What is "The Gut"?

Ellie Podojil

Mar 28, 2024 — 13 min read

Introduction

After listening to hundreds of hours of podcasts and skimming through thousands of pages of books, I can confidently say that there is one thing that almost all health experts agree on: the importance of gut health. I used to think that the gut was solely responsible for digesting food and eliminating waste, but a closer analysis of the functions of the gut reveals that it affects nearly every bodily process. In fact, I’ve come to learn that most of the health struggles I’ve dealt with are caused specifically by poor gut health. This includes hormonal imbalances, bloating, acne, fatigue, brain fog, and so on and so on.

The problem is that many people are oblivious to the fact that 90% of their health issues stem from their gut. Instead of addressing the root cause, they seek to treat each ailment individually. People with acne adopt a 20-step skincare routine, those who feel bloated after meals drink “magic” debloating green powders, and women with painful menstrual cycles take birth control. In reality, all of these issues can be fixed simultaneously simply by improving gut health. So why do people opt for testing a million different solutions rather than implementing one all-encompassing solution? It seems illogical to anybody with common sense. The reason is that these individualized solutions promise quick results with little effort. Skin creams claim to clear up blemishes overnight, debloating concoctions promise to keep your stomach flat all day long, and all you have to do to get rid of those painful period cramps is pop a tiny pill. On the other hand, the road to a healthy gut requires effort, consistency, and lifestyle changes. The results won’t come overnight, but they will be permanent.

Like many others, I was sucked into the scam of “quick fixes” for years, spending thousands of dollars on products that promised to fix all of my health problems with the snap of a finger. When I finally realized that they didn’t work or caused side effects that were worse than the original problem to begin with, I decided to put the control back in my hands and get to the root of my health issues - the gut.

Now that we’ve established the importance of gut health, let’s get into the nitty gritty. Unfortunately I’m one of those people that needs to know the “why” behind everything, so my journey to optimal gut health started with the basics. Before diving into how to fix my gut, I needed to know what the gut actually was. I found an excellent explanation in the book titled Athlete's Gut by Patrick Wilson. In this blog, I will break down what I’ve learned about the basic anatomy of the gut and the process of digestion in a way that makes sense to me, and hopefully many others.

What is “The Gut"?

To put it plainly, the gut is the network of tubing in between the mouth and the anus. This includes the esophagus, the stomach, the small intestine, the large intestine and other organs that play a role in the digestive process like the pancreas, liver, and gallbladder. Each section of the gut plays different but equally critical roles in breaking down food and absorbing nutrients. Think of it like an assembly line - if just one “worker” fails to keep up, the entire system falls out of whack. That’s why it’s important to understand the functions of each worker on the assembly line that makes up the gut; starting at the mouth and ending at the anus (or whatever you like to call it).

The Mouth

The Short Version

When food is chewed in the mouth, it becomes easier for the other sections of the gut to break down and absorb the available nutrients.

The Long Version

As a bite of food is crushed into smaller particles between the teeth, mechanical breakdown occurs. This mechanical breakdown makes chemical breakdown easier for the rest of the gut because “mashing up” the food increases the surface area that enzymes have to work with. I like to think of the pre-chewed food as a bowling ball and the post-chewed food as a thousand tiny marbles. The enzymes that break down proteins, carbs, and fats from food would much rather work on the marbles than the bowling ball; not only because they are smaller, but because they have a greater surface area for enzymes to cling to.

Although mechanical breakdown of food is the main function of the mouth, it also provides small amounts of chemical breakdown via enzymes found in saliva:

Amylase: Breaks down large carbohydrate molecules into shorter chains
Lingual Lipase: Initiates fat digestion

Overall, not much chemical digestion occurs in the mouth, and its main function is to mash food into smaller pieces.

My Real Life Take-Away:

Thoroughly chewing food will help with digestion and nutrient absorption because it will make the enzymes’ job a whole lot easier. Swallowing big portions of food or eating too fast causes discomfort because the enzymes in the gut cannot break down the “bowling ball” efficiently.

The Esophagus

The Short Version

The esophagus serves as a tunnel for food to travel from the mouth to the stomach.

The Long Version

The esophagus is pretty straight forward. Its main job is to transport chewed up food from the mouth to the stomach via a process called peristalsis.

Peristalsis: A wave like motion that occurs due to the constriction and relaxing of smooth muscle cells in the gut wall - think of squeezing toothpaste out of the bottle.

At the top and bottom of the esophagus are circular rings of muscle called esophageal sphincters that open and close when stimulated. Think of them like bodyguards that allow food to pass from the mouth to the esophagus, then from the esophagus to the small intestine (the next section of the gut). These sphincters ensure that food is flowing in the right direction, so as you can imagine, when they fail to carry out their job, food flows straight back up. This phenomenon is more commonly known as Gastroesophageal Reflux Disorder or GERD. As you can tell, the job of the esophagus is simple but important.

My Real Life Take-Away

Yes, the job of the esophagus and the esophageal sphincters is to make sure food is flowing in the right direction, but there are things we can do to help this process flow smoothly.

