text: is it true that marathon runners tend to poo themselves? and why? yes it does happen but i think less often these days.    in the extreme exertion of a marathon the body loses fluids, electrolytes (salts) and tends to overheat.  if a runner gets to a state of extreme dehydration and exhaustion the muscles that retain the 'poo' simply let go involuntarily.  with more knowlegeable runners, proper diet and hydration i think that this phenomena is now mostly behind us.