Diarrhea When Running: Why It Happens and How to Stop It

Running redirects blood away from your gut and toward your working muscles, and that sudden drop in intestinal blood flow is the main reason so many runners experience diarrhea mid-run or immediately after. The problem is common enough that it has its own informal name, “runner’s trots,” yet it went largely unmentioned in medical literature until the early 1980s. The causes turn out to involve more than just blood flow: mechanical jostling, stress hormones, pre-run meals, hydration choices, and even over-the-counter painkillers all play a role, and most of them are at least partly within your control.

How Blood Flow Shifts Cause Gut Problems

When you run, your body prioritizes delivering oxygen to your legs, heart, lungs, and skin (for cooling). To manage that increased demand, it sharply reduces blood flow to your abdominal organs, a state researchers call splanchnic hypoperfusion. During intense cycling in healthy volunteers, markers of gut blood flow dropped significantly by the end of the exercise bout, and markers of intestinal cell damage rose alongside an increase in small intestinal permeability compared to rest.¹ In plain terms, the lining of your intestines becomes temporarily leaky.

That leakiness matters because the gut lining normally acts as a tight barrier. When its integrity drops, fluid moves in directions it shouldn’t, bacteria and their byproducts can cross into the bloodstream, and local inflammation kicks in. The result is a cascade of unpleasant symptoms: cramping, urgency, loose stools, and sometimes outright diarrhea. The harder or longer you run, the more severe the blood-flow diversion tends to be, which is why the problem hits during races and hard tempo efforts more than easy jogs.²

Why Running Is Worse Than Other Sports

Cyclists and swimmers get gut symptoms too, but runners report them more frequently. One reason is mechanical: the repetitive up-and-down impact of running physically jostles the abdominal organs in a way that cycling or swimming does not. That bouncing stimulates the colon and speeds up the movement of contents through it.³ Combined with reduced blood flow, the mechanical shaking essentially pushes material through a gut that is already compromised, making urgency and diarrhea far more likely than during seated or horizontal exercise.

This also explains why lower-GI symptoms (diarrhea, cramping low in the abdomen, urgency to find a bathroom) dominate in runners, while upper-GI symptoms like nausea and bloating are more evenly spread across endurance sports. The colon sits in the lower abdomen and absorbs the brunt of the impact forces with every stride.

Stress Hormones and the Gut-Brain Connection

Hard running is not only a physical stress; it’s a psychological one. Race-day nerves, time-goal anxiety, and the general discomfort of pushing yourself all activate your body’s stress-response systems. Those systems release hormones that, among other things, slow gastric emptying (making you feel nauseous) while simultaneously speeding up motility in the large intestine, pushing contents toward the exit faster than normal.⁴

Research on ultramarathon runners found that elevated pre-race stress and anxiety were associated with worse GI symptoms during the event. The proposed pathway involves altered secretion of corticotropin-releasing factor, which both reduces blood flow to the gut and changes how fast things move through different segments of the intestine.⁵ So if you’ve ever noticed your gut is worst on race morning but fine during training runs of the same distance, anxiety is a plausible explanation.

What You Eat and Drink Before a Run

Your pre-run meal can either help or destroy your gut during the workout. Two dietary categories cause the most trouble: fiber and FODMAPs.

Insoluble fiber, the kind found in raw vegetables, whole grains, and bran, stimulates the colon to contract and push contents along. Soluble, viscous fibers can delay stomach emptying, which leaves more material sitting in your upper GI tract when you start bouncing. Both effects get worse during exercise.⁶ This is why many experienced runners shift to lower-fiber foods in the 24 hours before a long run or race, even if they eat plenty of fiber the rest of the week.

FODMAPs are a group of short-chain carbohydrates found in foods like apples, onions, garlic, wheat, dairy, and certain sweeteners. They are poorly absorbed even at rest, and during hard exercise, when the gut’s absorptive capacity is already reduced, they draw extra water into the intestines through osmosis and increase stool volume. The result is loose stools or outright diarrhea.⁷ A runner who has a bowl of high-FODMAP cereal with milk two hours before a tempo run is essentially loading the gut with osmotically active molecules right before reducing the gut’s ability to handle them.

Mid-Run Fueling Problems

Energy gels and sports drinks are another common trigger. Many gels are hypertonic, meaning they contain a higher concentration of sugar and other solutes than your blood does. When a hypertonic solution hits your small intestine, it pulls water into the gut lumen to dilute itself, which can produce bloating, nausea, and diarrhea.⁸ Taking a gel without enough water, or consuming too many gels in quick succession, magnifies the problem.

