The Greek physician Galen viewed the stomach as a feeling being that knew when it was empty or full. He believed that nature granted the stomach the ability to feel when there is a lack of food which then motivated a person to go out and seek food. Does the stomach feel?
Let's discuss what the organs sense and how they communicate in order to perform the complex tasks of emptying the organs, changing pH levels, releasing enzymes and more.
It may not work quite like Galen thought but there is an extensive network of hormones and neurotransmitters that send signals back & forth from the gut to the brain or from the gut to various organs. Even if there are many signals going back & forth the gut doesn't rely on the brain to operate. There are neurons in the gut that control mobility, nutrient absorption, and the release of enzymes so the digestion can keep going no matter how distracted we get. Hormones and neurotransmitters are the chemical signals that help keep digestion moving forward.
These hormones are released by endocrine cells, a collection of cells that secrete hormones in the body, in the gut and pancreas. The gut has more endocrine cells than anywhere else in the body. It is the largest endocrine organ. Think of hormones as communication signals. Taking a piece of information from one area and sending it to another. Like a telegram with encoded information. These messages regulate many of our bodily functions. Much of the signaling is done in response to the presence of nutrients. As if the entire body is saying "hey guys, dinner is ready, grab the crew & lets eat".
There are five (5) hormones present within all mammals that regulate digestion.
1. Gastrin
2. Secretin
3. Cholecystokinin
4. Gastric Inhibitory Peptide (GIP)
5. Motilin
When food arrives and expands the stomach, it triggers Gastrin, the messenger that tells the stomach to release hydrochloric and pepsin for digestion, to be secreted. It says, “hey guys lets sterilize and demolish this stuff’.
Gastrin keeps going until the PH is low enough to adequately do its job of decontamination and breaking down proteins. Pepsin loves to work in a low PH environment. Gastrin also stimulates the growth of the stomach lining and mobility and is also stimulated by caffeine, alcohol and undigested proteins. A low pH inhibits this hormone because when the pH is lowered it knows its job is done.
Secretin is a hormone that helps regulate water and pH. Once food passes from the stomach into the Duodenum, Secretin is notified that there is low pH coming its way in the form of mushed food.
It knows to stimulate the pancreas to release sodium bicarbonate to raise the pH, that way the enzyme needed for the next phase can activate as they prefer the higher pH. This also prepares the small intestine for absorption. Secretin stimulates bile production into the liver, this is in perfect concert with our next hormone that makes sure that bile production has a place to go since bile is needed to break down any fats along the way.
Cholecystokinin stimulates the gallbladder to release bile and the pancreas to release enzymes.
Cholecystokinin is also released in the Duodenum. This hormone stimulates the gallbladder to release its bile by sending a signal for it to contract. It also releases digestive enzymes in the pancreas when fat is present.
The next hormone Gastric Inhibitory Peptide (GIP) is an enzyme that slows down the churning of the stomach and inhibits the acid as it releases partially digested food or chyme into the small intestine. It also induces insulin. Insulin is a hormone that is made by the pancreas. Since sugar cannot go into cells directly, we rely on our friend the pancreas to help us out.
When we eat sugar, insulin attaches itself to the cell wall and helps it absorb the sugar. Sugar is a great energy source when eaten in moderation and from the right sources. Another great thing insulin does is signal the liver to store extra sugar for those cold winter nights. Now many of us don’t face that type of struggle or deprivation anymore but you can see why this was a useful trick for our ancestors and how it can be useful in terms of emergency.
Motlin increases the Myoelectric Motor Mobility. In the Duodenum, this hormone increases what we call the Migrating Myoelectric Motor Mobility, a wave of electrical activity that passes through the Intestines during a regular fasting period. Think of this as a night shift janitor. This janitor comes in & cleans and sweeps when no one else is around picking up the undigested pieces. The intestine likes to keep a clean house. The sound of growling that we have come to associate with hunger is actually the sound of the stomach being cleaned.
Let’s take a minute and look at this often-overlooked process.
Fasting was a part of many cultures for spiritual purpose or lack of resources, but only now are we starting to understand the importance of this phase of digestion in overall gut health. We know that during the migrating motor complex any bacteria that have traveled into the small intestine are ushered back into the large intestine. This is the time for the rest of the phase to rest & digest. This often-overlooked function is one of the most important.
Now that we have covered these 5 digestive hormones let’s talk about the famous hunger hormones, Ghrelin & Leptin. Ghrelin even just sounds like a hungry stomach.
Ghrelin is the hormone that increases hunger. Ghrelin is released by the stomach & targets the pituitary gland signaling the brain that the stomach is empty and needs to eat. Maybe this is what Galen was talking about. On the opposite team is Leptin, made by fat cells, this hormone decreases appetite.
As I mentioned in the beginning hormones & neurons regulate the digestive tract. These neurons are part of the enteric & central nervous system. Haven’t heard of the enteric nervous system? It’s the brain in your gut, the one that operates completely independently from the brain in your head. The enteric nervous system is the nervous system that governs the digestive tract.
The digestive tract has 100 million neurons from mouth to anus. These neurons coordinate what we call long & short reflexes. Long reflexes are controlled by sensory neurons that send signals to the central nervous system and to the brain. So, when you think about that freshly baked cookie in the bakery case & you start to salivate, it’s because the message was sent to your hippocampus that food is coming. That’s the long reflex.
The short reflexes are messages passed around the gut by hormones & neurons. These are messages that don’t have to go to the brain. They happen in the gut, their sent in the gut and they are received somewhere in the gut. All of this is done using its enteric nervous system. So, the reflex can happen quickly and automatically based on what’s needed in the moment.
So, for example, when your stomach distends because you just ate a few slices of pizza your body starts to secrete enzymes & hydrochloric acid. This all happens outside of your conscious awareness while your typing a report scarfing down your lunch. As many of us do you may even forget that you are eating or not notice that your full because your brain doesn’t have to tell your stomach to start digesting. This has its perks and it has its setbacks. There are also reflexes that control everything from movement to waste leaving the anus.
When we put together hormones & reflexes, we can see that digestion is influenced by these signals which regulate glands and instruct them to secrete enzymes and influence motility and movement to the food via muscle contraction.
Gastral intestinal hormones were discovered by William Bayliss & Ernest Starling in 1902. This is the same team that discovered that the gut could continue its process of reflexes for digestion even when no longer connected to the brain. For them this signified that the gut has its own nervous system.
We now know that the gut has more neurons than the entire spinal cord. Through this chemical network of hormones and neurons we are able to keep an incredibly complicated system running smoothly. Isn’t that impressive? We have a number of reflexes some that come from the brain and others that come from within the gut. Through this process we are able to sift through and digest just about anything.
Here’s a challenge for you – the next time you have a judgmental thought about your body think about how may parts and systems are perfectly coordinated inside of you every day making tasks like digestion look simple and easy. Your body is like a super intelligent computer.
Next week we'll discuss: How to change your relationship with food
TEACHABLE MONDAY