eat
Almost everyone blames breakfast or the gel. But in the marathon study, no statistically significant correlation was found between what the runners had eaten — before or during — and their digestive symptoms. 5 What did predict race-day symptoms was having had them in training. Food does matter — the other study did find that eating within the 30 minutes before the start increased bloating and urgency4 — but the underlying engine is circulatory, not dietary. That is why switching gels often fixes nothing.
3Part 3 of 6
When blood flow really fails: ischemia and bleeding
Here we move into serious territory. If the lack of blood flow is intense and prolonged enough, the gut does not merely get irritated: it becomes ischemic. The most typical form in athletes is ischemic colitis, presenting with the triad of abdominal pain, diarrhea and bleeding. 2 It is usually mild and reversible, but there are reported cases that required transfusion and, very rarely, surgery. 2 If you want the detail on this condition, I have it covered here: ischemic colitis.
It is neither anecdotal nor exclusive to ultrarunning: there are published cases in amateur marathon runners, 6 in competitive cyclists 7 and even after home-based high-intensity interval training. 8 There is a Spanish publication whose title sums it up better than I can: "running is not always good for you". 9
On bleeding, a 2025 systematic review gathered 51 articles and found 36 reported cases of visible gastrointestinal bleeding in long-distance runners; most were managed conservatively, four needed surgery and one death was recorded. The authors stress that it needs to be studied better. 10 Being uncommon does not make it normal: on the possible causes of blood in the stool, I explain it fully here: causes of blood in the stool.
Key idea: pain and diarrhea during effort are, in a way, a protective warning that forces you to stop. The real risk appears when you ignore them and carry on — which is exactly what the "push through it" culture encourages.
4Part 4 of 6
The most widespread mistake: ibuprofen before running
It is a changing-room habit in many clubs: a preventive anti-inflammatory before the long race. It is exactly the opposite of what helps, and there is an experiment that shows it elegantly. A group of athletes were given ibuprofen before cycling and small intestine damage was measured with a specific marker. The result: damage was clearly greater with ibuprofen + exercise than with exercise alone, ibuprofen at rest, or neither. Intestinal permeability also shot up. 11
The authors' conclusion is among the bluntest I have read in a sports paper: anti-inflammatory use by athletes is not harmless and should be discouraged. 11 A 2024 review in the American Journal of Medicine returns to the same point: combining anti-inflammatories with endurance exercise adds digestive risk. 12 It makes complete physiological sense: exercise is already reducing blood flow to the gut, and the anti-inflammatory blocks precisely the mechanisms that protect the lining. It is stacking two insults on the same tissue.
If you take anti-inflammatories often because of injuries, it is worth reviewing the strategy with your doctor; on gastric protection and its nuances I wrote here: about acid-suppressing drugs.
5Part 5 of 6
Ultrarunning and colon polyps: what the 2026 study says
This is the part circulating most at the moment and the one that needs most care, so I am going to tell it slowly and with its limits attached.
In 2026 a study was published that performed colonoscopy on 94 endurance runners aged 35 to 50 (they had completed at least two ultramarathons of 50 km or more, or at least five marathons) who had not had a colonoscopy in 10 years. The results were striking: adenomas were found in 41.5% and advanced adenomas in 15% (14 people), most of them on the right side of the colon. The benchmark they compared against — asymptomatic average-risk people aged 40 to 49 — is 1.2%. 13
Before you panic: what that study does NOT show
- No cancer was found. Not one.13
- The authors themselves call it "hypothesis-generating", literally, in the title. It is not a study designed to demonstrate causation.13
- It had no comparison group of its own. It was compared with a historical figure from a different population, and the authors warn that differences in symptoms and other risk factors limit direct comparison.13
- These are 94 people from a single centre, who moreover volunteered: it is quite possible that the runners who came forward were mainly those already having symptoms.
So what? The biological hypothesis is coherent: decades of repeated episodes of poor blood flow, mucosal damage and bleeding could leave a mark on the colon, and it fits that the lesions appeared mostly in the right colon, which is the territory with the poorest blood supply. 13 But from a coherent hypothesis to a demonstrated fact is a long way, and it remains true that physical activity reduces colon cancer risk in the general population. The honest position today is: worth investigating, doesn't change your screening.
What should take you to a doctor is the usual: the screening age that applies to you, family history and, above all, persistent symptoms. And if you want the context, colon cancer is indeed rising in young people, for reasons that almost certainly have little to do with running. On what a polyp really means, I explain it in what risk a polyp has of becoming cancer.
