The Dreaded Side Stitch

Depends largely on intensity of activity, but you've only got so much blood to go around.  If your legs want blood to move, you'll turn off other less important bodily functions. In our ancestral past, running meant surviving, so yeah it's ranked high on the priority list - higher than digestion.  Want a cramp in your side?  Make sure and eat/drink just before running ;).

From Trail Runner Magazine:

GOT CRAMPS?Avoid Painful race- and training-ending muscle spasmsBY Jason R. Karp, Ph.D
In high school, I was leading a cross-country race, with a pack of runners from another school right off my shoulder. I felt confident, and then with 800 meters to go, a cramp seized my calf. I stuttered for a few steps, and the entire pack flew by me as if I were standing still. 

Have you ever been running along enjoying a trail’s scenery, the smell of tall pines or eucalyptus and BAM—you get a nasty cramp? Whether it is a sharp stitch in the side of your abdomen or a sudden, piercing tightness in your calf or hamstring, cramps can downright destroy an otherwise great run. But they don’t have to if you understand how to deal with (and avoid) them.
Why Do You Get Cramps? 

Side stitches affect nearly 70 percent of runners, typically occur on the right side of the upper abdomen and are less prevalent in both older and fitter runners. Why cramps occur, though, is still somewhat of a mystery to physiologists and doctors. Scientists believe that side stitches, which are given the fancy name of exercise-related transient abdominal pain (ETAP), result from either eating or drinking too close to running, especially food and drink that has a high sugar content, or from the movement of internal organs inside the abdominal walls, causing their connective tissue to pull on the diaphragm, which moves with breathing. 

While many runners believe that muscle cramps are caused by dehydration and/or an electrolyte imbalance, which could theoretically affect a muscle’s ability to contract, research has shown that neither is the cause. Drinking a sports drink on your long trail runs, while helpful to maintain hydration, won’t prevent cramping. … Read More