This is one area where you have real control. Washing each gel down with water helps dilute it toward isotonic concentrations. Practicing your race-day fueling strategy during training lets you test what works for your gut specifically. And choosing gels or drink mixes labeled as isotonic or hypotonic can reduce the osmotic water draw that triggers diarrhea.

Ibuprofen and Other NSAIDs Make It Worse

Many runners pop ibuprofen before or during long races to manage aches. The research here is worth paying attention to, because the effect on the gut is surprisingly large. In a study that measured intestinal damage markers in athletes, cycling after taking ibuprofen produced roughly double the small intestinal injury compared to cycling alone, and gut barrier permeability increased substantially in the ibuprofen-plus-exercise condition compared to all other groups.⁹ Exercise already damages the gut lining through reduced blood flow; adding an NSAID on top appears to make that damage considerably worse.

A separate study of runners exercising in the heat found that ibuprofen did not additionally worsen markers of intestinal injury beyond what heat-stress running already caused, suggesting the effect may depend on context and conditions.¹⁰ Still, during the Chicago Marathon, ibuprofen use was linked to increased GI permeability and symptoms, while aspirin was not.¹¹ The safest move is to avoid NSAIDs around race efforts. If you need pain management during a long event, acetaminophen (Tylenol) does not have the same gut-barrier effects, though it comes with its own limits.

Heat and Dehydration Compound the Problem

Hot weather makes runner’s diarrhea worse because your body redirects even more blood to the skin for cooling, further starving the gut. Dehydration concentrates whatever you’ve consumed, raising the effective osmolality in your intestines and increasing the water-drawing effect described above. Running in heat causes significant elevations in intestinal injury markers regardless of NSAID use.¹⁰

When it comes to what you drink, the tonicity of the fluid matters. A meta-analysis of hydration studies found that hypotonic drinks (lower solute concentration than blood) maintained central hydration better than isotonic or hypertonic options during continuous exercise.¹² From a gut-comfort standpoint, hypotonic fluids also avoid the osmotic water pull that hypertonic sports drinks can cause. If you’ve been drinking a particularly sugary sports drink and getting cramps or diarrhea, diluting it or switching to a lower-concentration formula is a sensible first experiment.

Who Gets It and How Common Is It

Runner’s diarrhea is not evenly distributed. A large survey of recreational long-distance runners in China found that GI symptoms were more common in males (about 28%) than females (about 21%), and that runners aged 34 or younger reported symptoms at higher rates than older runners.¹³ The age finding may reflect training intensity: younger runners tend to push harder relative to their fitness, and intensity is one of the strongest predictors of GI trouble.

Prevalence estimates vary widely across studies depending on how questions are asked and what counts as a “symptom.” Some surveys of marathon runners put the rate of any lower-GI symptom (urgency, cramping, or diarrhea) as high as 30-50% during or immediately after a race. Most of these episodes are mild and self-limiting. Severe diarrhea that forces you to stop running is less common, but it’s common enough that every experienced distance runner either has their own story or knows someone who does.

Training Your Gut

One of the more encouraging findings in sports nutrition research is that the gut adapts. If you regularly practice consuming carbohydrates during training runs, your intestine upregulates the transporters that absorb those sugars, which means fewer unabsorbed carbohydrates sitting in your gut causing osmotic problems. A review of the evidence concluded that nutritional training can improve gastric emptying, increase absorption capacity, and reduce the chances of GI distress during endurance performance.¹⁴

In a more specific test, runners who did two weeks of repetitive gut challenges (consuming carbohydrates during runs) showed reduced gut discomfort, lower total GI symptoms, and about half the carbohydrate malabsorption compared to their baseline.¹⁵ A systematic review of gut-training studies found that a two-week repeated carbohydrate-feeding protocol reduced gut discomfort by an average of roughly 47% and cut carbohydrate malabsorption by 45-54%.¹⁶

The practical takeaway: never try a new gel, drink, or pre-race breakfast on race day. Use your training runs to rehearse your fueling plan at race intensity. Start with small amounts and build up over weeks. Your gut genuinely gets better at handling what you give it, but only if you give it practice.

