Stomach pain during running is extremely common, affecting up to 71% of long-distance runners at some point. The causes range from a harmless side stitch to reduced blood flow in your gut, and what you ate before your run plays a bigger role than most people realize. The good news: most running-related stomach pain is preventable with a few straightforward changes.
The Side Stitch: Most Common Culprit
That sharp, stabbing pain just below your ribs, usually on the right side, is called exercise-related transient abdominal pain, or a side stitch. It’s the single most recognized form of stomach pain in runners, and while scientists have debated its cause for decades, the leading explanation points to irritation of the parietal peritoneum, a thin membrane lining the inside of your abdominal wall and the underside of your diaphragm.
The theory, developed after studying roughly 600 people who experienced the pain, suggests that the repetitive jostling of running creates friction against this membrane. That friction triggers a localized, sharp pain that can range from mildly annoying to intense enough to make you stop. Older explanations blamed stress on the ligaments that suspend your liver and stomach from the diaphragm, and that mechanical tugging likely contributes too, especially when your stomach is full and heavy.
Side stitches tend to hit harder early in a run or when you’ve picked up your pace suddenly. They’re more common in younger runners and in people who ate or drank a large volume shortly before heading out. The pain almost always resolves on its own once you slow down or stop.
Your Gut Loses Most of Its Blood Supply
During intense running, your body redirects blood away from your digestive organs and toward your working muscles. This isn’t a subtle shift. Blood flow to the gut can drop to 40% of its resting level during hard exercise, and during all-out efforts it can plummet to as low as 20%. That means your intestines are temporarily running on a fraction of their normal oxygen supply.
This reduced blood flow, called splanchnic hypoperfusion, is a normal part of exercise physiology. But it explains why your stomach feels “off” during hard runs. Your gut simply can’t digest food efficiently when it’s getting so little blood. The result is cramping, nausea, bloating, or that heavy, sloshy feeling in your stomach. The harder you run, the more blood gets diverted, and the worse these symptoms become.
Interestingly, healthy athletes tolerate this blood flow reduction remarkably well compared to people with chronic circulation problems, who develop symptoms with far less reduction. But if you’re running hard on a full stomach, you’re asking your gut to do its most demanding work with its least resources.
What You Eat Before a Run Matters More Than You Think
Pre-run nutrition is one of the biggest controllable factors. Drinks and foods with high sugar concentrations slow gastric emptying considerably. A beverage with 10% glucose, for example, has an osmolality more than eight times higher than plain water, and that concentrated sugar sits in your stomach much longer. Sports drinks, fruit juice, energy gels taken with too little water, and sugary snacks are all common triggers.
High-fiber foods are another frequent offender. Fiber draws water into the intestines and produces gas as gut bacteria ferment it, both of which amplify cramping and bloating during the repetitive impact of running. Dairy products cause similar problems for many runners, particularly those with even mild lactose sensitivity that might not bother them at rest.
Timing matters as much as food choice. Eating a full meal less than two hours before a run leaves undigested food sitting in your stomach when blood flow drops. A small, low-fiber, low-fat snack 60 to 90 minutes before running is generally well tolerated. Larger meals need three to four hours to clear your stomach enough to run comfortably.
The Low-FODMAP Approach for Chronic Problems
If you regularly deal with bloating, gas, loose stools, or cramping during runs despite adjusting meal timing, a low-FODMAP diet may help. FODMAPs are short-chain carbohydrates found in foods like onions, garlic, wheat, apples, and certain dairy products. They ferment quickly in the gut and draw in extra water, a combination that becomes much more problematic when your digestive system is already stressed by exercise.
Research on endurance athletes shows that a low-FODMAP diet improves gut symptoms in 50 to 80% of those who try it. In runners specifically, following this approach significantly reduced flatulence, loose stools, diarrhea, and the urgent need to find a bathroom mid-run. Recreational athletes who adopted even a short-term low-FODMAP diet reported that both the frequency and intensity of their exercise improved because they weren’t battling their stomachs the whole time.
You don’t necessarily need to follow a strict FODMAP elimination permanently. Many runners find that simply reducing high-FODMAP foods in the 24 hours before a long or intense run is enough to make a noticeable difference.
How to Stop the Pain Mid-Run
For a side stitch that hits while you’re running, the fastest relief comes from slowing your pace and using a controlled breathing technique: take two quick inhales through your nose, then one long exhale through your mouth. Repeating this pattern two or three times changes how the nerves in your diaphragm area are firing and can reduce or eliminate the pain within 30 seconds, often without needing to stop completely.
Other strategies that work in the moment:
- Press into the pain. Pushing your fingers gently into the painful spot while bending slightly forward can relieve tension on the peritoneum and supporting ligaments.
- Exhale when the opposite foot strikes. If the stitch is on your right side, focus on exhaling when your left foot hits the ground. This reduces the downward force on the liver side of the diaphragm.
- Slow down before you stop. A side stitch that forces you to walk usually means you ramped up intensity too quickly. Gradually increasing pace at the start of a run helps your body adjust blood flow more smoothly.
For nausea or lower-gut cramping, slowing down is the primary fix. Reducing intensity even slightly allows more blood to return to your digestive organs. Sipping small amounts of plain water helps too, but avoid gulping large volumes, which can worsen stomach distension.
When Stomach Pain Signals Something Serious
The vast majority of running-related stomach pain is harmless and temporary. But certain symptoms cross a line. Bloody stool or dark red diarrhea during or after a run can indicate ischemic colitis, a condition where reduced blood flow to the colon actually damages the intestinal lining. One documented case involved a 20-year-old marathon runner who developed diarrhea at mile 12 and finished the race covered in bloody stool. She required medical treatment for ischemia-related colon damage.
This is rare in casual runners but becomes a real concern during prolonged endurance events, especially in hot weather when dehydration compounds the blood flow reduction. Persistent abdominal pain that doesn’t resolve after you stop running, pain accompanied by fever, or any visible blood in your stool warrants prompt medical evaluation. These aren’t the kind of symptoms you train through.