The Intestine's Cleaning Cycle
The migrating motor complex (MMC) is a remarkable pattern of muscle contractions that sweeps through the small intestine during fasting states, functioning as a housekeeping mechanism. The MMC operates in three distinct phases across 90-120 minute cycles. Phase I shows minimal contractions, allowing digestion. Phase II produces increasingly vigorous contractions. Phase III generates powerful peristaltic waves that propagate forcefully down the small intestine, physically sweeping the entire lumen clear.
This pattern depends on hormonal signalling, particularly motilin released from enteric cells during fasting. Motilin binds to receptors on smooth muscle, triggering coordinated contractions. Ghrelin also modulates MMC timing. When food enters the stomach, CCK and other hormones immediately suppress motilin signalling, terminating the MMC.
Continuous eating essentially prevents the MMC from completing full cycles. When snacking occurs every 2-3 hours, Phase III power strokes never fully traverse the small intestine. The mechanical clearing fails, and bacteria proliferate in the small bowel—small intestinal bacterial overgrowth (SIBO). SIBO produces gas, bloating, malabsorption, and pain.
Prokinetic agents enhance Phase III amplitude. Opioids suppress motilin signalling and devastate MMC function. Spacing between meals matters quantitatively. While humans generate partial MMC with 3-4 hour spacing, full Phase III power strokes require 4-5 hours of fasting. Three meals daily with 4-5 hour spacing optimises small bowel clearance.
Bacterial composition changes predictably with MMC dysfunction. SIBO shows overgrowth of facultative anaerobes and aerotolerant species (E. coli, Klebsiella) that thrive in the proximal small intestine's higher oxygen environment. These bacteria produce hydrogen and methane gas, detected on breath testing.