Marathon running places significant strain on the body, extending even to the digestive system. The common phenomenon of “runner’s diarrhea” is a distinct medical issue known as Exercise-Induced Gastrointestinal Syndrome (EIGS). This syndrome, reported by a large percentage of endurance athletes, involves a complex interplay of physiological and mechanical stresses. The urgent need to defecate during a long-distance run is a predictable reaction to the body’s internal priorities during extreme exertion.
Blood Shunting and Gut Ischemia
During sustained, intense exercise like a marathon, the body’s cardiovascular system drastically prioritizes blood flow to meet the energy demands of the working muscles, heart, and lungs. This survival mechanism involves blood shunting, diverting blood away from less active organs, including the splanchnic circulation that supplies the digestive tract. Blood flow to the gut can decrease significantly, sometimes by as much as 80% during intense, prolonged running.
This severe reduction in blood supply causes temporary ischemia, or oxygen deprivation, in the intestinal lining. When intestinal cells lack sufficient oxygen and nutrients, the mucosal barrier can become compromised. This damage increases intestinal permeability, often referred to as a “leaky gut,” allowing bacteria and toxins to pass into the bloodstream. The resulting inflammation and cellular distress within the gastrointestinal tract contribute to the urgent and rapid expulsion of intestinal contents. Dehydration further exacerbates this problem, as a lower overall blood volume makes the shunting effect more pronounced, increasing the severity of the hypoperfusion and subsequent ischemia.
Mechanical Stress and Accelerated Motility
The physical act of running itself imposes direct mechanical stress on the gastrointestinal system. The repetitive, high-impact motion of the feet striking the ground causes a constant jostling of the internal organs within the abdominal cavity. This physical trauma can damage the intestinal lining and trigger lower gastrointestinal symptoms.
Intense exercise also activates the body’s stress response, releasing hormones like catecholamines (adrenaline) and cortisol. This surge of stress hormones stimulates the sympathetic nervous system, which can have complex effects on gut function. While it may slow motility in the upper gut, the sympathetic drive can also increase the speed of contractions in the colon. This accelerated colon motility, combined with the physical stimulation from running, further speeds up the transit time of waste, leading directly to the sudden onset of diarrhea.
Dietary and Fluid Triggers
A runner’s consumption before and during a race plays a significant role in exacerbating EIGS. Many athletes rely on carbohydrate gels and sports drinks that contain high concentrations of sugar, which can create a hypertonic solution in the gut. When the concentration of solutes inside the intestine is higher than that in the blood, water is actively drawn from the body into the intestinal lumen to balance the concentration. This influx of water rapidly increases the volume of liquid waste, causing osmotic diarrhea and contributing to the urgent need to find a bathroom.
Certain components of a pre-race diet also act as powerful stimulants for gut motility. Consuming high-fiber foods just before a long run can be problematic because fiber is slow to digest and adds bulk to the stool, speeding up its passage through the intestines. Similarly, caffeine acts as a gastrointestinal stimulant and laxative, increasing the frequency of bowel movements by accelerating colon contractions.
Inadequate fluid intake or dehydration concentrates the contents of the gut and impairs the body’s ability to absorb nutrients and water. Dehydration can also reduce plasma volume, which intensifies the splanchnic hypoperfusion described earlier, placing the intestinal lining under greater stress. Avoiding foods high in fat or protein immediately before a run is also recommended, as these macronutrients require more digestive effort and time, which can slow gastric emptying and increase the likelihood of distress during exercise.