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Runner’s Stomach: What Causes It and How to Avoid It

By Molly Knudsen, MS, RDN, April 11, 2022

What causes runner's stomach and how to prevent  it

The scene is all too familiar. You start your run or workout and soon enough you have a stabbing pain in your side, become nauseous, or feel the urge to run to the bathroom. This phenomenon is commonly known as runner’s stomach, although it’s not necessarily specific to runners. 

Let’s dive into what runner’s stomach is, the causes, and what you can do to avoid it.

How to improve gut healthWhat is runner's stomach?

Runner’s stomach refers to the gastrointestinal (GI) distress that occurs during a run or bouts of exercise—resulting in cramping, bloating, nausea, diarrhea, vomiting, and pain. Runner’s trots and runner’s belly are other common names for these symptoms. 

Holley Samuel MEd, RDN, CPT and owner of Fit Cookie Nutrition, says that many of her clients also report a “sloshing feeling in their stomach, urgency to have a bowel movement and emergency bathroom stops while running, and the feeling that they can’t take in any hydration or fuel,” before, during or after a run. 

Runner’s stomach likely earned its name from the sheer prevalence of distance runners experiencing digestive related symptoms—it’s estimated to impact between 30-90% of distance runners, more commonly reported in younger individuals. [1, 2] Both males and females may experience these symptoms. 

“There’s no discernable difference when it comes to [biological] sex and GI upset,” says Stevie Lyn Smith, a board certified sports dietitian. “I, unintentionally, tend to work with more females than males in my practice, but I hear these complaints across the board from my clients.” 

Running isn’t the only type of exercise that can lead to these digestive symptoms, but it is the most documented. Some research suggests that team-based sports, high-intensity anaerobic exercise, and sprinting can all trigger GI issues. But only small, experimental studies have started to investigate those relationships. [3]

While exercise induced GI symptoms are very individualized—ranging from simply being a nuisance to impairing performance—there are commonalities among the causes of runner’s stomach. [3]

 

What causes runner's stomach? 

“There are many potential causes of GI distress in runners,” says Holley. Physiological changes, impact of exercise, posture, food, hydration, and supplements can spur or exacerbate these symptoms. 

Physiological changes from exercise

Some symptoms may be a result of physiologic changes that occur during exercise. Exercise and running can impact: [1, 2]

  • How the gut absorbs and regulates the movement of water and electrolytes between the digestive tract and tissues
  • Circulating levels of gut hormones
  • Blood flow (or lack thereof) to the intestines
  • Slowed gastric emptying (movement of food from stomach to intestines) 
  • Decrease in lower esophageal sphincter tone (AKA leading to increased risk of gastric reflux) 

These changes in gut function appear to be more prevalent during higher- versus lower-intensity exercises. However, more research is needed on the relationship of exercise-induced gut function changes on symptoms and why some people may be affected by them while others aren’t. [1] 

Impact of spot and posture of exercise

The high impact of running on the body physically jostles an already extended torso that can lead to abdominal discomfort (specifically lower GI symptoms like bloating and diarrhea). [1, 2] 

Cycling is a lower-impact endurance based sport, and the position of cyclists bodies are more highly associated with upper-GI symptoms like acid reflux compared to runners. [2, 4]

Food and improper timing around fueling 

Some foods, particularly when eaten too close to the start of exercise, can increase the likelihood of experiencing GI symptoms. 

“Eating foods high in fiber and fat as well as sugar alcohols and/or protein pre-run can contribute to GI symptoms on the run, because these foods take a long time to digest,” says Holley. “Another common nutrition error is poor timing of nutrition while running, such as taking a gel with a carbohydrate rich sports drink, which can be too concentrated in carbohydrates for some runners to have all at once.” 

In addition, taking in gels without any water may also contribute to GI distress, as most gels and chews are designed to be taken with water on the run.”These high-carbohydrate gels and drinks give runners much needed energy quickly. However, too much sugar too quickly can draw excess water into the gut and may lead to diarrhea. [1]

Underfueling 

While improper timing of food can contribute to runner’s stomach symptoms, so too can inadequate food. Stevie notices that, of her clients experiencing digestive symptoms when running, most of them are not meeting their daily energy needs—AKA underfueling

And during the run, research indicates that those who are not used to taking in food and fluid during exercise were twice as likely to develop GI symptoms compared to their fuel accustomed counterparts. [1]

Dehydration

While it may not directly lead to GI symptoms, dehydration during exercise may worsen symptoms that are already present. [1]

Supplements

Some supplements may also trigger digestive distress for individuals. Caffeine, a commonly ingested stimulant and performance enhancer, is associated with nausea during exercise. [Wilson, 2019] In addition, cyclists in a small study cited gut discomfort and higher perceived pain after supplementing with exogenous ketones. [5] 

Other supplements including iron or magnesium also have reported GI side effects. While these supplements are beneficial in maintaining overall health and performance for some, it may take the digestive tract some time to adjust. 

