Motility changes

1. MOTILITY CHANGES (1)

Motility = How fast the intestines contract and move their contents within them. Slow motility can lead to constipation, and fast motility can lead to diarrhea.

Motility of the gastrointestinal tract means the movements and contractions of the intestines that mix food content and move it forward. The enteric nervous system has the biggest say in how fast food moves through the gut, the second being our autonomic nervous system, which is affected by stress, relaxation and sleep. Also the immune system, gut microbiota and its fermentation products, and gut hormone secretions have a say in how quickly the intestines move.

What triggers the muscles in the intestinal walls to contract is most importantly stretching of the intestinal wall (due to food in the gut). So, if you are eating very little, or very little fiber, there will be less bulk inside the intestines and therefore less stretching of the intestinal walls, leading to less muscle contractions, especially in the colon, where stool changes shape from liquid consistency to solid. It is also more difficult to push small amounts of stool forward in the colon, which can lead to constipation. This is one reason why eating regular, normal sized meals is helpful for your digestion.

Additionally, in response to food in the stomach, gastrocolic reflex can trigger the colon to contract and begin mass peristalsis, which is like your stomach sending a signal down the road that more food is incoming, move on! Some people have a stronger gastrocolic reflex and especially fatty foods can trigger an urgent bowel movement even before finishing the meal. Once stool is moved to the rectum, defecation reflex is triggered. This requires the parasympathetic nervous system (also responsible for the rest-and-digest response) to then trigger muscle contractions in the colon and relax the internal anal sphincter so that stool can exit. This means that a relaxed state of mind is essential for the proper functioning of the gut.

Factors that can have an additional effect on motility:

Irritants inside the gut such as bile acids also trigger intestinal contractions. We absolutely need bile and bile acids for fat digestion, but if bile acids aren’t absorbed in the small intestine like they should, they end up in the colon and trigger diarrhea. Alcohol, caffeine and spicy food can also make food move faster through the intestines.

How much fluid the gut secretes into the intestines also affects motility. In addition to secretions from the stomach, pancreas, liver and the gallbladder, the intestine itself secretes water and electrolytes into the gut. These secretions are highly regulated, but problems with the balance of water secretion and water absorption can lead to motility changes. Only about 200-300 ml of water is excreted in feces, even though up to 9 liters of fluid passes through the intestine every day. With viral or bacterial diarrhea, gut secretions are also greatly increased. The same goes for a large number of particles in the gut – unabsorbed sugars and sugar alcohols for example attract fluid into the gut which can lead to diarrhea. For example, drinking a lot of fruit juice can cause this, as juice has a lot of fructose, a sugar that doesn’t absorb well from the gut and can stay in the intestine to attract water to itself.

The microbes in our colon affect the gut transit time through the microbiota-gut-brain axis. This means that in addition to the gut and the brain interacting with each other, the gut microbiota is also part of the discussion. Through this three-way communication, alterations in gut microbiota can lead to changes in motility.

Some bacterial species seem to make the transit time faster (eg. Actinobacteria) and some slower (eg. Faecalibacterium). The products of bacterial fermentation can also have an effect on motility, short chain fatty acids appearing to shorten transit time (which is typically desirable). The gases that the bacteria produce seem to also have an effect, although this is not completely clear yet. You can read more about the gut microbiota here.

Additionally, changes in intestinal motility commonly seen in IBS are thought to be affected by altered serotonin (5-HT) metabolism. Serotonin is released from the enterochromaffin cells of the gut enteric nervous system to stimulate gut movement and regulate gut secretions and blood flow to the intestine. Dysregulation of the enteric nervous system through the gut-brain axis can lead to increased or reduced secretion of 5-HT, which can manifest as diarrhea or constipation, respectively. (5)

In all, many things affect how quickly your gut moves its contents during digestion. A balanced, regular diet and creating a calm eating environment are some simple ways to promote healthy motility. Additionally, stress release methods calm the gut-brain connection and are also very helpful.

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