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Gut Function

by previlli

How what happens in your gut affects your whole body


If you’re like most people, you probably think of gut health as an isolated issue, affecting your digestion but not much else. Surprisingly, though, your gastrointestinal tract has four main functions — and only the first two have to do with digesting food. Plus, all four functions have effects that can be felt throughout your body. If something goes wrong in your gut, you could end up with problems as diverse and seemingly unrelated as brain fog, allergies, mood swings, respiratory challenges, sleep disruptions, weight gain, skin concerns, nutritional deficiencies, pain in your gastrointestinal tract, and respiratory, cardiovascular, or metabolic issues. In other words, what happens in the gut doesn’t stay in the gut. It reverberates throughout your entire body.

Gut Function #1: Nutrient Harvest


The most basic function of your gastrointestinal tract is to extract nutrients from the food you eat. That is why we eat, after all. To fully understand the marvel that is your body’s ability to harvest nutrients, let’s track what happens to a hamburger from the moment it enters your body to the moment it exits.
Digestion starts in your mouth in two ways: most obviously with chewing, the most mechanical part of the process, but also with the early stages of carbohydrate digestion. As soon as you take that first bite of burger, your mouth releases digestive enzymes that initiate the breakdown of simple carbohydrates in the bun down into simpler sugars. 
When you chew and swallow that bite, it descends through your esophagus and reaches your stomach. The presence of food there signals your stomach to fill with hydrochloric acid, which continues the process of breaking it down into smaller pieces. 
It is one of the wonders of gut structure that you have this caldron of bubbling acid in your stomach that is strong enough to dissolve a penny, yet it doesn’t harm you. Why? Because it is contained by the pink, protective mucin lining of your stomach, which is generated at an astonishingly quick pace. Your body does an amazing job of managing this potentially toxic environment such that typically, you only have an occasional burp.   
Also inside your belly, protein-specific enzymes break the protein in the burger down into amino acids, but again, without breaking down your stomach wall, thanks to that lining of mucin (also called your gut lining). You are only able to break down and digest food because your gut lining keeps you from breaking down and digesting yourself! 
After leaving your stomach, the digested protein, partially digested carbohydrates, and undigested fats pass into your small intestine — the longest part of your digestive tract. Like your tummy, it too is covered by your gut lining.  
Your gut lining is a bit of a chameleon, changing its characteristics for each stage of digestion to properly do its job In the small intestine, the mucosal structure is such that the cells of your intestinal wall produce a single layer of mucus to provide protection, but not inhibit absorption. In the large intestine, where most of your gut bacteria live, the cells lay down a double layer of mucin for extra protection and to provide a sticky surface for good bacteria to hang on to. 
It’s in your small intestine that your body digests the fat in your burger, and where it finishes the job of simple carbohydrate digestion. The process is still partly mechanical, through movements of the small intestine called peristalsis, and partly chemical, through digestive fluids. 
Your burger is now in tiny little pieces. The nutrients inside it have been freed and are ready to pass through the villi — tiny finger-like projections that cover the folds and hills of your small intestine lining. The structure of the small intestine is another of nature’s engineering wonders. The folds and villi turn your 20-foot small intestine tube into the surface area of a tennis court.
In addition to the villi, your digestive system is also equipped with microvilli — much smaller filaments that are tiny extensions of the cell membrane. Think of the microvilli as the doors from your small intestine to your bloodstream. They let the nutrients (e.g. carbs, protein, fat, vitamins and minerals) from your digested burger into your bloodstream, where they circulate through your whole body, fueling and nourishing every part of you.  
But it’s not just your digestive organs involved in digesting that burger. Your gut bacteria help by fermenting the insoluble fiber of the onions, lettuce, and tomato that top your burger into short-chain fatty acids (SCFAs) and gasses. While you may not welcome the gasses, you do want the SCFAs, because they supply five to ten percent of your daily energy requirements. That number goes up even higher if you eat a lot of fiber.
The major SCFAs in the gut are acetate, propionate, and butyrate, but not everyone makes the same SCFAs. The quantity and type of SCFAs produced in your gut depend on how old you are, what you eat, and how healthy your gut bacteria are. Healthier gut bacteria make better SCFAs. 

The Problems with a Permeable Gut


As you can see from our hamburger example, the structures of the gut play a vital role in protecting us from the harsh conditions necessary to extract nutrients from our food. A permeable gut is what happens when your gut lining may have holes or cracks, which open your bloodstreams to anything that’s capable of making it through. This results in things like partially digested food, toxins, harmful bacteria, and renegade immune molecules to sneak into your bloodstream and penetrate tissues and other parts of the body where they aren’t supposed to be.
Because the gut is exposed to so many harmful outside compounds; because we don’t always eat the healthy foods we should; because we live in a world where pollution and toxins cannot be completely avoided; everyone to some extent is battling day by day to maintain a healthy gut barrier. A lack of fiber, combined with heavy sugar, saturated fat and/or alcohol consumption, and certain medications, can intensify the permeability of your gut. To combat this, a change in diet, such as increasing consumption of fiber (as found in fruits and vegetables) and protective superfood polyphenols, while cutting back or removing alcohol, heavy sugars or saturated fat, can help secure your gut’s health.