6Part 6 of 6
How to train your gut (yes, it can be trained)
The good news is that almost all of this can be modulated. A 2025 systematic review of 29 studies went through the nutritional strategies, and these are the conclusions it supports: 14
| Strategy | Evidence |
|---|---|
| Gut training | Promising. Getting it progressively used to taking carbohydrate while you run improves symptoms over time1415 |
| Adjusting carbohydrate | Works. Following current recommendations on amount and type is associated with fewer and milder symptoms14 |
| Low-FODMAP diet in the days before | May help, but it is restrictive and can cost you in other ways. Not forever14 |
| "Hydrogel" gels | No demonstrated advantage over standard carbohydrate products14 |
| Probiotics | Mixed evidence: neither dismiss them nor count on them14 |
The practical part, in six points
- Hydrate well and watch the heat. This is the most powerful lever: dehydration and hyperthermia are the great amplifiers of the damage.3
- Don't try anything new on race day. Rehearse your nutrition in training, which is also where the gut gets "trained"; for ultra-distance there is even an official nutrition position stand.1518
- Avoid eating in the 30 minutes before, and go easy on fat, protein and fibre before competing.4
- No preventive anti-inflammatories.11
- Build the load gradually. The threshold is a matter of dose: more volume and more heat, more risk.1
- If severe pain appears, stop. Abdominal pain during effort is not something to "push through".2
Myths and facts
The five I hear most from runners in clinic.
Myth: "The more exercise, the better for your health"
Fact: for the gut the relationship is not linear. Moderate exercise improves the intestinal barrier, whereas very prolonged and intense exercise damages it temporarily: it is a dose phenomenon, what physiology calls hormesis.16 The dose makes the poison here too.
Myth: "Runner's diarrhea is about what you ate"
Fact: it contributes, but it is not the engine. In marathon runners there was no significant correlation between intake and symptoms; what best predicted them was already having had them in training.5 The underlying cause is the drop in blood flow.
Myth: "An ibuprofen beforehand prevents discomfort"
Fact: it increases it. With ibuprofen, measured intestinal damage was nearly double that of exercise alone, and the barrier became more permeable.11
Myth: "A bit of bleeding after a long race is normal in athletes"
Fact: it is common, which is not the same as normal. There are published cases that required transfusion, four that ended in surgery and one death.10 Recurrent bleeding must be investigated, athlete or not.
Myth: "Running ultramarathons causes colon cancer"
Fact: that is not demonstrated. The 2026 study found more advanced adenomas than expected, but no cancer, it involved 94 people with no control group, and its authors label it hypothesis-generating.13 It is a signal to investigate, not a diagnosis.
Warning signs: when to stop normalising it
Passing discomfort that settles when you stop is expected. Seek advice if any of these appear:
- Red blood, clots or black stools after exercise, especially if it recurs.10
- Severe abdominal pain that does not settle when you stop, or that worsens over the following hours: that is the pattern of ischemia.2
- Vomiting that stops you rehydrating after the event.
- Fever, marked abdominal distension or feeling generally unwell after an extreme effort.
- Dizziness or fainting with abdominal pain: ischemia and severe dehydration must be ruled out.17
- Anemia or iron deficiency on blood tests with no clear cause. In athletes this is attributed to sport far too quickly, and sometimes it hides something else.
- Unintentional weight loss or a persistent change in bowel habit beyond training.
My approach in clinic
With athletes I take particular care not to fall into either extreme. Neither blaming everything on the sport — "well, you run a lot", which is how diagnoses get missed — nor alarming someone unnecessarily when all they have is a physiological and entirely expected symptom.
In practice: if there is bleeding, it gets investigated, however much you run. If there is anemia or iron deficiency, it gets investigated. And if what there is are bothersome symptoms with no red flags, the work is almost always fine-tuning — hydration, heat, timing of intake, dropping the anti-inflammatories — rather than tests. My aim is never for you to stop training, but for you to train without paying for it.
In-person or online consultation, and colonoscopy if your case calls for it.
Related reading
A personal note
This topic interests me because it runs against everyone's intuition, including my own. We spend our clinics encouraging people to move — and rightly so — so it feels uncomfortable to explain that there is a point at which that same exercise attacks the gut. But the message is not "run less": it is that the body has physiological limits, and that ignoring a pain signal or a bleed because "I'm an athlete" is the worst possible way to look after yourself.
Frequently asked questions
Is running bad for your gut?
No. Moderate exercise is one of the best things you can do for your digestive system and for preventing colon cancer. What changes things is intensity and duration: from around two hours of sustained effort at 60% of your maximum capacity, measurable changes start to appear in the gut, and this happens even in very well-trained people. It is a dose effect, not a switch: going out for a half-hour run has nothing to do with stringing together four hours of running in the heat. 1
Why do I get diarrhea when I run?
Because during intense effort your body diverts blood to the muscles and takes it away from the gut. That lack of blood flow temporarily damages the intestinal barrier, and to this you add the mechanical jolting of running, dehydration and heat. That is why so-called runner's diarrhea does not depend only on what you have eaten: in a study of marathon runners no relationship was found between what they had consumed and their symptoms. Food matters, but the underlying mechanism is circulatory. 5
Can I take ibuprofen before a long race?
