Every runner has experienced it---the crampy pain or "side stitch" that grabs you in the gut, puts you off your stride, and can even cause you to cut your run short. It's got a fancy name, ETAP or exercise-related transient abdominal pain, and lots of work has been done to investigate its causes. There's pretty good consensus on the following.
Abdominal pain while running is most often due to muscle cramps, altered motility of the GI tract, or a combination of the two. In some cases, cramps can be a symptom of heat exhaustion, which can be quite serious.
Muscle cramps can be due to electrolyte imbalances, dehydration, muscle fiber fatigue, reduced blood flow to the abdominal wall muscles and diaphragm, and irritation due to mechanical trauma from running.
Blood flow to the GI tract decreases dramatically during vigorous exercise---as much as 80 percent after an hour in one study of cyclists---as the muscles demand more oxygen and fuel to keep you moving. This can cause pain, especially if the gut is busy trying to digest a meal. Although the stomach empties more slowly during exercise, the large and small intestine become more active. Cramping and even diarrhea may result.
It's important to avoid dehydration and electrolyte imbalance. Water is your best choice, but sports drinks are okay; sodas and other hypertonic drinks can upset your gut and cause fluid imbalance. Allow enough time between eating and running for your stomach to empty (at least 2 hours for a full meal; 60 to 90 minutes for a snack or light meal) and avoid foods that are high in fat, fiber or protein. Stretching can help too, but be careful not to overdo it when your muscles aren't warmed up. Alternating walking and running for a minute at a time during the first 10 minutes of a run works for many runners.
Try breathing more deeply and at a slower rate. Running more slowly may help too. Sometimes synchronizing breathing with your stride works---the idea is to exhale when your foot on the side that does not hurt hits the ground (so for a right-sided cramp, exhale when your left foot hits the pavement). If these don't work, stop and rest. You may be dehydrated, have low sodium or potassium levels, or be experiencing early heat exhaustion. If you don't feel better after cooling off and rehydrating, seek medical attention.
Article Written By Peggy Hansen
Peggy Hansen holds a Bachelor of Science in biology from UC San Diego, Doctor of Medicine from UCLA, and completed postgraduate training at Stanford, Duke and Harvard. An award-winning writer and photographer, her work has been featured in Catnip, Herbalgram, Porter Gulch Review, and many online pieces. She's also a commentator for KQED-FM