How to Not Poop During a Marathon, According to the Research
Djokovic threw up his way out of the US Open and said it has been every match this year; Berlin, Chicago and New York are weeks away. What hard exercise does to a gut has a number — 80% less blood — and, unlike most running problems, two controlled trials that tested a fix.
Gut trouble is the marathon's most common companion: in the foundational survey of 471 Belfast City Marathon entrants, 83 percent reported gastrointestinal disturbances during or right after running, 53 percent an urge to have a bowel movement and 38 percent diarrhea. The mechanism is blood flow. Working muscles borrow it from the gut, and in trained men cycling at 70 percent of maximum, portal-vein flow fell 80 percent within an hour; at that level of under-perfusion a marker of intestinal cell injury roughly doubled and the gut became more permeable, and Boston Marathon finishers showed the same marker sharply elevated with symptoms tracking it. Around two hours at 60 percent of maximal effort is the threshold where this becomes significant regardless of fitness, worse in heat and on foot. Pre-race nerves are a separate mechanism: in 44 ultramarathoners, race gut symptoms correlated with pre-race stress and anxiety, and colonic manometry shows an hour of psychological stress increases the colon's propulsive contractions. The stakes are measured: athletes with gut symptoms in competition had sevenfold higher odds of withdrawing. Two interventions have controlled trials. Gut training, two weeks of daily one-hour runs while taking 90 grams of carbohydrate an hour, reduced lower-gut symptoms and cut breath hydrogen from 13 to 6 ppm in 18 runners; six days low-FODMAP reduced daily symptoms, urge and loose stool in 11 runners, though not symptoms during exercise itself. Coffee at 150 milligrams of caffeine stimulates the colon like a 1,000-calorie meal, so take it early. Loperamide is not on the 2026 WADA list but belongs in a clinician conversation. Educational, not medical advice.
Ten days ago at the US Open, Novak Djokovic lost a first-round match for the first time in his career, and the story was not the score. He vomited repeatedly on court, cramped, and afterwards told reporters it was “something that I’ve been carrying, unfortunately, pretty much every match this year” — “vomiting, throwing up, a serious issue there. Then my whole body starts to collapse, basically cramping and pain.” Two weeks earlier in Cincinnati he had gone to his hands and knees mid-match and said the problem is worst “when there’s a lot of humidity and heat.” He has not disclosed a diagnosis, and we are not going to guess at one. But the most famous stomach in sport has put a question in front of a lot of people who are about to run 26.2 miles: what does hard exercise actually do to a gut?
It is a timely question. Berlin is September 27, Chicago is October 11, New York is November 1, and the phrase people type into search in the weeks before a marathon is not “exercise-induced gastrointestinal syndrome.” It is “how to not poop during a marathon.” So here is the research that answers it: how common the problem is, the number that explains the mechanism, and the two controlled trials that actually tested a fix.
The prevalenceEighty-three percent, since 1986
The foundational study is old and blunt. In 1986, researchers sent a questionnaire to every entrant of the Belfast City Marathon and got 471 back. Eighty-three percent said they occasionally or frequently suffered a gut disturbance during or immediately after running. More than half reported the urge to have a bowel movement; 38% reported diarrhea. Nearly a third said it had hurt their performance. And the remedies runners had already invented for themselves, forty years before Reddit, are the same ones people trade today: 35% ran on an empty stomach, and a fifth depended on going before they left the house.
| What 471 marathoners reported (Belfast, 1986) | Share |
|---|---|
| Any gut disturbance during or right after running | 83% |
| Urge to have a bowel movement | 53% |
| Diarrhea | 38% |
| Urge or diarrhea, women only (38 respondents) | 74% / 68% |
| Said it hurt their performance | 29% |
| Already ran on an empty stomach to manage it | 35% |
| Relied on a pre-run bowel movement | 20% |
The problem has since been given a proper name — exercise-induced gastrointestinal syndrome — and a threshold: around two hours of exercise at 60% of maximal oxygen uptake is where gut disturbance becomes significant regardless of fitness, with heat and running (as opposed to cycling) making it worse. A marathon, for almost everyone, is two-plus hours at roughly that intensity, on foot. It is the experiment.
The mechanismThe number is eighty percent
Why does running do this? Because the gut is what the body borrows blood from. In a study of eight trained men cycling at 70% of their maximum, blood flow in the portal vein — the vessel draining the intestines — had fallen by 80% after an hour. The gut runs its digestive and barrier functions on that blood, and at 20% supply things begin to go wrong in measurable ways: in healthy men after an hour of hard cycling, a blood marker of intestinal cell injury roughly doubled and the gut became measurably more permeable, and both tracked how badly the gut had been under-perfused. Costa’s group calls this the circulatory pathway; a second, neuroendocrine pathway runs through stress hormones, and the two arrive together on race day.
