He's had it for so long, he's typically able to tell when its starting, but before it gets very bad. He'll just take it at that point and it stops it from happening. I suppose it would be possible to take it if you were just hungry, but it seems to me like a better option in that case would be just to carry a snack bar or a protein bar around and just use the Donnatal if you feel like your cramps are coming on.

A doctor will of course be able to give you real medical advice. This is just anecdotal, but the no eating causing stomach/intestinal cramps sounds very similar.