Never lay down right after eating (I think this one is common sense)
Avoid eating too fast: the esophageal sphincters may become overwhelmed and delay opening, causing food to “get stuck” or travel in the opposite direction
Maintain a healthy weight: Excess weight puts pressure on the abdomen, pushing up on the stomach and causing acid to seep into the esophagus

The Stomach

The Short Version

Mechanical digestion continues in the stomach as peristalsis churns and mixes food boluses, but a major portion of chemical digestion also occurs in the stomach through the release of enzymes and acid.

The Long Version

The stomach is where food first encounters the big guns of digestion. I don’t want to get too sciency here, but I do want to offer a brief description of the enzymes that play a key role in the stomach.

Gastric Lipase: The lingual lipase found in saliva follows the food bolus to the stomach where it continues the breakdown of fat molecules. The stomach releases a more abundant enzyme - gastric lipase - to help with the process.
Amylase: The amylase released by the salivary glands also follows the food bolus into the stomach, however, the acidic environment of the stomach kills this amylase, leaving most carbohydrate breakdown to be handled by the small intestine
Hydrochloric Acid: A heavy portion of protein breakdown occurs in the stomach due to the presence of hydrochloric acid. This protein breakdown occurs in three important steps:

Hydrochloric acid denatures - or unravels - the structure of proteins which allows enzymes to attack the bonds that hold the molecules together
Hydrochloric acid activates the protein-digesting enzyme called Pepsin. Without the presence of hydrochloric acid, it would remain in its inactive state called Pepsinogen
Pepsin breaks down protein into smaller peptides and amino acids that can be absorbed in the small intestine and used for other processes in the body like repairing muscle tissue and regulating hormones

Although the functions of enzymes and acids are very complex, the main point I want to get across is that the stomach is the first section of the gut to initiate major chemical breakdown of food: especially protein and fats.

I also want to briefly touch on another function of the stomach, which is its capacity for expansion. The stomach is the only section of the gut that has the ability to expand - and by expand I mean it can increase 50-70% in volume! Stomach expansion is a natural process that occurs when eating food, however, people who habitually eat larger portion sizes can tolerate greater stomach expansion before feeling discomfort than those who eat healthy portion sizes. Yes, that means obese individuals or individuals who binge eat have trained their stomach to hold larger volumes of food, so they can eat more before they get to that point of feeling “stuffed”. It makes me regret all those trips to Golden Corral in high school.

My Real Life Take-Away

Upping the amount of protein in a meal increases the time it takes for Pepsin to break down the protein molecules and delays stomach emptying. That’s why eating meals high in protein makes you feel fuller for longer and curbs sugar cravings.
Continuing to eat after the point of stomach discomfort will train your stomach to expand so it can hold more food. As your stomach continues to expand, it will require more food to make you feel “full”, naturally leading to higher food consumption.

The Small Intestine

The Short Version

The small intestine chemically breaks down proteins, fats, and carbohydrates and transports the resulting products into the bloodstream where they are shipped off to be used in other vital bodily functions.

The Long Version

The small intestine is the longest portion of the gut by far, with an average length of 3-7 meters depending on the size of the individual it sits in. This long strand of intestine must take on a coiled shape to fit inside the abdomen, and it’s the most important location for nutrient absorption. A lot of this nutrient breakdown can be accredited to the pancreas, also known as the “workhouse” of the digestive system. I will insert a short step-by-step explanation of the specific role of the pancreas because it’s that important to the digestive process.

How the Pancreas Works in the Digestive Tract

The stomach acid that’s mixed in with the food bolus comes into contact with the cells lining the small intestine
These cells secrete hormones that travel to the pancreas
The hormones trigger the pancreas to dump its “juices” into the small intestine

The pancreas spews out about 2.5 liters of these “juices” per day, and they contain enzymes that are vital to chemical digestion.

Pancreatic Lipase: Continues the breakdown of fats
Pancreatic Amylase: Handles the majority of carbohydrate breakdown
Trypsin and Chymotrypsin: Continues breakdown of protein

At this point, the substance that first entered the mouth as food has been battered and beaten into nutrients that can now be used by other cells. Luckily, the lining of the small intestine is made up of numerous circular folds that facilitate transportation into the bloodstream. More commonly known as microvilli, these finger-like membrane protrusions increase the surface area where absorption of nutrients can occur. Once in the bloodstream, the nutrients are packaged up and shipped to another destination. I know it’s all very confusing, so I’m going to lay it out in a step-by-step fashion that makes sense to me, starting with a good old-fashioned cheeseburger.

