I always take immodium before a long distance events mainly because when I first started running it was a real problem. More recently it's less of an issue but I still take the tablets an hour before the start.
A couple of years ago I started a thread on this subject and I got this terrific response...
..."the scientific explanation, just to add to Bryan's summing up: there are pressure receptors in your gut that register when there is some bulk present and react by contracting to push it along. That is what causes the normal movement of food and waste through the entire length of the gut. The sensitivity of the receptors differs between people and can be affected by numerous other factors (eg. abuse of laxatives reduces the sensitivity and so can lead to constipation or a dependency on the laxatives, ignoring the "urge" can deaden the sensitivity and so also cause constipation).
When running there is a jostling of the gut and so an increase in the pressure from the contents of the gut. The gut responds quite normally and starts contracting to move everything along. Another normal response to the pressure in the gut is secretion of mucous, to aid the movement of the bowel contents. This mucous secretion is also enhanced during running, again because of the increase in pressure on the gut lining. This alone can cause an increased urge to defaecate but the whole thing is worsened by the fact that, due to the "fight or flight" mechanism, blood is being diverted from the gut to the muscles. This means that less water is being absorbed from the bowel contents and so the waste has a higher volume than if absorption were happening normally.
End result - chaos!
It really is just a dead normal healthy reaction. Things that can make it worse are any sort of food sensitivity (lactose intolerance is a fairly common one) or a high bulk of waste anywhere in the gut before starting running (eg. big meal). Something that can help is firming up the abdominal muscles to lessen the jostling of the abdominal contents.
HTH!"
Hope this helps.
Edited: 24/04/2010 at 09:47