There has not been done research on the cause of side stitches but there are various theories. One of them says that they are caused by a lack of oxygen in the respiratory muscles due to a reduced blood flow. However, side stitches often occur due to mechanical jerky movements.

Another theory claims that physical activity leads to a reduction in the size of internal organs and the resulting pain reaction causes us to feel side stitches. 

But the stitch can also be favored by carbohydrate intake before physical activity.

Tip: Run slowly and not on a full stomach and it is best to breathe calmly and deeply. (1)