You take a big bite of a juicy cheeseburger
As the bite of food travels through the gut…
1. Amylase breaks down the carbohydrates in the bun
2. Lipase breaks down the fat in the cheese
3. Pepsinogen/Trypsin/Chymotrypsin breaks down the protein in the beef
After the enzymes have worked their magic…
1. The carbohydrates have turned into simple sugars (e.g. glucose)
2. The fats have broken down into fatty acids (e.g. linolenic acid)
3. Proteins have broken down into amino acids (e.g. glycine)
These molecules are transported into the bloodstream via the microvilli on the lining of the small intestine
Once in the bloodstream, they are shipped to other areas of the body to carry out vital processes. For example:
1. Glucose is deposited into nearly every body cell as a source of energy
2. Linolenic acid can be used to form the membrane barrier of cells in the body
3. Glycine travels to the muscles to help synthesize body protein (muscle mass)

It makes a little more sense when you understand the gut as an assembly line that starts with a piece of food and ends with nutrients in their most basic form.

My Real Life Take-Away

If you think about it, the lining of the small intestine determines what external substances are allowed to enter our body. This lining is only one cell thick, so it's extremely susceptible to damage. When we consume things that are harmful to the gut lining like alcohol, antibiotics, and processed foods, the barrier is damaged and becomes more permeable to toxins that it would normally keep out. This process is what you might know as Leaky Gut Syndrome and it can wreak havoc on the body. In order to maintain integrity of the small intestinal lining, it's important to avoid foods and environmental contaminants that cause inflammation and cell damage. I think we all have a basic understanding of what those might be.

The Large Intestine (Colon)

The Short Version

After enduring mechanical and chemical breakdown, the last bit of the food bolus makes its way into the large intestine in the form of undigested food, fiber, water, and electrolytes. The large intestine is like a pawn dealer, it picks out the valuable goods to be reabsorbed in the body, and leaves the junk to be excreted.

The Long Version

Unabsorbed substances from the small intestine, or chyme, travel through the ileocecal valve and into the large intestine where the good stuff is separated from the bad stuff. The “good stuff” usually refers to water and electrolytes, of which most is reabsorbed back into the body. In fact, the large intestine absorbs 90% of fluid that passes into it in order to maintain ideal hydration levels in the body. After this reabsorption of the “good stuff”, we’re left with a lump of stool that mostly consists of fiber - a nondigestible carbohydrate. This lump of stool travels to the rectum where it is eventually excreted (hopefully in the bathroom).

Another key function of the large intestine is its role in housing our gut microbiome - the trillions of living microorganisms that live inside our gut walls. The large intestine is a jungle of organisms, compared to the stomach and small intestine which have unwelcoming environments for microbes due to stomach acid, bile, pancreas juices, and antimicrobial agents. Although the gut microbiome consists of many different organisms, it is overwhelmingly populated by bacteria. As many as 40 trillion bacterial cells reside in the large intestine, which is more than the total number of human cells in your body. That probably sounds like a disaster waiting to happen since we’ve all been conditioned to view bacteria as a harmful pathogen. However, recent discoveries show that bacteria residing in the large intestine play vital roles in overall health. Most importantly, healthy bacteria can break down substances that are indigestible by human enzymes such as polyphenols and complex carbohydrates. The byproducts of these substances are antioxidants, cholesterol regulators, anti-inflammatory agents, anti-obesity agents, and they even have cancer-fighting properties. The gut microbiome is an in-depth topic to be discussed separately, but for now just understand that the quantity and quality of your gut bacteria has a significant correlation to health.

My Real Life Take-Away

Eating fiber-rich foods will facilitate the process of emptying stool from the large intestine and allow for more regular bowel movements.
Since the large intestine hosts trillions of bacteria that significantly impact our health, it might be a good idea to feed these bacteria foods that help them thrive - AKA probiotics and prebiotics. Probiotics are foods that actually contain live microorganisms such as yogurt, sauerkraut, and kombucha. Prebiotics are foods that foster growth of already existing bacteria such as bananas, garlic, and sweet potatoes.

The Rectum

The Short Version

The rectum is the gateway for emptying stool.

The Long Version

As stool or gas enters the rectum from the large intestine, receptors are activated that send signals to the brain that are interpreted as discomfort or the need to blow up the bathroom. Luckily the nerve endings in the rectum allow us to differentiate between solids, liquids, and gasses - which ultimately prevents us from shitting our pants or sharting. That’s pretty much all you need to know.

My Real Life Take-Away

I really hope the nerve endings in my rectum are working properly.

Conclusion

I am not claiming to be an expert on the anatomy or functions of the gut, this is just a summary of what I learned from reading a book written by someone way smarter than me. If you want a more detailed layout, I highly recommend reading Athlete's Gut by Patrick Wilson, especially if you struggle with gastrointestinal discomfort during exercise. One thing I do know is that I have been convinced that the state of the gut has a huge impact on the rest of the body, and having a basic knowledge of the digestive system will allow me to better understand more specific processes related to the gut. For example, I want to to understand how gut health affects hormones, the immune system, and even mood. Those are all much more complex topics to tackle, but I feel better-equipped now that I have a basic understanding of the gut.

I hope after reading this post you can confidently answer anyone who asks you "What is The Gut?". Next week we will take a deeper dive into the gut microbiome and the wide range of roles it plays in the body. Thanks for reading!