Healthy human functioning is a very ordered activity. The gastrointestinal tract is a clear example. Intake occurs at one end and output at another.
Ingestion and excretion have been studied for centuries. Over the last half century, the process in between, gut transit, has become a subject for research, especially with the increase in transit disorders such as irritable bowel syndrome (IBS) and diverticulosis.
Until recently, the research in these areas has been quite inconclusive. For example, analysis of the cells that line the gut has shown no difference between healthy tissue and the tissue of patients who have diverticulosis. Even the latest postulations about the gut microbiome fail to clarify whether it causes IBS or is a consequence of it.
Unfortunately, like in our cities, not enough consideration has been paid to infrastructure, in this case, the gastrointestinal muscular highway and how it may precipitate or alleviate problems. The intestines are semi-automatic muscles, like our heart, and therefore not very responsive to our conscious direction. The way they direct traffic is precise. First, there are standing contractions or stationary contractions. These contractions, discovered over a century ago, work like a stationary pump to allow for the mixing of ingested food and digestive juice. The more mixing and dilution of food, the greater the possibility of absorption.
Then there are circular contractions to propel the contents of gut along the pathway. There are two types: ripples, or rhythmic, shallow contractions, which only move the contents short distances – both backwards and forwards and quite fast. Ripples are also designed to aid mixing and digesting. The other type of contractions are large waves, which are much gentler and designed to propel the waste products to the exit.
Not only do the type of nutrients we ingest stimulate these muscles differently, but also the feasting and fasting cycle during the day influences which muscles are working when. Non-nutritive ingestibles, such as fiber, abolish the activity of the muscles that encourage absorption. Instead, they stimulate propulsive wave contractions, consequently increasing intestinal evacuation. Perhaps that is why high fiber diets can increase intestinal discomfort. The same type of cleaning contractions occurs in fasting.
The mechanisms that lead to changes in motility patterns are not well understood. However, some interrelationships between contractions and certain dietary elements have been observed. Opiates, their derivates, and proteins slow gut motility, though the specific site and which muscles they slow has not been described. Surprisingly fats have no effect. Conversely, caffeine has been shown to stimulate colonic propulsive muscles.
Transit time in our intestines is very variable. Where the stationary pumps and ripples are active, it averages 4 hours. In the colon where wave motion is used it can vary between 18 and 34 hours.
Strategies for altering ingestion and excretion by diet has been based on trying to minimise symptoms reported by patients, which can vary enormously. More attention should be devoted to how the work of the gut muscles can help us better control our digestive systems and the problems they can cause. This might lead to better treatments for gastrointestinal maladies.