It is precisely what you should not do. In a study where athletes were given ibuprofen before cycling, small intestine damage was clearly greater than with exercise alone, and the intestinal barrier became more permeable. The authors concluded, in so many words, that anti-inflammatory use by athletes is not harmless and should be discouraged. If you are in pain, discuss it with your doctor and look for alternatives: a preventive anti-inflammatory before a long race adds risk rather than removing it. 11
If I run marathons, should I have a colonoscopy?
As things stand, running is not on its own a reason to bring your colonoscopy forward. A study published in 2026 found a strikingly high proportion of advanced adenomas in endurance runners, but its own authors describe it as hypothesis-generating: it had no comparison group of its own and no cancer was detected. What should send you to a doctor is having symptoms, above all bleeding that recurs, abdominal pain that does not settle or anemia, and meeting the usual screening criteria for your age or family history. 13
References (clickable)
- Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease. Aliment Pharmacol Ther (2017). PMID: 28589631 (opens in a new tab)
- Moses FM. Exercise-associated intestinal ischemia. Curr Sports Med Rep (2005). PMID: 15763045 (opens in a new tab)
- Pires W, Veneroso CE, Wanner SP, et al. Association Between Exercise-Induced Hyperthermia and Intestinal Permeability: A Systematic Review. Sports Med (2017). PMID: 27943148 (opens in a new tab)
- Zhao X, et al. Gastrointestinal symptoms among recreational long distance runners in China: prevalence, severity, and contributing factors. Front Nutr (2025). PMID: 40771206 (opens in a new tab)
- Pugh JN, Kirk B, Fearn R, et al. Prevalence, Severity and Potential Nutritional Causes of Gastrointestinal Symptoms during a Marathon in Recreational Runners. Nutrients (2018). PMID: 29937533 (opens in a new tab)
- Benmassaoud A, et al. Exercise-induced ischemic colitis in an amateur marathon runner. Endoscopy (2014). PMID: 25314207 (opens in a new tab)
- Mavrelis PG, et al. Exercise-Induced Ischemic Colitis from Competitive Cycling. Curr Sports Med Rep (2021). PMID: 34752429 (opens in a new tab)
- Masterman BJ, et al. Exercise-Induced Ischemic Colitis From Home-Based High-Intensity Interval Training (HIIT). Cureus (2024). PMID: 38586719 (opens in a new tab)
- Horta D, et al. Ischemic colitis in an athlete: running is not always good for you. Rev Esp Enferm Dig (2016). PMID: 27643713 (opens in a new tab)
- Vujasinovic M, et al. Gastrointestinal bleeding in long-distance runners: a systematic review. Eur J Gastroenterol Hepatol (2025). PMID: 39976001 (opens in a new tab)
- Van Wijck K, Lenaerts K, Van Bijnen AA, et al. Aggravation of exercise-induced intestinal injury by Ibuprofen in athletes. Med Sci Sports Exerc (2012). PMID: 22776871 (opens in a new tab)
- Langland JT, et al. The Gastrointestinal Risks of Combining NSAIDs and Endurance Exercise. Am J Med (2024). PMID: 38788824 (opens in a new tab)
- Swain WR, et al. Advanced adenomas among young endurance runners: A prospective hypothesis-generating prevalence study. Cancer Epidemiol (2026). PMID: 42066739 (opens in a new tab)
- Mlinaric J, et al. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. J Int Soc Sports Nutr (2025). PMID: 40650376 (opens in a new tab)
- Martinez IG, et al. The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Med (2023). PMID: 37061651 (opens in a new tab)
- Keirns BH, et al. Exercise and intestinal permeability: another form of exercise-induced hormesis? Am J Physiol Gastrointest Liver Physiol (2020). PMID: 32845171 (opens in a new tab)
- Armstrong LE, et al. Interactions of Gut Microbiota, Endotoxemia, Immune Function, and Diet in Exertional Heatstroke. J Sports Med (Hindawi Publ Corp) (2018). PMID: 29850597 (opens in a new tab)
- Tiller NB, et al. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. J Int Soc Sports Nutr (2019). PMID: 31699159 (opens in a new tab)
- Braschler L, et al. Physiology and Pathophysiology of Marathon Running: A narrative Review. Sports Med Open (2025). PMID: 39871014 (opens in a new tab)
- Lambert GP. Intestinal barrier dysfunction, endotoxemia, and gastrointestinal symptoms: the 'canary in the coal mine' during exercise-heat stress? Med Sport Sci (2008). PMID: 19208999 (opens in a new tab)
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Transparency: this article is not sponsored. The recommendations are based on the evidence cited, and where a finding is preliminary this is stated explicitly.
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