Runner’s stomach is the common, non-medical term for exercise-induced gastrointestinal (GI) distress experienced by runners. Symptoms range from discomfort and nausea to an urgent need for a bathroom, often disrupting training or races. This problem is widespread, affecting an estimated 30% to 90% of long-distance runners during or after a run. Addressing this requires understanding the body’s internal response to exercise, diet, and hydration.
Identifying the Symptoms of Runner’s Stomach
The symptoms are broadly categorized based on where they occur in the digestive tract. Upper GI issues include heartburn, belching, nausea, and sometimes vomiting or upper abdominal pain. These symptoms are common and may be experienced by up to 70% of endurance athletes.
Lower GI symptoms are often the most disruptive, involving abdominal cramping, bloating, and gas. They frequently include diarrhea or a sudden, urgent need to defecate, often called “runner’s trots.” The constant physical jarring of running is particularly linked to these lower intestinal disturbances. These symptoms can persist for several hours after the run is complete.
The Physiological Causes of Exercise-Induced GI Issues
The main physiological cause of exercise-induced GI problems is splanchnic hypoperfusion. During intense exercise, the body redirects blood flow away from the digestive organs toward the active skeletal muscles, heart, and lungs. This redirection can reduce circulation to the gut by as much as 80%, causing temporary oxygen deprivation (ischemia) in the intestinal lining. This reduced blood supply compromises the gut’s function and integrity, leading to symptoms like cramping, nausea, and diarrhea.
The mechanical stress of the running motion further exacerbates this vulnerability. Repetitive, high-impact bouncing physically jostles the internal organs, stimulating gut motility and causing lower abdominal discomfort. This physical trauma, combined with insufficient blood flow, may cause intestinal injury, reflected by increased intestinal permeability. Additionally, the body releases stress hormones, such as catecholamines, during intense exercise, which accelerate digestive tract movement.
Dietary and Hydration Triggers
Certain consumables increase the likelihood of GI distress by adding stress to an already compromised digestive system. Foods high in fiber, such as whole grains and certain vegetables, are difficult to digest quickly and can lead to bloating and cramping if consumed too close to a run. Similarly, high-fat foods slow gastric emptying, remaining in the stomach longer and increasing the risk of digestive upset during exercise.
High-sugar solutions, like certain gels or sports drinks, can worsen symptoms due to their osmotic load. These hypertonic solutions draw excess water into the intestines to dilute the concentration, causing cramping and diarrhea. Beyond nutrition, non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, are irritants that contribute to digestive issues and should be used with caution before a run. Both dehydration and overhydration impair regular gut function.
Actionable Prevention and Management Strategies
One effective strategy involves adjusting the timing and composition of pre-run meals to allow for adequate digestion. A substantial meal should be eaten two to four hours before exercise; closer fueling should consist of a small, easily digestible snack. Pre-exercise meals should be high in carbohydrates but low in fiber, fat, and protein, as these components take longer to process.
Runners can improve gut tolerance through “gut training,” which involves strategically practicing their fueling plan during training runs. This means gradually introducing the specific drinks and foods intended for race day to condition the digestive system. Proper hydration is achieved by consistently sipping water throughout the day, avoiding a large volume immediately before a run, and using electrolyte-infused beverages during longer efforts. If symptoms begin during a run, reducing the pace can immediately lessen physiological demand and decrease mechanical jostling.