Once again, if fast food is causing you noticeable gastrointestinal distress you should probably see a doctor.
If fast food seriously, consistently caused the effects for people that are constantly being bemoaned, they wouldn't be a massive international industry.
Fast food can absolutely give you gastrointestinal problems if you're not accustomed to eating it. I LOVE fast food, and when I'm eating fast food regularly, I have no stomach problems. But when I eat cleanly for months (vegetables, meats, fruits), then eat my beloved Popeyes or generally something breaded and deep fried, I do have stomach aches and diarrhea. But if I continue to eat that type of food, my stomach adapts to it. So I believe people who say they have diarrhea when they eat fast food because I assume they hadn't eaten it for a long time.