The migrating motor complexThe migrating motor complex is actually housekeeping mode for your digestive tract. The basics of it are simple. Throughout your digestive tract, particularly the stomach and duodenum, there are receptors that detect nutrients. When these receptors no longer detect nutrients and pressure in the duodenum increases through light contractions of the smooth muscle, the migrating motor complex is initiated. Serotonin builds up in the duodenum and causes secretion of the hormone motilin. Motilin causes strong contractions starting in the stomach that work their way down the digestive tract to the end of the small intestine, called the ileum. If you're interested in the hard, dirty science of it, the schematic below should help:
These rhythmic contractions of the digestive tract occur during interdigestive periods to move undigestible material, leftover particles of food, and unwanted bacteria from the stomach and small intestine in to the colon where they can be eliminated in feces, a process known as peristalsis. During this process, there is also an increase in enzymatic secretions that aid in cleansing the digestive tract by breaking down undigested food particles and killing bacteria. It is interesting to note that people with small intestinal bacterial overgrowth(SIBO) have a defective migrating motor complex. Unfortunately, this is a classic chicken or the egg scenario so we don't know if it's cause or effect, but it's likely both.
There are 4 phases in the migrating motor complex, phase 3 being the most important as it is the period of greatest motor and secretory activity. When you feel that rumbling in your belly, those are phase 3 contractions of the stomach. Phase 3 lasts between 5-15 minutes and occurs roughly every 90-120 minutes. Since this is the migrating motor complex, and the end goal is to move contents of the digestive tract to the end for eventual elimination, phase 3 contractions do not all happen at the same time. The contractions typically begin in the stomach and move progressively through the beginning(duodenum), middle(jejunum), and end(ilieum) of the small intestine. Sometimes the phase 3 contractions can bypass the stomach and begin in the duodenum or jejunum, but the vast majority begin in the stomach.
Factors that affect the migrating motor complexMultiple factors have an impact on the migrating motor complex. The primary factor regulating it is the presence of food in the digestive tract. Once food is consumed, phase 3 contractions cease no matter which part of the digestive tract they are happening in. This is because the types of muscular contractions that occur during digestion are distinct from those of the migrating motor complex. While the contractions of the migrating motor complex are meant to clean and move food out of the digestive tract, the contractions that occur during digestion are meant to mix and grind the food you eat, which occurs in a distinctly different fashion.
Another factor is stress. The digestive tract contains its own, fully functioning nervous system called the enteric nervous system. The enteric nervous system can function on its own without input from the central nervous system, but the central nervous system does interface with the enteric nervous system and imparts some control over it through the autonomic nervous system, your stress management headquarters. Specifically, the vagus nerve and prevertebral ganglia interface with the enteric nervous system. While this control is not necessary, it is certainly optimal as the vagus nerve controls acid secretion and motility of the stomach. Specifically with regard to the migrating motor complex, stress delays phase 3 contractions in the stomach and likely impacts gastric acid and pepsin secretion.
The final factor we will discuss that impacts the migrating motor complex is sleep. Sleep slows down the migrating motor complex. This is unfortunate because the vast majority of people spend most, if not all, of their time fasting asleep. This means that, in most people, digestive housekeeping more like Animal House and less like Grandma's house.
Modern eating patterns are no good for a clean GI tractIf you think about it, the eating patterns of most people probably doesn't lend itself well to this entire housekeeping process, including those that are living a "healthy" lifestyle. If you eat 5 times a day with the first meal being as soon as you wake up, exactly when do these interdigestive periods begin? It takes approximately 2-6 hours for food to leave the stomach and an additional 3-5 hours for food to leave the small intestine. Given the fact that each migrating motor complex takes, at minimum, 90 minutes to complete, you'll likely get no full cycles during the day. This means you are leaving the entirety of the housekeeping process to the times when it is least active, at night.
In my opinion, 5 meals a day really serves no purpose. For one, the thermic effect of food, the energy demanding process that would be impacted by what you eat, isn't even dictated by how often you eat, it's dictated by how much you eat. Those same receptors in your stomach that detect nutrients use that information to determine how much digestion needs to take place, how many types you eat doesn't even factor in to the equation. Even eating 3 times a day may be too much because there really isn't an interdigestive period during the day. In certain circumstances it may be necessary to ingest smaller meals, particularly if you have poor digestion. However, this may not be optimal and a better approach may be to ingest digestive enzymes with your food until your digestion gets back on track.
Probably the most effective way to get all you can out of this housekeeping process without drastically changing your lifestyle is to withhold ingesting anything with calories until at least 3 hours after you wake up. Notice I didn't say eating, I said ingesting anything with calories. You can drink coffee, just stop dumping butter or coconut oil in it. Black coffee has a stimulative effect on peristalsis and may increase gastric secretions because caffeine blocks the breakdown of acetylcholine. This would, in theory, improve the effectiveness of the migrating motor complex by strengthening contractions and enzymatic secretions. To fully optimize the process, withhold eating anything at least 3 hours before you go to bed.