What Causes Runner's Trots and Research-Backed Solutions
If you plan your runs around where the bathrooms are, this guide explains why running sends you there, and what to change in your food, your timing and your routine to stop it.
Jeff Gaudette, MSUpdated Oct 2, 2026
If you've ever cut a run short to find a bathroom, you're one of the 30 to 50% of endurance athletes who have stomach or bathroom problems when they exercise.
Most runners blame the last thing they ate. Food does matter, but the bigger cause starts with your first steps: within 10 minutes, blood flow to your gut drops about 20%, and on long or hard runs it can drop by 80% or more.
That drop explains the cramping, the urgency and the first-mile bathroom stops, and it points to the fixes that work. In this guide, we'll look at:
- 01Why your gut loses up to 80% of its blood flow on a hard run, and what that does to digestion
- 02How heat, coffee, dehydration, race nerves and ibuprofen make it worse, with the change that removes each one
- 03What every triathlete with race-day cramps had eaten in one study, and when to eat instead
- 04The probiotic strains that gave marathon runners 57% fewer GI symptoms, plus 3 more fixes that work alongside your food
The quick summary
- Runner's trots is the sudden need to use the bathroom during or right after a run, most often in the first few miles.
- Studies suggest 30 to 50% of endurance athletes have stomach or bathroom problems when they exercise.
- The main cause is blood flow. On long or hard runs, blood flow to your gut can drop by 80% or more, so digestion slows, the gut lining gets damaged, and food moves through faster than normal.
- Heat, dehydration, caffeine, race nerves and ibuprofen all add to the problem.
- The fix you can make tomorrow is food selection and timing: what you eat the day before, and how long you wait after eating before you run.
- A daily supplement for runner's trots, with psyllium and probiotic strains tested in runners, adds a second layer on top of those food changes.
Less Blood Reaches Your Gut When You Run
Your digestive system needs a steady blood supply to digest food and move it through at a normal pace. As soon as you start running, your body redirects blood to your working muscles, your heart and your skin.
Researchers have measured how much blood your gut loses.
Blood flow to your gut drops about 20% within 10 minutes of starting to run. A 2012 review of gut blood flow during exercise found that on long or hard efforts it can drop by 80% or more.
That means your intestines are trying to digest your last meal with a fraction of their normal oxygen supply.
Without enough oxygen, the cells lining your intestines start to get damaged. In a 2011 study, healthy men exercised for 60 minutes at 70% of their maximum workload, and a blood marker of damage to the gut lining doubled.
That means an hour at a steady, moderately hard effort is enough to injure the lining of a healthy gut.
A damaged, oxygen-starved gut absorbs less water and moves food through faster than normal. Add the up-and-down jostling of every stride, and you get the cramping, loose stools and sudden urgency that send you looking for a bathroom.
Heat Pulls Even More Blood Away From Your Gut
The longer and harder you run, the more heat your body produces, and hot weather adds to it. To cool you down, your body sends more blood to your skin, which leaves even less for your gut.
A 2017 review found that the higher runners' core temperature climbed, the leakier their gut lining became. Every time core temperature went above 39°C (102°F), gut permeability went up.
Most marathoners get close to that line: 70% of runners reach a core temperature of 38.5°C (101°F) during a marathon.
That means a hot long run or a summer race puts more stress on your gut than the same effort on a cool morning.
If your race is likely to be hot, the 10 to 14 day heat acclimation protocol gets your body used to running in the heat before race day.
A Leaky Gut Lets Bacteria Into Your Blood
You may have heard the term "leaky gut" before. During hard or hot runs, small gaps open between the cells that line your intestines.
Those gaps let endotoxins, fragments of the bacteria that normally live in your gut, pass into your bloodstream.
A 2016 review describes what happens next. Your immune system treats the endotoxins as a threat and starts an inflammatory response, releasing more interleukin-6 as the endotoxin load climbs.
That inflammation is part of what causes the cramping, nausea and urgent need to go during and after hard runs.
We cover the cramping side in more detail in what causes stomach cramps and nausea while running.
Hard or hot runs open small gaps in your gut lining.
Bacterial fragments pass into your bloodstream.
Your immune system responds and releases interleukin-6.
Cramping, nausea and the urgent need to go.
