Side stitches while running are often the result of not enough flexibility in the abdominal muscles.
This. It's not nutrition, but mechanics. When you breathe to one side, you are stretching and taxing the muscles on that side more than the other. For example, if you're right-handed, you'll typically breathe "in" when your right foot is striking (assuming you're working hard, such that your body is seeking rhythm and trying to compensate for the load demand) and breathe out on the left foot. So the answer becomes almost apparent; you're over-taxing the right side.
Try this. On your next hard run, try changing the in-breath to the other foot. That sounds simple, but you might actually stumble or stutter-step while trying to change. The point is, being able to change the in-breath to the alternate foot will often eliminate side stitches. For me (n=1), I try to change the in-breath side every mile in a (more than 5k) race.