Many of you here have surely experienced the debilitating phenomenon of "runners craps"... uou know, where you get diahrrea(sp?) in the middle of your run and have to jump in the bushes and whipe yourself with leaves..... a real PAIN IN THE ASS (get it - "pain in the ass"?)
Having suffered from this problem for years, I have established that the only reliable way to avoid the problem is to have a thorough bowel movement within the 3-4 hours before my run. The problem is that I can't control when I'm going to have a bowel movement; it's something that is involuntarily dictated by your nervous system. Usually, for me, the "natural time" is shortly after my second or third meal of the day, which is a problem since I usually train before lunch. I'll try to have a dump at 11am right before I train, but usually I simply am not able to go that early in the day... in which case I have to do a "pre-run" run just to get the crap out before my real run. Again, a pain in the ass.
Recently, I started taking a prescription stimulant called Adderall for ADD, and found that it makes me have a bowel movement within 30 minutes of ingesting the pill. So, if I take the stimulant at 8am with breakfast, I'll have a bowel movement by 9 o'clock, and be able to run crap-free after that. It's been a huge help.
I figure this works by stimulating the autonomic nervous system, which controlls involuntary processes such as digestion. I don't know if this will work with other stimulants such as coffee or caffiene, but it's definitely something that I would reccommend trying if you suffer from frequent runners craps