“More marathons are won or lost in the porta-toilets than at the dinner table.”

This memorable quote from marathon legend Bill Rodgers highlights a universal challenge for distance runners: gastrointestinal (GI) issues. Studies estimate that 30–90% of endurance athletes experience some form of stomach trouble during events, depending on conditions.

Symptoms like nausea, cramps, diarrhea, and even bloody stools can derail the best-prepared runner. While some individuals may be genetically predisposed to these problems, adopting the right strategies can significantly reduce their impact.

What is Runner’s Stomach?

A runner’s stomach is not a medical diagnosis but a collection of symptoms that runners commonly report during exercise. These include:

• Diarrhea during a run.
• Abdominal cramping and pain.
• Side stitches (also known as exercise-related transient abdominal pain).
• Each person’s experience may vary, but the causes behind these symptoms often overlap.

How Exercise Can Cause an Upset Stomach

One major culprit is the reduced blood flow to the gut during exercise. At high intensities, up to 80% of the blood that normally supports the gastrointestinal system is redirected to working muscles and organs like the lungs. This lack of oxygen can damage the gut lining, leading to symptoms such as nausea, vomiting, or diarrhea.

However, regular training can improve your gut’s tolerance, as trained athletes experience less blood flow restriction during exercise. Lower-intensity workouts are also less likely to trigger symptoms.

Other factors include:

Acid reflux: High-intensity exercise can weaken the esophageal valve, allowing stomach acid to travel upward.

Posture and impact: Activities like running, with repetitive jostling, may irritate the intestines more than low-impact sports like cycling.

Nutritional Causes of Stomach Problems

What you eat before and during exercise significantly affects GI discomfort. 

For instance:

High-fiber, fatty, or protein-rich foods delay digestion and increase the likelihood of nausea or diarrhea.

Concentrated carbohydrate solutions can draw water into the gut, causing discomfort.

Runners need carbohydrates for energy, but consuming a mix of sugars (e.g., glucose and fructose) is better tolerated, as these are absorbed through different pathways. Proper hydration is essential, as dehydration exacerbates blood flow reduction to the gut.

10 Steps to Manage Stomach Problems While Running

1. Avoid NSAIDs and Aspirin:

Refrain from taking aspirin or NSAIDs (like ibuprofen) before or during training and races, as they can irritate your stomach.

2. Skip High-Fiber Foods Before Running:

While fiber is essential for a healthy digestive system, avoid high-fiber, spicy, or fatty foods the night before and on race day. These can upset your stomach during runs.

3. Limit High-Fructose Foods and Drinks:

Opt for drinks with a mix of fructose and other carbs instead of high-fructose options. These combinations are easier on the stomach.

4. Hydrate When Consuming Carbs:

Pair carbohydrate intake with plenty of water or choose lower-carb-concentration drinks for quick absorption. For single-carb solutions like glucose, stick to 30–60g per hour (6% concentration). For combined fructose-glucose drinks, athletes can tolerate up to 90–105g per hour (8–10% concentration). Avoid solid carbs, which are harder to digest mid-run.

5. Don’t Overdo Carbs for Short Runs:

Carbs improve performance, but rinsing your mouth with a carbohydrate solution can boost performance without ingestion during less than an hour of exercise. Start fully carbo-loaded for best results.

6. Stay Hydrated:

Begin your runs well-hydrated and drink according to your thirst during the race. Avoid both dehydration and overhydration.

7. Practice Nutrition During Training:

Test your fueling strategies in training runs. This helps your gut adapt to digesting food and drink during exercise and highlights foods that may cause issues.

8. Explore Fat Adaptation (With Caution):

Emerging research suggests training your body to rely more on fat for energy during endurance events, reducing the need for carbs. However, more studies are needed before solid recommendations can be made.

9. Try Nitrate-Rich Foods:

Beetroot juice, a natural nitrate source, may help reduce GI discomfort by improving blood flow to the gut. However, natural foods can sometimes cause digestive issues, so test them cautiously.

10. Adjust Training Intensity:

If stomach problems persist, lower your training intensity until your digestive system adapts. Gradually increase the intensity as your stomach becomes more tolerant.

How to Relieve Stomach Pain After Running

The best long-term fix for post-run stomach pain is optimizing your fueling strategy. Train your digestive system to handle fuel and hydration effectively during runs, such as endurance-building.

Tips for Success:

• Start with easily digestible, low-fiber carbs (e.g., half a banana) 20–40 minutes before a morning run. Gradually increase intake as your gut adjusts.
• For runs later in the day, reduce fiber and fat content in earlier meals and have a small, carb-rich snack 30–60 minutes before running.

What are the Expert Insights

Holley Samuel, MEd, RDN, CPT

Holley notes that many runners experience stomach pain or an urgent need for a bowel movement during runs. Others report being unable to tolerate hydration or fuel before, during, or after running.

Stevie Lyn Smith, Sports Dietitian

Stevie observes that GI distress is common in runners regardless of gender. She emphasizes that while running is the most studied exercise causing GI symptoms, other high-intensity activities like team sports and sprinting can also trigger issues.