Coffee Sends You to the Bathroom Within Minutes
Coffee contracts your colon (which is what moves food through your digestive system) within minutes after drinking it, which is why the urge to go so often hits on the way out the door.
You can keep the coffee but drink it early enough that the bathroom trip happens at home.
Starting Dehydrated Leaves Less Blood for Your Gut
Starting a run low on fluid leaves less blood available for your gut once your muscles start pulling it away. In a 1990 study, 37.5% of runners had stomach problems when they ran after losing 4% of their body weight in fluid. When the same runners started hydrated, none did.
That means drinking enough the evening before and the morning of a long run protects your gut as well as your pace.
Runners with stomach problems on the run
Rehrer et al., Medicine & Science in Sports & Exercise, 1990
Race Nerves Make Your Stomach Worse on Race Day
If your stomach is worse on race morning than on training runs at the same pace, nerves are part of the reason. In a 1992 survey of distance runners, 43% said they got "nervous" diarrhea before competition.
That means your race-morning routine should be one you've already practiced before long runs, so the only new thing on race day is the race.
Ibuprofen Adds to the Gut Damage Running Causes
Taking ibuprofen before a long run or race to head off soreness increases the gut damage running causes. In a 2012 study, athletes who took ibuprofen before exercise had a gut-damage marker nearly twice as high as when they did the same session without it.
The day before counts too. In a 2006 study of a 160-km race, runners who took ibuprofen the day before and during the race finished with more endotoxin in their blood. Skip it before and during long runs and races.
When to skip ibuprofen
So, How Do You Stop Runner's Trots?
Start with what you eat and when you eat it. It's the change you can make before your next run, and the other fixes in this guide build on it.
You can't stop running from pulling blood away from your gut. What you can change is how much work your gut has to do while that blood is gone.
Food that is still being digested when you start running, and food that is hard to digest in the first place, is what your gut struggles with once blood flow drops.
Researchers have tracked this in race conditions.
A 1992 study of half-Ironman triathletes compared what they ate before the race with the symptoms they had during it. Every athlete who had intestinal cramps had eaten fiber-rich foods in their pre-race meal. Only 10% of the athletes without cramps had.
That means the fiber in your pre-race meal is one of the biggest things you control.
Timing showed up in the same study. Every triathlete who ate within 30 minutes of the start vomited during the swim, because that food was still sitting in their stomach when blood flow dropped.
Some foods are hard on a runner's gut because of the type of carbohydrate they contain. Researchers call these FODMAPs, and they're found in onions, garlic, beans, apples, milk, and sugar-free products sweetened with sugar alcohols.
In a 2018 study, runners with a history of stomach problems on runs ate a low-FODMAP diet for 6 days. Compared with 6 days of high-FODMAP eating, they had less day-to-day urgency, fewer loose stools and less diarrhea.
That means cutting high-FODMAP foods for a few days before a race or key long run can calm your gut before you ever start. A 2017 review of runner's diarrhea reaches the same conclusion, recommending runners avoid fiber, fat, protein, FODMAPs and caffeine before they run.
Everything in this plan works on cause 1: how much food is in your gut when blood flow drops. It does nothing for the heat damage from section 2 or the bacteria that cross your gut lining in section 3, and it only limits what is already moving through loose and fast.
Your takeaway
You can't control how much blood leaves your gut when you run, but you can control how much food is sitting in it. Cut fiber-rich and high-FODMAP foods the day before long runs and races, finish your last full meal 2 to 3 hours before you start, and drink your coffee early enough to use the bathroom at home.
If planning every run this way is too much, just do it before your long runs and races, the runs where a bathroom stop matters most.
Food timing limits how much is in your gut when you start. It can't firm up what is already moving through loose and fast once blood flow drops. Luckily, there is a natural ingredient in MAS Flush that can: psyllium husk.
Psyllium is a different type of fiber from the beans and bran you cut the day before. It's a soluble fiber that absorbs water in your bowel and forms a gel, which firms up loose stools and makes bowel movements more regular.
In a 2001 trial of adults prone to loose, urgent stools, the people taking a fiber supplement had firmer stools and less than half as many accidents as the placebo group.
That means taking psyllium in the evening helps you have a complete bowel movement before your morning run.
