The main function of the small intestine is digestion and absorption of nutrients. In this process, the role of small bowel motility is to mix food products with the digestive enzymes, to promote contact of chyme with the absorptive cells over a sufficient length of bowel and finally to propel remnants into the colon. Well-organized motility patterns occur in the small intestine to accomplish these goals in the fed as well as the fasting state. During fasting, a migrating motor complex (MMC) exists. This complex is characterized by a front of intense spiking activity (phase III activity) that migrates down the entire small intestine; as the front reaches the terminal ileum, another front develops in the gastroduodenal area and progresses down the intestine. The purpose of this phase III myoelectric and contractile activity is to sweep remnants of the previous meal into the colon and prevent stagnation and bacterial overgrowth. The MMC often starts in the lower esophagus. Sweeping through the stomach, it removes debris and residual material not emptied with the last meal. Absence of phase III activity is associated with bacterial overgrowth and diarrhea. Thus, the small bowel is active even during fasting.

During meals, this cycle is interrupted and the motility pattern in the small bowel becomes an irregular spiking activity called the fed pattern. This fed pattern of motility does not seem to move intestinal contents forward to any great extent but does mix these contents with digestive juices, spreading them again and again over the absorptive surface of the brush border. Diarrhea can thus occur when this normal fed pattern is replaced by aggressive propulsive contractions.