I know that marathon runners avoid eating before a race, specifically to avoid diarrhea during the race.
I used to avoid eating before karate practice, because it was uncomfortable to do heavy exercise after a full meal.
I think it really depends on how heavy the exercise is. If it's diverting too much blood from your digestive system, then you are likely to have some problems. I wouldn't worry about it, if it's not causing gas, gas pain, bloating, or diarrhea, though.