“Runner’s stomach” is a common term used to describe a variety of gastrointestinal (GI) issues that affect endurance athletes, particularly runners. Estimates suggest that between 30% and 90% of long-distance runners encounter some form of GI distress during training or competition. It is not a formal medical diagnosis but rather an umbrella phrase encompassing exercise-induced digestive complaints. These issues can range from mild discomfort to symptoms severe enough to significantly impair performance.
What Is Runner’s Stomach
Runner’s stomach refers to a spectrum of digestive symptoms that manifest during or immediately following a run. These symptoms are categorized into upper and lower gastrointestinal complaints. Upper GI issues often include nausea, vomiting, acid reflux, and bloating.
Lower GI symptoms are frequently reported and involve abdominal cramping, flatulence, and an urgent need to defecate, often resulting in diarrhea. The constant, repetitive jostling motion of running contributes particularly to these lower GI problems.
Typical Duration and Recovery
The duration of runner’s stomach is highly variable, depending on the severity of the symptoms and the underlying cause. For many athletes, acute symptoms like cramping or nausea begin to resolve shortly after they stop running, often within one to two hours. Once the body returns to a resting state, blood flow is restored to the digestive organs, allowing normal function to resume.
More persistent or severe episodes, especially those involving significant dehydration or intense endurance events, can result in symptoms lasting for 12 to 24 hours post-run. Residual effects like abdominal discomfort, bloating, or loose stools may linger during this time. Seek medical attention if symptoms are severe, include bloody stool or vomit, or continue past a 24-hour period, as this could indicate a more serious condition like ischemic colitis.
Common Causes of Gastrointestinal Distress
The primary physiological trigger is the redirection of blood flow away from the gut to the working skeletal muscles. During intense exercise, up to 80% of blood flow is diverted from the splanchnic circulation, which supplies the digestive tract, in a process known as ischemia. This reduced blood supply impairs the gut’s function, leading to delayed gastric emptying and compromised mucosal integrity, which causes many uncomfortable symptoms.
Mechanical factors also play a role, as the physical, repetitive high-impact motion of running causes the internal organs to be constantly jolted. This mechanical trauma can irritate the lining of the intestines and contributes to lower GI issues like urgency and diarrhea. Runners experience more lower GI distress compared to athletes in lower-impact sports like cycling.
Fluid and electrolyte imbalances further complicate the issue. Dehydration reduces blood volume, which exacerbates the restricted blood flow to the digestive system. Sweat loss without adequate replacement intensifies physiological stress, increasing the risk of symptoms like nausea and cramping. Excessive consumption of highly concentrated carbohydrate solutions, such as overly sweet sports drinks or gels, can also draw excess water into the intestine, leading to osmotic diarrhea.
Dietary choices immediately before and during a run are another significant factor. Foods high in fiber, fat, or protein slow down the rate of gastric emptying, meaning they sit in the stomach longer. This increases the chance of discomfort when combined with exercise, as these nutrients are more difficult to digest under the stress of running.
Practical Steps for Prevention
Runners can proactively manage their risk by strategically planning their nutrition and hydration leading up to and during a run.
Nutrition Strategies
Pre-run meals should focus on easily digestible, low-fiber, and low-fat carbohydrates, such as a banana or white toast, consumed one to four hours before exercise. Avoiding very large meals just before a run allows sufficient time for gastric emptying. For longer efforts, consuming carbohydrates at a rate of 30 to 60 grams per hour is recommended, using easily digestible forms like sports drinks or gels.
Hydration and Training
Maintaining a consistent hydration schedule throughout the day is important to avoid starting a run dehydrated. During the run, consuming fluids and electrolytes steadily is preferable to “chugging” large volumes, which can lead to stomach upset. A process known as “gut training” can help the digestive system adapt to fueling during exercise by gradually introducing fluids and carbohydrates during training runs. Experimenting with different fuel types and timings during practice, not on race day, is the best way to determine individual tolerance. Reducing the intake of high-FODMAP foods, which are fermentable carbohydrates, a few days before a race may help sensitive runners.