Why should you wait to run after you've eaten?

One reason you should wait to run after you've eaten is because after you eat, insulin is released into your blood to lower your glucose levels. This release actually causes a dip in homeostatic glucose levels. So when you eat your glucose levels go up shortly and then go down below where it was before you at. It takes some time for your glucose level to rise back up slightly to the normal level. If you run right after you eat, that dip in glucose will occur while your running and you won't have the energy (glucose) in your blood that helps you run.