This is probably it -- after exhausting the contents of the digestive tract, the body resorts to glycogen reserves in muscle and the liver. Your glycogen reserv

This is probably it -- after exhausting the contents of the digestive tract, the body resorts to glycogen reserves in muscle and the liver. Your glycogen reserves are your "blood-sugar" and consuming them requires a burst of oxygen, so depleted oxygen typically follows low blood-sugar, ergo yawning. Energy gels are usually high-energy but low-everything-else, which is probably what the parent is referring to ("eat glucose"). This is to supplement blood-sugar levels without affecting the digestive process too much, or, more to the point, without affecting their marathon performance.