Runner’s diarrhea may occur because exercise can cause the intestines to allow more substances to pass into the bloodstream. Avoiding foods like FODMAPs, protein, and dairy may help.

Runner’s diarrhea, also called “runner’s colitis” and “runner’s trots,” refers to a condition that affects runners during and after a run.

Long-distance runners (those who run 3 miles or more at a given time) are the most likely to experience this. When you have runner’s diarrhea, you experience frequent bowel movements during and right after your run.

About 30% to 90% of endurance runners experience exercise-induced gastrointestinal disturbances, according to one 2014 review of earlier research.

While experts aren’t completely clear on what causes this, there are ways to identify and manage runner’s diarrhea.

Keep reading to learn what experts think may cause runner’s diarrhea, the symptoms and how long they last, and tips for preventing it, including foods and drinks to avoid before a run.

While researchers do not understand exactly what causes runner’s diarrhea, it may be related to changes in intestinal permeability that happen when you’re running or doing other challenging endurance exercise, such as:

  • cycling
  • swimming
  • skiing
  • multisport training

According to a 2017 study, running at a challenging pace for 90 minutes can temporarily damage the small intestine, increasing the substances that pass from it into your bloodstream. However, not all participants experienced digestive symptoms, so it’s unclear if this intestinal permeability is the cause, one of mulitple contributing factors, or unrelated.

Other research from 2023 suggests that endurance exercise may:

  • decrease gastric motility and emptying (the rate at which nutrients pass through your intestines)
  • increase bacterial translocution (the process by which bacteria from the gastrointestinal tract pass through the intestinal wall to other parts of the body, like the lymph nodes, liver, and bloodstream)
  • alter intestinal permeability (affecting the amount of substances that pass from it into your bloodstream)

Together, these factors may cause exercise-induced nutrient malabsorption and contribute to gastrointestinal symptoms.

Actual diarrhea is only one of the symptoms of runner’s diarrhea. Other symptoms you may experience include:

  • gas
  • acid reflux
  • nausea
  • cramping

It’s also possible to have gastrointestinal symptoms from running without having diarrhea.

The symptoms of runner’s diarrhea will typically begin during your workout and may continue in the hours after you’re done running.

Runner’s diarrhea shouldn’t last for more than 24 hours. If you get diarrhea during a run and your loose bowel movements don’t stop, it may be due to another medical condition, such as an infection, food intolerance, or health condition affecting your digestive system.

If you regularly get runner’s diarrhea, you may be able to prevent or manage it so it doesn’t get in the way of your runs.

Taking over-the-counter (OTC) treatments before a run

OTC treatments, such as bismuth subsalicylate (Pepto Bismol) and loperamide (Imodium), might be an option to stall the diarrhea until after your run.

Note that taking these medications on an empty stomach may make you feel ill.

Making changes to your diet before and after a run

There are steps that you can take to decrease your chances of experiencing runner’s diarrhea.

Most of the treatment relates to your diet, particularly what you choose to eat in the hours and days prior to a long run.

In the days before a run, endurance runners in a 2020 survey commonly avoided:

  • meat (32%)
  • milk products (31%)
  • fish/seafood (28%)
  • poultry (24%)
  • high-fiber foods (23%)

However, it’s possible that some of these foods may not bother you and that others may. What foods you avoid before a run can also depend on any food intolerances you may have.

Avoiding tight clothing

You may also consider what you wear while you’re running, as clothing that’s too tight around the midsection can constrict blood flow to your digestive tract and make your symptoms more uncomfortable.

Treating underlying conditions

You might also want to try to determine if you have lactose intolerance or an underlying condition such as irritable bowel syndrome (IBS).

You may be able to prevent runner’s diarrhea by adjusting your diet in the days leading up to a run.

What to avoid eating

According to 2017 research, this can include avoiding:

What to eat before a run

In the two hours before your run is set to begin, try to avoid eating anything besides a quick energy-boosting snack, like whole wheat toast or a banana.

Avoid caffeine of any kind in the time period immediately before you run, as it works as a diuretic. If you tend to have runner’s diarrhea, try cutting back on artificial sweeteners, sugars, and alcohol the evening prior to your run.

Avoid energy gel packs and supplements that are supposed to provide easy, portable “fuel” during a run. Many contain artificial sweeteners and preservatives that can trigger diarrhea.

Above all, always stay hydrated before, during, and after your run. Staying hydrated can help improve your athletic performance and prevent dehydration-related symptoms, like muscle cramps.

If you regularly experience diarrhea that is affecting your ability to run, it’s best to talk with a healthcare professional, such as a primary care physician, dietitian, or a sports medicine specialist.

A sports medicine specialist is a doctor who specializes in health conditions affecting those who play sports who may be able to recommend changes to your running routine that can help reduce bothersome symptoms, including diarrhea.

If you experience diarrhea, you should take steps to prevent for dehydration. Diarrhea causes you to lose more water through your stool than is typical. Along with sweating from running, it may increase your risk of dehydration, which can cause symptoms like:

You can counteract mild dehydration by drinking electrolyte-containing liquids, like certain sports drinks and coconut water.

If you experience symptoms of severe dehydration, you may need urgent medical attention to help you rehydrate using an IV solution.

Symptoms of severe dehydration can include:

Runner’s diarrhea is common and may affect between 30% to 90% of endurance runners.

You can take OTC medications like bismuth subsalicylate (Pepto Bismol) and loperamide (Imodium) to prevent runner’s diarrhea. However, taking either on an empty stomach may cause other digestive symptoms.

The dumping syndrome after running refers to runner’s diarrhea. Endurance running can affect the digestive system and cause malabsorption that results in loose, watery stools during and after running.

By experimenting with what you eat and what time of day you run, you may be able to stop runner’s diarrhea from happening during your runs.

Make sure to drink enough fluids to avoid dehydration.

The sweat you lose during a long run, in addition to diarrhea, can compound your risk of losing too much fluid. If you have tried to change your diet and your running habits and you’re still experiencing runner’s diarrhea, you may need to speak with a healthcare professional, such as a primary care doctor, dietitian, or a sports medicine specialist.