You have two main nervous systems when it comes to energy usage / conservation.
- Constriction of pupil
- Constriction of bronchioles
- Slowing of heart rate
- Increase secretion of digestive glands
Tabers Cyclopedic Medical Dictionary (21)
When you decide to eat, the parasympatheic nervous system begins conserving energy so that more of it goes to digesting food and other bodily functions.
When you decide to exercise, the sympathic nervous system begins expending energy so that you can do exercise x.
In both of these, the other nervous system is reduced, permitting more energy to be given as needed.
When you both consume food and exercise the body is placed in a tricky position. It can't say no to digesting food, it will begin to rot (gas anyone?) and it needs that food to generate more energy later on. But it can't say no to exercise, that could very realistically kill you in a survival situation.
So it does a compromise, both will function at decreased levels.
This doesn't mean you can't eat food while exercising. As otherwise mentioned, marathoners do consume food. But it's unlikely they will eat complex foods like fat or protein and go straight to simple foods like carbohydrates since it is easier for the body to break it down. Despite simple foods, energy must be used to digest so it makes exercising harder. But the benefit of the energy is more quickly greater then not consuming it. Consuming too much can override this though. 1 There aren't any long term marathoners I know of that eat a thanksgiving dinner prior and expect to make it very far.