Psyllium goes in with your evening capsules.
It absorbs water in your bowel and firms up loose stools.
A complete bowel movement before you head out.
In MAS's in-house study of runners using Flush, 96% reported immediate relief from runner's trots and 74% had less stomach pain during workouts and races.
Food timing does nothing for the bacteria from section 3. When gaps open in your gut lining on hard or hot runs, fragments of those bacteria cross into your blood and set off the inflammation behind the cramping and urgency.
Probiotics add live bacteria to the ones already in your gut. Each strain behaves differently, so a result from one strain tells you nothing about another, and most probiotics were never tested on the stress running puts on your gut.
Researchers have tested specific strains in marathon runners. A 2019 trial gave marathon runners a probiotic with 2 Lactobacillus acidophilus strains and 2 Bifidobacterium strains for 4 weeks before their race. In the last third of the marathon, they had 57% fewer GI symptoms than the placebo group.
That means the runners taking the probiotic lost about half as much pace late in the race, slowing 7.9% over the last third against 14.2% on the placebo.
So we built MAS Flush around the species tested in runners. It contains Lactobacillus acidophilus and Bifidobacterium animalis, 2 of the 3 species used in that trial, plus Bifidobacterium longum.
The same capsule carries 4 digestive enzymes (protease, amylase, lipase and cellulase). They break down the protein, carbohydrate, fat and fiber in your last meal, so less of it is still being digested when blood flow drops.
“I honestly thought needing to go to the bathroom within the first few miles of a run were just something all runners needed to deal with. It’s been working every day for the last 3 weeks.”
That leaves the heat from section 2. Heat further slows the digestive system and significantly increases nausea, stomach pain and GI issues. Luckily, there is an innovative natural ingredient found in MAS Flush that can mitigate this: blackcurrant.
Blackcurrant extract is rich in anthocyanins, plant compounds that help protect the gut lining when you run hot.
A 2025 study had runners do a hot treadmill run after 7 days of blackcurrant or placebo. On placebo, 11 of 12 runners reported stomach symptoms. After 7 days of blackcurrant, 4 did.
In a 2022 study, men ran for an hour in 34°C (93°F) heat after 7 days of blackcurrant extract. Damage to their gut lining was about 40% lower than after 7 days of placebo. In a separate study, blackcurrant was shown to reduce total gastric distress symptoms in runners by 73%.
That means blackcurrant targets the heat damage from section 2, which food timing can't fix.
On long runs and races there's one more source of food in your gut: the gels and sports drinks you take along the way. They land in a gut running on a fraction of its normal blood flow, and the carbohydrate your gut can't absorb stays there, where it can upset your stomach late in the race.
Your gut absorbs glucose through a transporter protein called SGLT1. Certain probiotic strains increase how much SGLT1 your gut makes and how active it is, so more of the carbohydrate you take in gets into your blood.
Researchers have tested this in endurance athletes. In a 2020 trial, 4 weeks of a multi-strain probiotic increased the carbohydrate they burned from 1.87 to 2.20 grams per minute during 2-hour endurance sessions, an 18% increase. For a 4-hour marathoner, that's about 40g of additional usable fuel.
That means more of each gel goes to your muscles as fuel, so your body leans on the carbohydrate you take in and saves more of your muscle glycogen for the end of the race.
That's the second job the probiotic strains in MAS Flush do on race day: fewer stomach symptoms, and more of each gel absorbed.
“Gels gave me horrible stomach cramps on three different occasions so I was desperate to try anything. Race day was the first marathon I had where my stomach wasn’t churning in the final miles.”
“It made me scared to go to group runs or even try out new trails for fear of not having a bathroom nearby. I’ve been using for 3 weeks now and not once have I had to go to the bathroom once I started running.”
“There were times after hard workouts that it would take hours for my stomach to feel normal. MAS Flush changed everything. I’ve been using it for a month and I haven’t had any stomach pain after workouts.”
Introducing MAS Flush
The daily probiotic built for runners' stomachs
- Start runs with an empty gut. Psyllium in the evening helps you have a complete bowel movement before you head out.
- Fewer late-race stomach problems, and a last meal broken down. Probiotic species tested in marathon runners, plus 4 digestive enzymes for protein, carbohydrate, fat and fiber.
