For those of you who are lucky enough to not have experienced this, quite literally, shitty side effect from running, allow me to bitch about it to you. Runner's trots is basically getting diarrhea while running. It sucks quite a bit. I get it about a third of the time, especially during long runs.
It's caused by several different factors. Besides theories regarding psychological causes, its generally happens because: A) running tends to jostle up food in your stomach and intestines, leading to flawed digestion, and B) there is a reduced blood flow to the intestines as the body diverts blood to working muscles.
I have not yet found a full out solution, though I've done things that have (I think) helped. For one I've cut out all lactose. I drink lactose-free milk and don't eat cheese or ice cream anymore. Giving up ice cream was hard but any relief from constant diarrhea is worth the trouble. I also, recently, realized that a particularly bad case of the trots I've had this past couple weeks coincided with us getting a box of raisin bran crunch, which is incredibly high in fiber. Sure enough, after doing some research, I found that its recommended the victims of the trots don't eat high-fiber cereals. There's also drugs that slow down intestinal activity, but I'd rather take a more naturalistic approach to the issue. Plus I don't want to be popping pills before I run (5-6 days a week).
So yeah, hope this helped out any fellow chronic shitters.