What causes this?
It seems that just when you reach your running peak, blood-supplying oxygen is diverted to the body parts most needing oxygen: the muscles. Also, blood is rushed to the skin to play its part in the body’s temperature control.
The more intense the run, the more blood flow is diverted away from the GI tract. In addition, the longer the run, the longer the GI tract is deprived of its usual blood flow (Bolon).
That means the oxygen-bearing blood is “pulled away” from the gut, creating an inflammatory-type response to the colon. This inflammation is what causes the cramps and subsequent diarrhea.
Susan Paul informs us the diarrhea may be bloody. She explains that the blood comes from the colon’s “sloughing off” its inflamed lining. While the lining of the colon is shed daily, the risk of permanent damage to a healthy colon by a recurrent inflammatory response from exercise is unknown.