Why Does Running Upset Your Stomach? Understanding the Runner's Gut

Have you ever mapped out your running route based on bathroom availability? Or felt your stomach churn halfway through a long run? If so, you're not alone.

Digestive issues are incredibly common among runners. Studies suggest that anywhere from 30-90% of endurance athletes experience gastrointestinal (GI) symptoms during training or racing. Symptoms can range from mild discomfort to a race-ending sprint to the nearest porta potty.

The good news? While digestive issues can be frustrating, they're often manageable. Understanding why they happen is the first step toward finding strategies that work for your body.

Digestive issues are common on runners. Understanding them is the first step in prevention.

What Is "Runner's Gut"?

"Runner's gut" is a term used to describe digestive symptoms that occur during or after exercise, particularly running.

Common symptoms include:

• Urgency to have a bowel movement

• Diarrhea or loose stools

• Abdominal cramping

• Bloating

• Nausea

• Reflux or heartburn

• Excessive gas

• Side stitches or abdominal discomfort

Some runners experience symptoms only during long runs or races, while others notice them during shorter efforts as well.

Why Does Running Affect Digestion?

Several factors come together during exercise that can make your digestive system less comfortable.

1. Blood Flow Gets Redirected

During exercise, your body prioritizes delivering blood and oxygen to your working muscles, heart, and lungs. As a result, blood flow away from your digestive tract.

This is a normal physiological response, but it can slow digestion and contribute to symptoms like cramping, nausea, or an upset stomach.

2. The Mechanical Effect of Running

Running involves repeated impact with every stride. Your stomach and intestines are literally bouncing along with you.

For some runners, this mechanical jostling can contribute to urgency, cramping, or the infamous "runner's trots."

3. Exercise Can Speed Up Gut Motility

Running stimulates movement in the digestive tract. As your heart rate and breathing increases this stimulates the natural contractions in your intestinal muscles. While this can be helpful for someone dealing with constipation, it can also create an urgent need to find a bathroom during a run.

4. Nutrition and Hydration Play a Role

What you eat and drink before exercise can influence how your gut feels during exercise.

Some common triggers include:

• Large meals too close to a run

• High-fat foods before exercise

• Very high-fiber meals shortly before running

• Trying new foods on race day

• Inadequate hydration

• Consuming more sports nutrition products than your gut is accustomed to

Importantly, this doesn't mean these foods are "bad" or that you need to avoid them forever. Timing often matters more than the food itself.

Not Every Runner's Gut Is the Same

One of the biggest mistakes runners make is assuming there is a single "perfect" fueling plan.

The truth is that digestive tolerance is highly individual.

One runner may tolerate a bagel and peanut butter before a run. Another may feel better with a banana and sports drink. Neither approach is inherently right or wrong.

Your gut is trainable! Just like you train for a race by increasing miles and doing speed workouts, you also can add nutrition - both quantity and diversity - to your training regimen. Work with your dietitian to develop your nutrition training program.

Eating adequately before during and after training is essential to supporting your gut and training it for more intense or longer endurance events.

Can Under-Fueling Cause GI Problems?

Yes, yes, yes.

Many runners assume digestive issues are caused by eating too much. In reality, not eating enough can also contribute to GI symptoms.

Low energy availability, inconsistent fueling, inadequate fluids and long gaps between meals may increase stress on the body and negatively affect both performance and recovery.

This is one reason I encourage runners to practice fueling before, during, and after exercise rather than viewing food as something that needs to be "earned."

Your body performs best when it has access to adequate energy.

When Should You Seek Additional Support?

While occasional digestive discomfort is common, persistent symptoms deserve attention.

Consider talking with a healthcare provider or registered dietitian if you experience:

• Frequent diarrhea

• Significant abdominal pain

• Blood in the stool

• Unintended weight loss

• Symptoms that interfere with training or daily life

• Ongoing bloating, constipation, or GI distress

  
  

Sometimes runner's gut is simply a fueling issue. Other times, conditions such as IBS, food intolerances, reflux, or other gastrointestinal disorders may be contributing.

The Bottom Line

Digestive issues are common among runners, but they are not something you simply have to "push through."

Runner's gut can result from changes in blood flow, the physical impact of running, hydration, nutrition choices, and individual digestive sensitivity. Understanding these factors can help you identify patterns and develop a fueling strategy that supports both performance and comfort.

In the next article in this series, we'll dive into practical nutrition strategies—including what to eat before, during, and after a run to help reduce GI distress and keep you feeling your best on the road, trail, or track.