Side Stitches: The Most Common Pain Runners Ignore Until It Hits
A side stitch is a sharp, localised pain in the side of the abdomen that strikes during running or other repetitive-torso activities. The medical term is exercise-related transient abdominal pain, or ETAP. It affects roughly 70% of runners in a given year, and during any single running event approximately 1 in 5 participants will experience it. Despite how common it is, the exact mechanism has been debated for decades. The most supported theory, based on a 2015 systematic review in Sports Medicine, is that ETAP results from irritation of the parietal peritoneum, the thin membrane that lines the inside of the abdominal wall. When this membrane is stressed by repetitive torso movement, fluid shifts, or pressure changes during running, it produces the sharp, well-localised pain you feel in your side. The pain is real, but it is not dangerous, and it is almost always preventable.
What Causes a Side Stitch: The Science Behind ETAP
Peritoneal irritation: the leading explanation. Your abdominal organs are covered by a two-layered membrane called the peritoneum. The visceral layer wraps the organs; the parietal layer lines the abdominal wall. Between them is a thin film of fluid that allows the layers to glide smoothly. During running, the repetitive impact and torso rotation can disrupt this fluid layer, causing friction between the two membranes. The parietal peritoneum is innervated by somatic nerves that produce sharp, well-localised pain, which is why a stitch feels like a knife point rather than a dull ache. This theory explains why ETAP is most common in sports with repetitive torso movement (running 69%, horse riding 62%) and less common in activities like cycling (32%).
Pre-exercise food and drink amplify risk. Eating or drinking large volumes within 2 hours of running is one of the strongest risk factors for ETAP. Hypertonic beverages (fruit juice, soft drinks, concentrated sports drinks) are particularly problematic because they draw fluid into the gut, distend the stomach, and increase the mechanical load on the peritoneal ligaments. Research shows that isotonic and water-based fluids cause significantly less ETAP than hypertonic alternatives. The type of food matters too: high-fat and high-fibre meals take longer to empty from the stomach, increasing the chance of discomfort during a run.
Age and fitness level matter. ETAP is more common in younger runners and decreases significantly with age and training experience. This may reflect improvements in core stability, posture, and visceral fat distribution that occur with sustained training. Beginner runners report side stitches far more frequently than experienced runners, which aligns with the theory that improved fitness reduces peritoneal stress through better torso control and breathing mechanics.
Posture and breathing play a role. Kyphotic posture (rounded upper back) is associated with higher ETAP incidence. Poor thoracic posture compresses the abdominal cavity and may increase peritoneal friction during running. Shallow, chest-dominant breathing also correlates with side stitches. Diaphragmatic breathing and an upright running posture help maintain space in the abdominal cavity and reduce mechanical stress on the peritoneum.
How to Prevent Side Stitches When Running
- Time your pre-run nutrition. Avoid large meals for 2-3 hours before running. If you must eat closer to your run, keep it to a small, low-fat, low-fibre snack (a banana, a few crackers, or a small energy bar). Avoid hypertonic drinks (fruit juice, full-strength sports drinks, soft drinks) within 2 hours of running. Water or diluted isotonic drinks are the safest options. This single change eliminates the most controllable risk factor for ETAP.
- Improve your thoracic posture. Rounded upper back posture compresses the abdominal cavity and increases peritoneal irritation during running. Thoracic extension exercises, foam rolling the upper back, and wall slides improve the mobility needed for an upright running posture. Run tall: imagine a string pulling the crown of your head toward the sky. This creates more space in the abdominal cavity and reduces the mechanical stress that triggers stitches.
- Strengthen your core. A strong, stable core controls torso rotation and reduces the excessive movement that irritates the peritoneum. Planks, dead bugs, Pallof presses, and anti-rotation exercises build the specific core stability runners need. Aim for 2-3 core sessions per week, 10-15 minutes each. Focus on endurance over maximum effort: 30-60 second holds and 12-15 rep sets train the sustained control needed during a run.
- Practice diaphragmatic breathing. Breathing into your diaphragm rather than your chest reduces pressure fluctuations in the abdominal cavity. Practice lying on your back with one hand on your chest and one on your belly: only the belly hand should rise as you inhale. Transfer this pattern to easy runs first, then maintain it as intensity increases. Rhythmic breathing patterns like 3:2 (inhale for 3 steps, exhale for 2) also help by creating a predictable, controlled pressure cycle in the abdomen.
- Build running volume gradually. Because ETAP decreases with fitness and experience, progressive training is itself a prevention strategy. Increasing weekly mileage by no more than 10% per week allows your body, including your core and postural muscles, to adapt to the repetitive impact of running. Runners who jump in volume too quickly are more susceptible to stitches during the build phase. Monitoring your gait with tools like Arion Running Analysis can reveal asymmetries in torso rotation or foot strike that may contribute to uneven abdominal loading and higher stitch risk.
FAQ
What causes a side stitch when running?
The most supported explanation is irritation of the parietal peritoneum, the membrane lining your abdominal wall. During running, repetitive torso movement and impact can disrupt the fluid layer between the peritoneal membranes, causing friction and sharp, localised pain. Risk factors include eating or drinking large volumes before running (especially hypertonic beverages), poor thoracic posture, weak core muscles, and lower fitness levels. The pain is usually on the right side but can occur on either side.
How do I prevent side stitches while running?
The most effective prevention strategies: avoid large meals and hypertonic drinks for 2-3 hours before running, improve thoracic posture through mobility work, strengthen your core with 2-3 sessions per week, and practice diaphragmatic breathing. Build running volume gradually, as ETAP occurrence decreases with improved fitness. Some runners also find that warming up thoroughly before hard efforts reduces stitch frequency, as the gradual increase in intensity allows the abdominal cavity to adapt to the impact progressively.
How do I get rid of a side stitch mid-run?
The most effective techniques reported by nearly 600 runners in a research survey: deep breathing or exhaling forcefully (used by 40%), pressing firmly on the affected area (31%), stretching the affected side by raising the arm overhead and leaning away (22%), and bending forward at the waist (18%). You can also slow to a walk, focus on belly breathing, and gradually resume running once the pain subsides. Most stitches resolve within 30-60 seconds with these interventions. If a stitch recurs immediately, walk until it fully resolves before running again.
Does eating before running cause side stitches?
Eating and drinking before running is one of the strongest modifiable risk factors for side stitches. Large meals, high-fat and high-fibre foods, and hypertonic beverages (fruit juice, soft drinks, concentrated sports drinks) are the most problematic. They distend the stomach, slow gastric emptying, and increase mechanical stress on the peritoneum during running. Allow 2-3 hours after a full meal before running. If you need fuel closer to your run, choose small, easily digestible options and stick to water or diluted isotonic drinks.
Do side stitches go away as you get fitter?
Yes. Research consistently shows that ETAP occurrence decreases significantly with age and improved fitness. Younger, less experienced runners report stitches far more frequently than veteran runners. This likely reflects improvements in core stability, thoracic posture, breathing mechanics, and visceral adaptation that come with sustained training. If you are a beginner experiencing frequent stitches, they will almost certainly become less common as your fitness improves, provided you also address nutrition timing and posture.
