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How do runners get side stitches? And how can they make them go away while running?

The short answer to the first part of the question is that side stitches are most likely caused by a muscle cramp of the diaphragm. The diaphragm is a muscle that separates the chest cavity from the abdominal cavity. The diaphragm helps us breathe: when we inhale, we move the muscle down: when we exhale, it moves up.

The internal organs in the abdominal cavity are connected with ligaments to the diaphragm. These organs are bounced around during running and they pull down on the diaphragm as we exhale (and move the diaphragm up). Interestingly, most people experience side stitches on the right side. This is due to the fact that the liver, the largest organ in the abdominal cavity, is located on the right side.

There are numbers of different methods for preventing, or getting rid of side stitches. The most effective way is to breathe deeply when running. Shallow breathing keeps the diaphragm in a high position and doesn't allow the connective ligaments to relax. Some runners use a technique called "belly breathing"--this method allows the diaphragm to fully lower and reduces the stress on it.

Another effective technique to reduce stress on the diaphragm is to exhale as your left foot hits the ground. The organs attached to the diaphragm are smaller on the left side of your body than on the right side. Finally, some runners "purse" their lips while exhaling. Apparently, a slower release of the pressure in the lungs helps reduce stress on the diaphragm.