This is not so much about stomach exercise, but about exercising in general - to your body, it doesn't really matters which muscles groups you are working on, and the abdomen muscles you are working on has nothing to with the internal stomach muscles that has to do with digesting.

I think - eat something small and high on energy, like a banana or a granola bar.  exercising is always better when the body have some energy from food, it gets all your systems working better and faster. However, if you eat a lot you might feel heavy, also, the pressure on your belly from the food might make you feel heavy and nauseous. 

So, it's a balance between eating enough to energize your body, and not eating too much that you feel nauseous.