What causes the sharp pain in the abdomen called stitches?

Stitches are sudden, sharp pains in the upper abdomen. No athlete is immune to them, and their cause is still a matter of debate, although the consensus is that they're caused by cramps in the diaphragm, the large muscle that aids in breathing. These cramps are probably caused by blocked blood flow to the diaphragm or upper abdominal viscera, perhaps due to spasm of the arteries, or by pressure from intestinal gas, eating too soon before exercise, milk intolerance, or weakness in the abdominal muscles. To prevent stitches, don't eat before you exercise and avoid foods that cause gas. Some hypothesize that exercises such as "crunches" or bent-knee sit-ups are effective in preventing stitches, but these exercises are so helpful for posture and for protecting the lower back from injury that they're a good idea whether you have stitches or not.