Gut Function #2: Waste Disposal


What goes in must eventually come out. The last step of digestion is elimination. 
While waste is in your small intestine, 90 percent of its water content is reabsorbed. The large intestine absorbs most of the remainder, so the waste can be converted into solid stool. (This is why when waste passes quickly through your system — when you have diarrhea — it is liquid. The small intestine doesn’t have time to absorb the water.)
Again, the specialized cells of the large intestine and your gut bacteria play a big role in the final process of breaking down any proteins and carbohydrates in your burger that were not completely digested in the small intestine. Bonus: intestinal bacteria (or their byproducts) also synthesize several B vitamins and vitamin K. 
You can get a good idea of how important gut bacteria are in the large intestine by how populous they are. The further along the digestive tract, the more gut bacteria there are. Per gram of intestinal surface, there are 10 10 to 10 11 microbial cells in the colon — the very last section of the large intestine.¹ Given this, it’s not surprising that imbalances in gut bacteria can lead to problems with elimination, such as constipation, diarrhea, or more serious bowel problems.
Now that your burger’s completely digested, waste proceeds from your large intestine to your rectum and anus, where it leaves your body as feces. Feces is more than just undigested food, though. It also contains dead and live gut bacteria, cells that have been shed from the mucus layer lining the gut, inorganic salts, and enough water to soften the waste and smooth its passage. 
Changes in the color, shape, consistency, size, and frequency of your bowel movements can signal problems with your digestion or other bowel health issues. The Bristol Stool Scale is a helpful tool to help you evaluate your bowel movements and identify some common digestive ailments.

Gut Function #3: Immune Signaling


Digestion and elimination are the most well-known functions of the gut, but there’s more happening there than you probably ever suspected. 
For starters, most of your immune cells (70 to 80 percent) reside in your gut.² There’s good reason for this. Because your gastrointestinal tract consists of a series of interconnected hollow organs that go from your mouth to your anus, your gut is in constant contact with the outside world, more so than any other part of your body — even your skin. That’s why it needs its own personal army. 
This army is called the gut-associated lymphoid tissue (or GALT for short). Its job is to protect your body from a gut invasion. 
The GALT is home to a whole slew of immune cells, such as B cells, T cells, macrophages, and dendritic cells. Should a pathogen make it into your GI tract — from say, eating an undercooked burger contaminated with E. coli bacteria — those immune cells are loaded and ready to fire upon the intruder.
Your GALT doesn’t work alone though. Your gut mucosa — that layer of cells covered in mucus or the lining your gastrointestinal tract — has evolved to help your GALT cells and your beneficial gut bacteria work together to protect your body from pathogens. 
How do good gut bacteria ward off bad? One of the most important tasks your immune system mastered as it matured during your infancy was learning to tell the difference between beneficial and harmful substances, or “self” versus “not self.” Your gut bacteria provided a sort of training course for your immune system.
Another important role your gut plays outside of digestion and elimination is that of gatekeeper. The surface of your gut serves as a barrier between foreign substances passing through your gut and the rest of the body. However, if that barrier becomes permeable, things that don’t belong in your bloodstream can end up there. Immune cells, recognizing something is out of place, mount an attack, which can lead to a whole host of issues, from food allergies to more serious bowel problems. 
So what weakens the gut barrier? The top offenders include poor diet, illness, stress, hormonal imbalances, and certain medicines. And to make matters worse, when your gut is not self-contained, the problems associated with an excess of harmful bacteria go beyond digestive ills. They can affect your blood sugar, your weight, your heart, your bronchial passageways, your skin, and even your outlook on life.

Gut Function #4: Hormone Production


To recap, your gut harvests nutrients, disposes of waste, and protects you from pathogens. Your gut is also a factory, but instead of making widgets, it makes hormones. In fact, the vast majority of your endocrine (or hormone) system lives in your gut, in the form of enteroendocrine cells.
About 1 percent of the cells in the top layer of your intestines are enteroendocrine cells. They are like German Shepherds for your gut — they have a keen ability to sense which nutrients are hanging out there and then respond appropriately. 
Since the goal of enteroendocrine cells is to help the other cells of your intestinal lining with nutrient absorption, responding appropriately means secreting hormones that will help digest whatever food you just ate. For example, if you’ve just enjoyed an omelet, your enteroendocrine cells will produce gastrin — a hormone that stimulates the release of protein-digesting gastric and pancreatic juices. If you’ve just noshed on a round of brie cheese, they’ll make cholecystokinin — a hormone that causes the secretion of fat-digesting bile.³
Enteroendocrine cells are about more than just sensing what kind of food needs to be broken down. They also secrete hormones that regulate hunger and satiety. When you’re craving a late-night snack, you can blame your enteroendocrine cells for releasing the appetite-increasing hormone ghrelin. (Although, to be fair, they’re just trying to help you out. If you’re up late, they figure you need more fuel.) When you’re feeling sated, you can thank those same cells for secreting the stop-eating hormone leptin. 
Enteroendocrine cells also aid intestinal motility, the movement of food through your gastrointestinal tract. If you’re feeling the need to have a bowel movement, your enteroendocrine cells have secreted the hormone motilin, which causes the contractions in your intestines that move waste along.
It’s not just your gut cells that produce hormones. Your gut bacteria do too. They can turn the stress hormone cortisol into male sex hormones like testosterone.⁴ They can produce an enzyme that turns the female sex hormone estrogen into its active form.⁵ And they can manufacture neurotransmitters that act a lot like hormones, such as serotonin (which regulates mood), dopamine (which controls pleasure and reward), and norepinephrine (which prepares the body for action).⁶