- Less gut damage in the heat. Blackcurrant extract, studied in runners on hot treadmill runs.
- More fuel from every gel. Probiotic strains that help your gut absorb more of the carbohydrate you take in mid-race.
Your Runner's Trots Checklist
Here's a chart of each cause of runner's trots and what to change in your training and nutrition.
| Cause | Why it happens | What to do |
|---|---|---|
| Less blood to your gut | Blood flow to your gut can drop 80% or more on long or hard runs | Finish your last full meal 2 to 3 hours before you run |
| Fiber and FODMAPs | Hard-to-digest food is still in your gut when blood flow drops | Cut fiber-rich and high-FODMAP foods the day before long runs and races |
| Heat | More blood goes to your skin, and your gut lining gets leakier above 39°C (102°F) | Run in the cooler part of the day and heat-acclimate before hot races |
| Caffeine | Coffee contracts your colon within minutes after you drink it | Drink it early enough to use the bathroom before you leave |
| Dehydration | Less blood volume leaves less blood for your gut | Start long runs and races hydrated |
| Race nerves | Stress speeds up your gut before you start | Practice your race-morning routine before long runs |
| Ibuprofen | Nearly doubles gut-lining damage during exercise | Skip it the day before and during long runs and races |
If your stomach trouble starts after you take gels or sports drinks mid-run, that's a fueling problem with its own fix: training your stomach to handle gels and fluids.
Frequently Asked Questions
Why do I always need to poop when I run?
Running sends blood away from your gut to your muscles, heart and skin, and on long or hard runs gut blood flow can drop by 80% or more. With less blood, your gut lining gets damaged and absorbs less water, so food moves through faster than normal. The jostling of every stride adds to it, which is why the urge often hits in the first few miles.
Is it normal to need to poop every time you run?
It's common. Studies suggest 30 to 50% of endurance athletes have stomach or bathroom problems when they exercise. Most runners can reduce it by changing what they eat the day before and how long they wait after eating before they run. See a doctor if you notice blood in your stool, weight loss, or symptoms on days you don't run.
What should I eat before a run so I don't need the bathroom?
Keep the day before and your pre-run meal low in fiber, fat and FODMAPs. White rice, pasta, potatoes and lean meat are easier on your gut than beans, bran, big salads, onions, garlic or sugar-free products. In one study of triathletes, every athlete with intestinal cramps had eaten fiber-rich foods in their pre-race meal.
How long should I wait after eating to run?
Finish a full meal 2 to 3 hours before you run. If you need something closer to the start, keep it small and simple, like white toast with jam or a sports drink 30 to 60 minutes before. Food that is still in your stomach when you start is what your gut struggles with once blood flow drops.
Does coffee make runner's trots worse?
It can. Coffee contracts your colon (which is what moves food through your digestive system) within minutes after drinking it, which is why the urge to go so often hits on the way out the door. You can keep the coffee but drink it early enough that the bathroom trip happens at home.
Why is my stomach worse on race day?
Race nerves act directly on your gut, and in one survey 43% of distance runners said they got "nervous" diarrhea before competition. Racing also means running harder than on most training days, which cuts blood flow to your gut even more. Practice your race-morning breakfast, coffee and bathroom timing before your long runs so the routine is familiar on race day.
Does taking ibuprofen before a run cause stomach problems?
Yes. In a 2012 study, athletes who took ibuprofen before exercise had a gut-damage marker nearly twice as high as when they exercised without it. Runners who took ibuprofen the day before and during a 160-km race also finished with more endotoxin in their blood. Skip it before and during long runs and races.
Do probiotics help with runner's trots?
Probiotic strains tested in runners can help. In a 2019 trial, marathon runners who took a probiotic with Lactobacillus and Bifidobacterium strains for 4 weeks had fewer stomach symptoms in the last third of the race and slowed by 7.9%, compared with 14.2% for the placebo group. A probiotic works best on top of the food and timing changes.
Jeff is the co-founder of MAS and a former Olympic Trials qualifier. He's spent the last 15 years breaking down complicated training and nutrition concepts into actionable advice for everyday runners. His writings and research can be found in journals, magazines and across the web.
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