An hour of hard exercise cut blood flow to the intestines by 80%. The gut runs on what is left, and a marathon is two to five hours of that.
Real races confirm the lab. Forty finishers of the Boston Marathon showed that same injury marker sharply elevated immediately after the race — at several times the resting values seen in the lab studies — and still elevated a day later, and their gut symptom scores rose from near zero to a clear signal — with the marker and the symptoms correlating. Heat multiplies it: in a study published this year of 56 endurance athletes exercising two hours in 35°C, inflammatory markers explained part of the urge-to-defecate scores, and the authors concluded that symptoms alone probably underestimate what the gut is going through. Which brings us back to a tennis player in New York humidity.
The nervesStart-line stomach is real, and separate
Pre-race urgency is a different mechanism from mid-race urgency, and it also has data. In a study of 44 ultramarathoners, gut symptoms during the race correlated with pre-race stress (a coefficient of 0.50) and anxiety (0.41), and pre-race stress, anxiety and symptoms together explained more than a third of the variance in what happened on course. The physiology is old news to gastroenterologists: in a colonic-manometry experiment, an hour of psychological stress increased propagated contractions — the colon’s forward-moving waves — in healthy volunteers, and the activity stayed high after the stress ended. A start corral is an hour of psychological stress with a countdown. That said, honesty requires one caveat from the largest diary study of runners: across 150 runners logging a month of runs, stress and anxiety correlated with gut distress overall, but the link was weakest, after adjustment, for the lower-gut symptoms specifically. Nerves are part of it. They are not most of it.
The stakesA measured DNF risk
If this reads like a nuisance rather than a performance issue, a 2026 survey of 727 endurance and ultra-trail athletes is the corrective. Athletes who had gut symptoms in training were about four times as likely to have them in competition (adjusted odds ratio 3.96), and athletes with gut symptoms in competition were seven times as likely to withdraw from a race (adjusted odds ratio 7.04), with events over six hours carrying their own added risk. The gut is one of the most common reasons a marathon ends early, and, unlike a hamstring, it is one you can train.
The fixesTwo things that were actually tested
Train the gut. The best-supported intervention has a boring name and a real trial. In an 18-runner study, participants ran an hour a day at 60% of maximum for two weeks while taking 90 grams of carbohydrate an hour — the gel-heavy race intake most people only attempt on race day — or a placebo. The gut-trained group finished with less gut discomfort, fewer lower-gut symptoms, a breath-hydrogen peak (a marker of carbohydrate reaching the colon unabsorbed) that fell from 13 to 6 parts per million, and ran further in a timed effort bout. The review that popularised the idea is literally titled “Training the Gut for Athletes.” The principle: never let race day be the first time your intestines meet your race fuel.
Go low-FODMAP in race week, with a caveat. In a crossover trial of eleven recreational runners, six days on a low-FODMAP diet cut daily gut symptoms substantially compared with six days high-FODMAP, with urge to defecate, loose stool and diarrhea all lower. The caveat matters: symptoms during exercise did not differ between the diets. What the diet fixes is the pre-race state of the bowel, which is a real target — the less fermentable material sitting in the colon at the start line, the less there is to become an emergency at mile eighteen. The same logic underwrites the cuts everyone makes in the last day or two: less fiber, less fat, less protein. Those are consensus practice, and the review literature is candid that controlled evidence for them is lacking. They are sensible. They are not proven.
Use the coffee, then time it. Coffee is a race-morning fixture because it works on the colon: in the classic manometry study, a cup carrying 150 milligrams of caffeine stimulated colonic motor activity comparable to a 1,000-calorie meal — 60% more than water. That is the mechanism behind the 20% of Belfast runners who relied on a pre-run bowel movement, and the reason to take the coffee early enough that its effect is spent before the gun. The full physiology is here.
The pharmacy option, briefly. Runners talk about loperamide, the over-the-counter anti-diarrheal. Two facts and a caveat: it is not on the 2026 WADA Prohibited List, there is no published data on how many runners use it, and slowing a gut that is already under-perfused and dehydrated is a conversation to have with a clinician or sports dietitian, not a start-line improvisation.
The company you keepEveryone, including the champions
If it happens anyway, it has happened to the best. Paula Radcliffe stopped at the roadside during the 2005 London Marathon and won the race. Davis Clarke, the Boston-based creator, ran 2:56 at Boston in 2024 and told his followers exactly what had happened on the way. The physiology above is not a personal failing; it is what 80% less blood does to an organ that was not consulted about the race. Train the gut, empty it before the start, keep the last two days plain, and take the coffee early. And if gut symptoms are showing up in ordinary training runs, that is the strongest predictor in the literature of trouble on race day — and worth a clinician’s look before you taper. This article is educational, describes what the cited studies measured, and is not medical advice.