How much water to drink when exercising

Can you treat or prevent GI distress during exercise?

There are several ways you can lower your chances of developing runner's stomach. 

If you think your pain or discomfort stems from exercise intensity, “slowing down to a light jog or walk can help relieve some of the pain—sometimes called stitching—or cramping,” says Stevie. She also adds that, since dehydration can contribute to GI discomfort, runners should aim to stay as hydrated as possible throughout the day and during runs. “Start with sips of water and if you can tolerate that, try to add in sips of a sports drink to keep up on carbohydrates and electrolytes.”

However, the best long term solution is to optimize your nutrition fueling. “Improving runner’s stomach is possible by being intentional about pre-run fuel, timing intra-run nutrition properly, gradually training the digestive system to take on fuel and hydration over time, and avoiding any foods that are known to trigger symptoms before and during runs,” says Holley. “Fueling for your runs is one of the most significant things you can do to get the most out of your training session and prevent injury by providing your body with adequate energy to complete the workout safely and repair muscles.”

However, many people enter a run or workout completely fasted. “I hear from about 75% of my clients when we start working together that they can’t tolerate or can only tolerate small amounts of fuel before their runs,” says Stevie.

So what happens when you physically feel like you can’t fuel? You train yourself how to.

Train your gut to tolerate food and water before exercise 

Just like a runner’s training plan gradually builds mileage over time, the digestive system also needs to be trained to take on fuel, says Holley. “When starting to incorporate pre-run nutrition for morning workouts, choose simple carbohydrate rich foods low in fiber and start with a small volume, perhaps about a 15-20g serving of carbohydrate anywhere from 20-40 minutes pre-run.” For reference, that’s about half of a large banana. From there, you can then begin to work up to 30g to 60 g of carbohydrates with time. 

Holley advises that, “If your workout isn’t right when you wake up, consider reducing the fiber and fat content of the biggest meal before the run (like lunch or dinner) and add in a small carbohydrate rich snack 30-60 minutes pre run. 

 

What to eat before exercise to fuel and avoid stomach discomfort 

If you’re really struggling with taking in any food before a run, Stevie recommends using a sports drink for easily digested pre-workout fuel to provide carbohydrates and a hydration boost. 

Other pre-run foods Holley and Stevie often recommend include:

  • 1-2 graham cracker rectangles
  • 1 applesauce packet
  • 1 gel or 1 serving sports nutrition chews
  • 1 banana (or ½ the banana to start)
  • 2-3 dates
  • 1-2 servings dry cereal
  • 1 small cereal or granola bar (or ½ the bar to start)
  • 1 serving gummy candy
  • English muffin with jelly 

 

Keep this in mind

Experiencing some type of digestive discomfort during exercise, especially high intensity aerobic exercise, is quite common. Symptoms can range from a minor nuisance that resolves itself quickly to performance altering or debilitating symptoms. Avoiding triggers, properly timing fuel, and training your gut to handle that fuel can help you ward off cramping, bloating, diarrhea, and nausea. 

However, there are times when a runner's stomach is actually more than a pesky disturbance during exercise. “If there is persisting GI distress despite implementing sound sports nutrition principles, or if there is significant pain or blood in the stool, consult your doctor to rule out other potential causes of these symptoms,” says Holley.

Personalized testing to improve gut health

InsideTracker and gut health

There’s no easy cure for runner’s stomach. And while working with a dietitian or running coach can help you pinpoint exactly what may be triggering your specific symptoms, you can also take steps to improve your gut health to optimize it for processing and absorbing the pre-workout and recovery fuel you’re taking in: no stool test required. 

InsideTracker’s gut health goal—compatible with the Ultimate and Immunity plans—advises you on how to optimize your gut microbiome using data from blood tests and wearable sleep tracking devices to generate recommendations for foods, supplements, and stress management techniques that have been scientifically shown to support gut function. For example, hsCRP is a blood biomarker for inflammation that has a two-way relationship with gut health. If inflammation levels are elevated, it may be indicative of poor gut health. As part of your gut health goal Action Plan, InsideTracker prioritizes recommendations, such as eating fermented foods, to help improve that biomarker. Getting a blood test every three to six months allows you to track and iterate on how to best manage your gut health based on your body's current needs. 






Molly Knudsen1Molly Knudsen, MS, RDN
Molly is a Content Writer and Team Nutritionist at InsideTracker. As a Registered Dietitian, Molly enjoys connecting people to the food they eat and how it influences their biomarkers. When she’s not writing about the latest nutrition science, she’s likely in the middle of a yoga flow or at the beach with a good book.

References:

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4008808/ 

[2] https://www.uptodate.com/contents/exercise-related-gastrointestinal-disorders#H3 

[3] https://pubmed.ncbi.nlm.nih.gov/28253039/

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4281377/ 

[5] https://pubmed.ncbi.nlm.nih.gov/29109686/