My husband works across the street from McD's and eats lunch there often. Every time he eats there, no matter WHAT menu item(s) he chooses, it gives him gas and we have a horrible evening after he's eaten there. I told him that if he can't pack his lunch, find somewhere else to eat.
There's something in their food, all their food (even their salads) that gives us both gawd-awful gas, which is why we do not eat there. No other fast food place does that to us. We loooove us some Jack In The Box, tho.