Side cramps while running, called a "stitch," can leave a runner bent over, trying to alleviate the pain. According to America's Council of Exercise, a stitch is likely caused by the jarring and stretching of ligaments at the point where the diaphragm meets the stomach. Once a stitch develops, there is little you can do to stop it until you have had some time to rest. While the mechanism for avoiding a stitch still is not entirely understood, there are some methods runners employ that seem to be effective.
Warmup properly and increase intensity gradually. Not only do you need to properly loosen the ligaments and muscles before each running session, you need to build stamina for more intense workouts over time. Start slow and speed things up as you know your body can handle it.
Limit the amount you eat and drink three hours prior to running. While you want to remain hydrated, an engorged gut might increase ligament pulling while running, according to a 1999 study by Plunkett and Hopkins published in "Medicine and Science in Sports and Exercise." Bring water with you to sip as you run, taking in smaller portions at any given time.
Alternate your breathing to reduce the jarring on one side. Most people breath in synchronicity with running steps, meaning you exhale on the same leg repeatedly, which fatigues one side. Monitor your breathing and try exhaling on the other foot.
Breathe deeply while running. Shallow breathing not only deprives your muscles of oxygen during aerobic exercise, but also keeps the diaphragm in an elevated, contracted position, preventing it from relaxing. This can lead to a spasm, causing the stitch.
Walk down the hills as you are building stamina.
Avoid running downhill if you are not conditioned to do so as this increases the chance and intensity of stitches.