I used to get incredibly bad diarrhea during runs nearly EVERY time out. It was horrible and it completely f***ed up my training. The only way I could avoid it was by taking a complete, thorough dump before running, but I usually couldn't do that because as we all know you can't just take a huge crap whenever ya want. However, after years of frustration, I finally discovered the ultimate solution to runner's craps: STIMULANT DRUGS. Taking a stimulant drug such as those prescribed for ADHD (adderrall, dexedrine, ritalin, etc.) will usually induce a bowel movement when it kicks in. Pop a pill about an hour before running, defecate thoroughly, and you'll be fine on your run.