Bicycling magazine had an article this month on stomach upsets, you should look for it. Basically, it said that people shouldn't overdo the carbohydrates while riding--sports drinks or food carbs, but diagnosed this guy's problem as his system couldn't handle too much. I personally eat sparsely before and while riding or I feel really bloated and slow. I also stick to simple food, mostly fruit, don't do processed or junky food while riding. I feel much stronger and faster.
I was amazed on a week long 500 mile tour that some people would down big cafe breakfasts of pancakes and bacon and eggs, coffee, etc. and get on their bikes and ride. I would start w/a banana and eat another or half a cup of cereal and milk by 25 miles. Then they would repeat the process with lunch stops. I just can't do it, much as I love food.