Running redirects blood away from your digestive system and toward your working muscles, and that shift is the single biggest reason your stomach hurts during or after a run. About 26% of long-distance runners report gastrointestinal symptoms during races, with stomach pain, bloating, and the urge to defecate topping the list. The good news: most post-run stomach pain is temporary and preventable once you understand what’s triggering it.
Your Gut Loses Blood Flow During a Run
When you run, your body rapidly redirects blood from your digestive organs to your legs, heart, and lungs. This drop in blood supply to the gut, called splanchnic hypoperfusion, is the root cause of most running-related stomach problems. With less blood flowing through, the intestinal lining becomes more permeable. Think of it like a fence with gaps widening between the boards: substances that normally stay inside the gut can leak through, triggering inflammation, cramping, and nausea.
The harder you run, the more dramatic the blood flow shift becomes. That’s why easy jogs rarely cause issues, but tempo runs, intervals, or races are far more likely to leave your stomach in knots. Heat makes it worse, too, because your body sends extra blood to the skin for cooling, leaving even less for digestion.
Side Stitches Have a Different Cause
That sharp, stabbing pain just below your ribs isn’t the same thing as general stomach distress. Known formally as exercise-related transient abdominal pain (ETAP), side stitches are one of the most common complaints among runners. The leading explanation is irritation of the parietal peritoneum, the thin membrane lining the inside of your abdominal wall. An older theory blamed stress on the ligaments connecting your abdominal organs to the diaphragm, but the peritoneal irritation model better accounts for where the pain shows up and how it behaves.
Side stitches tend to strike when you’re running shortly after eating or drinking, especially large volumes of fluid. They’re more common in less experienced runners and usually fade as fitness improves. Slowing your pace, pressing gently on the painful spot, and exhaling forcefully can help a stitch pass mid-run.
Running Speeds Up Digestion
Exercise doesn’t just reduce blood flow to the gut. It also makes food move through your system faster. In one study, mild treadmill walking shortened the time it took for a meal to travel from the mouth to the large intestine from 98 minutes at rest to just 75 minutes during exercise. The researchers found that cortisol, a stress hormone, rose significantly during the activity, while other gut hormones stayed the same, suggesting the acceleration may be driven by the body’s general stress response rather than a single digestive signal.
This faster transit means partially digested food reaches parts of the intestine that aren’t ready for it, which can cause cramping, bloating, gas, and urgent trips to the bathroom. Runners who eat high-fiber or high-fat meals before running are especially vulnerable because those foods already take longer to break down.
What You Eat and Drink Matters
Concentrated sugary drinks are a frequent culprit. When a drink’s carbohydrate content climbs above about 10%, it empties from the stomach at a slower rate and draws water into the intestine, causing bloating and cramping. Many fruit juices and some poorly formulated sports drinks exceed this threshold. Standard sports drinks designed for exercise typically stay at or below 6 to 8% carbohydrate, which is closer to the rate at which plain water leaves the stomach.
Timing matters as much as content. Eating a large meal within one to two hours of a run gives your body a competing demand: it needs blood in the gut to digest, but it also needs blood in the muscles to move. The result is often nausea, stomach pain, or both. Smaller, low-fiber, low-fat snacks eaten two to three hours before running give your stomach enough time to mostly empty before your feet hit the pavement.
Pain Relievers Can Make It Worse
Many runners pop ibuprofen or other anti-inflammatory painkillers before a long run or race, assuming it will help with aches. This can backfire badly on the stomach. These drugs work by blocking enzymes that produce compounds involved in inflammation, but those same compounds also protect the stomach lining. Without that protection, the gut wall becomes more vulnerable to the damage already being caused by reduced blood flow during exercise.
Common side effects include indigestion, bloating, upper abdominal pain, heartburn, and acid reflux. These drugs can also weaken the valve between the esophagus and stomach, making reflux more likely. If you’re already prone to stomach problems during runs, taking a painkiller beforehand is likely to make the situation noticeably worse, not better.
You Can Train Your Gut Like a Muscle
One of the more useful findings in sports nutrition research is that your gut adapts to what you ask it to do. Repeatedly practicing eating or drinking during training runs teaches your digestive system to handle fuel under stress. In controlled studies, runners who followed a two-week gut-training protocol, consuming carbohydrate gels or food every 20 minutes during hour-long runs, saw gut discomfort drop by 44 to 49% compared to runners taking a placebo. Even simply practicing drinking fluids during runs reduced discomfort by 26% over several sessions.
The key is consistency. Protocols in the research ranged from 4 to 28 days, but two weeks of regular practice during training runs appears to be enough for most runners to notice a meaningful difference. Start with small amounts of whatever you plan to use on race day and gradually increase. Your intestinal lining and the muscles controlling digestion literally adapt to tolerate more input while blood flow is compromised.
Common Symptoms by Type
Among long-distance runners surveyed about race-day symptoms, the breakdown is revealing:
- Bloating was the most common complaint overall at 18.6%
- Urge to defecate affected 17.8% of runners
- Stomach pain hit 16.5%
- Heartburn and belching each affected about 15%
- Diarrhea showed up in 15.4%, more commonly in men
- Nausea and vomiting were reported by about 14% each
Men and women experience these symptoms somewhat differently. Men are more likely to report diarrhea as a lower GI symptom, while women more frequently report side stitches and abdominal pain. In both groups, bloating was the leading upper GI complaint.
Practical Ways to Reduce Post-Run Stomach Pain
Most runners can eliminate or significantly reduce stomach pain by adjusting a few habits. Avoid eating large or high-fiber meals within two to three hours of running. Keep any drinks consumed during a run at or below a 10% carbohydrate concentration. Skip anti-inflammatory painkillers before runs. And if you plan to fuel during longer efforts, practice doing so in training rather than trying it for the first time on race day.
Running in cooler conditions or slowing your pace on hot days also helps, since heat compounds the blood flow problem. Staying hydrated throughout the day, rather than gulping large amounts right before a run, prevents the stomach from sloshing and reduces the chance of cramping.
If stomach pain persists despite these adjustments, it may point to an underlying digestive condition like irritable bowel syndrome or colitis. Persistent symptoms that don’t respond to changes in timing, diet, and intensity are worth investigating with a gastroenterologist, who can rule out conditions that running simply aggravates rather than causes.