Practical Prevention Strategies

Because runner’s diarrhea has multiple causes acting at once, the most effective prevention combines several small adjustments rather than relying on any single fix. Here are the strategies with the best evidence or strongest physiological rationale:

Meal timing: Allow at least two to three hours between your last solid meal and a hard run. The meal itself should be low in fiber, low in fat, and moderate in easily digestible carbohydrates. Many runners find that a small portion of white rice, a banana, or plain toast works well.
Reduce FODMAPs pre-run: Avoid high-FODMAP foods like dairy, apples, onions, garlic, wheat-based cereals, and artificial sweeteners (especially sorbitol and mannitol) in the hours before running.
Hydrate with hypotonic or isotonic fluids: Avoid highly concentrated sports drinks. If using gels, always chase them with water to dilute the sugar load hitting your gut.
Skip the ibuprofen: If you need pain relief before a race, acetaminophen is gentler on the gut lining. Better yet, address the underlying injury rather than masking it with pre-race painkillers.
Gut-train with carbs: During training, practice consuming the same type and amount of fuel you plan to use on race day. Gradually increase the amount over several weeks.
Manage pre-race anxiety: Familiar routines, adequate sleep, and realistic goal-setting can reduce the stress-hormone surge that accelerates colonic motility on race morning.
Adjust intensity gradually: GI symptoms are strongly linked to exercise intensity. If you consistently have trouble, backing off the pace slightly during the early miles and building into the effort can help.

When Loperamide Might Help

Loperamide (Imodium) is the most commonly used over-the-counter anti-diarrheal, and there is some evidence it works specifically for exercise-related gut issues. In a study where participants exercised after taking loperamide, the drug completely abolished the acceleration of gut transit time that exercise normally causes.¹⁷ Interestingly, loperamide did not significantly slow gut transit at rest, meaning it primarily blocked the exercise-specific speed-up without causing constipation when you’re not moving.

Some runners use a low dose of loperamide prophylactically before a race they know will be problematic. This is a conversation to have with a doctor rather than a blanket recommendation, because loperamide can mask symptoms of a more serious condition, and relying on it habitually may not be ideal. But for a runner who has tried dietary and timing adjustments and still faces disruptive diarrhea in key events, it’s a reasonable tool to discuss.

After the Run: Why Symptoms Sometimes Hit Post-Finish

Some runners feel fine during their run but develop diarrhea within 30 minutes of stopping. This happens because of a phenomenon sometimes called reperfusion. During the run, blood flow to the gut was severely restricted. When you stop running, blood rushes back into the intestinal vessels. That sudden return of oxygenated blood to tissues that were briefly oxygen-deprived generates a burst of reactive oxygen species and local inflammation, which can damage the gut lining further and trigger symptoms that didn’t appear while you were running.²

This post-exercise window is also when impaired nutrient and fluid absorption is at its worst, which can slow recovery. Cooling down gradually rather than stopping abruptly, and waiting 15-20 minutes before consuming large amounts of food or concentrated fluids, may ease the transition. Starting with small sips of water or a dilute electrolyte drink and adding solid food incrementally tends to be better tolerated than diving straight into a post-race meal.

Red Flags Worth Investigating

Runner’s diarrhea is overwhelmingly benign and self-limiting. Most episodes resolve within an hour of finishing your run and leave no lasting effects. But certain patterns deserve medical attention rather than being dismissed as normal runner’s trots:

Blood in the stool: Small amounts of GI bleeding can occur after very long events, but visible blood, especially if it happens repeatedly, should be evaluated. It may indicate ischemic colitis or another condition exacerbated by exercise.
Persistent diarrhea: If symptoms continue for hours after every run or appear on rest days too, the cause may not be exercise-related at all. Celiac disease, inflammatory bowel disease, and microscopic colitis can all present as exercise-triggered diarrhea before they become obvious at rest.
Unintended weight loss: Chronic malabsorption from a GI condition can mimic runner’s gut problems while also causing nutritional deficiencies.
Fever or signs of infection: Exercise-induced gut permeability can, in rare cases, allow bacterial translocation serious enough to cause a systemic inflammatory response. Fever, chills, or feeling profoundly unwell after a long event warrants prompt evaluation.

The existence of runner’s diarrhea as a common phenomenon sometimes leads runners to normalize symptoms that actually point to something else. If dietary changes, timing adjustments, and gut training don’t help after several weeks, getting checked out is worth the time. A basic workup can rule out conditions that happen to flare with exercise but require their own treatment.