The Problems with Gut Dysbiosis


If your gut wall is in tip top shape, several things go right. Your gut barrier is tight, so your gut microbes stay inside the lumen (the hollow space inside your intestines) instead of traveling where they don’t belong. Your gut microbiota is balanced, so the bad bacteria stay in check. You produce just enough of the right hormones at the right time, so everything runs smoothly. And your immune system is finely tuned, so you’re ready for an attack. 
But if it’s out of balance — if you suffer from gut dysbiosis — things can go equally wrong. Symptoms of gut dysbiosis can vary wildly depending on the imbalance. Looking at hormones as one example, if you have too much of the appetite-stimulating hormone ghrelin, you might start craving food more often, regardless of whether you’re actually hungry. Meanwhile, too little of the satiating hormone leptin could mean that you’re not feeling hungry even when you really do need to eat. This is yet another reason why maintaining a healthy gut structure and balance of gut bacteria is so important to your health.

How Gut Functions Affect Your Health


A healthy gut helps build a healthy body, while an unhealthy gut can cause all sorts of problems from the gut on out. Here are just some of the ways your gut health affects your overall health:

How gut health affects your brain

Your gut health is intimately related to your brain health. As mentioned previously, your gut bacteria can produce over 30 kinds of neurotransmitters such as serotonin, dopamine, and norepinephrine. These brain chemicals regulate your mood, pleasure/reward system, and action, which heavily influence your state of mind, brain activity, and decision-making.

How gut health affects your mental health

In addition to your brain function, your gut health also affects your mental well-being. There is strong speculation in neuropsychology that certain mental disorders are linked with imbalances in gut bacteria. This suggests that maintaining a healthy gut — including a healthy balance of gut bacteria — could be a simple way to promote mental health. In contrast, activities such as consumption of antibiotics, a poor diet, heavy alcohol consumption, chemotherapy, and radiation treatment can interfere with your gut bacteria and affect your mental well-being. Certainly, your gut health has a significant impact on how you perceive and manage stress.

How gut health affects your skin

An imbalance in your gut bacteria, such as a lack of good bacteria and/or an overabundance of bad bacteria, can negatively affect your skin condition. Generally, people who have chronic skin conditions also tend to have issues related to digestion and gut inflammation. 

How gut health affects your mood

Maintaining a healthy balance of gut bacteria can positively impact your mood. That’s because your gut bacteria produce many mood-regulating neurochemicals, including the neurotransmitter serotonin. In fact, 90 percent of your body’s serotonin is made in your gut. If you have an unhealthy gut, you may not have the microbes you need to produce adequate serotonin, leaving you feeling anxious, sad, and irritable. Maintaining a healthy gut helps ensure you have plentiful supplies of serotonin. 

How gut health affects your weight

Your weight can be heavily affected by your gut health. That’s because gut bacteria affect nutrient absorption — particularly how much fat you absorb from the foods you eat. They also impact energy storage — in other words, whether you burn those calories you just ate or they end up stored on your hips. In fact, how much weight you lose on a particular diet is influenced by which types of bacteria are taking up residence in your gut. Research shows that if you eat the same high-fiber, whole grain diet as your friend, but you have more of the bacteria phylum Prevotella in your gut while your friend has more Bacteroidetes, statistically speaking, you will lose 5.1 more pounds of body fat over a six month period. 

The World Within You


Did you notice one commonality of all four gut functions? They all rely on your gut structure, which houses your gut bacteria!
There are tens of trillions of microorganisms living inside your gut, going about their business as you go about yours. Even if you don’t think about them much, they influence how your body and mind function every day. If you are digesting and eliminating your food comfortably, and enjoying good immune health and hormonal balance, thank the legions of tiny guests in your digestive tract. 
You can keep your gut wall strong and your bacteria happy by eating fiber-rich foods that nourish them, staying hydrated and well-rested, reducing your sugar intake, exercising, not smoking, spending time outdoors, and avoiding the overuse of antibiotics (these kill the good ones along with the bad ones). When it comes to your health, you and your gut bacteria are in it together.

References
  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5745014/
  2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2515351/
  3. https://www.news-medical.net/health/What-are-Enteroendocrine-Cells.aspx
  4. https://phys.org/news/2013-09-specialized-bacterial-cells-gut-steroid.html
  5. https://kresserinstitute.com/gut-hormone-connection-gut-microbes-influence-estrogen-levels/
  6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4259177/
  7. https://www.medicalnewstoday.com/articles/325293.php
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