How Does Digestion Work?
Today we're going to talk about how digestion works from looking at food to when it comes out as stool. And it's complex, a lot involved on average around the world.
Every human eats about 2.2 to six pounds of food a day.
It all goes through the gut to get converted into energy to support the function of your body.
And we have 10 organs to assist us in these nine meters and 20 different cell types that we know of.
So digestion starts by us looking at food and the mouth starts to produce saliva. It produces about 1.5 liters of saliva every day. That's about six cups.
And then also when you chew food and the mouth is releasing enzymes like amylase and the breakdown of starch begins in the mouth.
The food is then converted into something called a bolus where it travels to the pharynx where you have an epiglottis that closes off the respiratory tract.
So if you've ever had water go down that tract and you cough a little bit, went down the wrong tube, that's the epiglottis, and that completely close off the respiratory tract. So it should be closing that, and it should be traveling just down the esophagus, which is about 10 inches long.
And some nerves get simulated where then peristalsis, this movement of the esophagus, this food shoot pulls the food down, passed another flap between the esophagus into the stomach, into the very acidic stomach.
So that's the purpose of this flap between the esophagus and the stomach, because that acid, we don't want coming up into our esophagus.
And I want you to make note that if you're on an acid suppressant medication, please schedule with our clinic because there are really scary long-term side effects so being on those medications for too long because it's suppressing your stomach acid, your stomach needs to be acidic so that you're properly pulling nutrients out of foods.
And basically, one thing that can happen is, that it could cause a vitamin B12 deficiency, which can lead to actual brain damage and memory issues so this is important.
I kind of got off on a roll there, but I want to rewind a little and properly introduce myself.
So I'm Ashley Oswald, Gut Health Dietitian, and founder of Oswald Digestive Clinic, where we help people improve and eliminate bothersome gut issues like gas, bloat, diarrhea, constipation, and more.
So let's restart from the stomach.
How Does Digestion Work
Okay. So now the food is in the stomach. It's very acidic and that's intentional because it needs to be acidic to properly break down the foods. So when you hear these things like, oh, your body's acidic, you need it more alkaline.
It's different in different parts of your body. Your stomach should not be alkaline, should not be basic needs to be acidic, but we'll talk about it once we get to the small intestine, that's where it becomes more basic.
So the stomach also has really strong muscles to help turn that bolus, to create it into something called chyme, which is just the food getting broken down further.
Chyme is more broken down than the bolus, which is created from chewing in your mouth. Now hormones also help to release gastric juices, including enzymes that support the further start of the breakdown of these foods. So like, Pepsin starts the breakdown of the proteins.
Not a lot of nutrients get absorbed in the stomach. Mostly some water gets absorbed there, alcohol, and some minerals like iodine and copper. And then from the stomach, the food gets trickled past the pyloric sphincter.
So right. We're going down the esophagus stomach and pass the pyloric sphincter, usually around right here for people, and into the small intestines. So the stomach expands, it can expand a lot and it holds that food. It keeps churning that food while it slowly trickles into the small intestine, which is a more narrow tube.
And I want to add that when the food gets to the stomach, these hormones also trigger the pancreas and the liver, and the gall bladder to start creating their juices, to support the breakdown and digestion once the food gets into the small intestine and mixes with those pancreatic enzymes, and then that bile is created in the liver.
It gets stored in the gallbladder and that gallbladder releases a bolus at meals of that bile, which is basically for fat absorption. So now the food is trickling through that pyloric sphincter into the small intestine. And that is where most of the absorption happens.
So most of your vitamins and minerals and the macronutrient, fat, protein, and carbohydrate in food are getting absorbed in the small intestine.
So our body was created in a very smart way where we have all these villi and microvilli. So they do look like humps and on those humps are micro humps. And that increases the surface area of the small intestine to improve the number of nutrients that can get absorbed.
So the small intestine is in total about 20 feet long and it's broken into 3 parts, the Duodenum, which is about a foot long, the Jejunum, which is about 8 feet long, and the Ileum, which is about 12 feet long.
The first part comes right from the stomach into the Duodenum, you might have heard people pronounce it as, that is where all these, juices from the pancreas like the bicarb and those enzymes come in. And that's when that food is being turned from that acidic 3.5 pH to a more basic 8.5 pH and then also the liver and the gallbladder are bringing bile in as well to support that breakdown of the fats.
So that food is going be moving through, mixing with all these juices to support the further breakdown. The liver needs proper absorption of different nutrients to function properly.
There are three different phases in the liver that rely heavily on vitamins and minerals, and amino acids to function properly. So it's a full circle here.
And then the fats are going to be broken down into free fatty acids in the glycerol. The proteins are broken down into amino acids, and the carbohydrates are broken down into glucose. So their smallest forms so that they can then get absorbed into the body.
Now, the fats mostly go into the lymphatic system, and then, the medium-chain triglyceride fat, so they can bypass the lymphatic system, and the carbohydrates and amino acids are going to pulled into the bloodstream and then transported to the different organs in the body.
And so most of that absorption is happening in the second part of the small intestine that jejunum, we have these and these microvilli on the inside of that intestine wall, like rolling Hills and all the rolling Hills, are rolling Hills themselves.
And that just really increases the surface area to support as much absorption from the food as possible, mostly happening in that second part of the intestine, the Jejunum, interestingly, most people don't know this, but the cells on our small intestine turn over about every 14 to 21 days. So it's constantly rejuvenating itself.
And about 70% of our immune system is in the intestine and these Peyer's patches. So we must be feeding it properly and supporting it properly because it's going to influence our immune system and our overall health in that way.
In the third part of the intestine, the Ileum, a lot of the fat gets absorbed there. We're heading through this, Ileocecal valve into the large intestine, which is home to most of our bugs.
Most of it, like bacteria and yeast and protozoa and viruses, we have this whole macrobiotic community in our large intestine, which then helps to feed half of like the fibers that we're eating and creates things like short-chain fatty acids like butyrate we've talked about in other topics, which is very anti-inflammatory and support some of the water of absorption, reabsorption, some reabsorption of salt in the large intestine, but it's mostly this kind of fermentation tank of our microbiota.
Most of our bacteria should be in our large intestine, and should not be coming up through that Ileocecal valve to our small intestine. That's one possible reason why people might be having a lot of gut issues. If you're having like excessive gas and bloating and just malabsorption that could be one possible thing that is going on.
So let's talk a little bit more about the large intestine. When the small intestine comes into the large intestine, we have that appendix there. And for the longest time, we just thought that it does nothing, that we don't need it.
There is some newer research showing that maybe it's helpful for our immune system if we get a bacterial infection, maybe what it does is bolus good probiotics, and good bacteria to help us fight off any possible infections.
And this theory and I talk a lot more about the appendix in another topic. If you're interested in learning the current understanding of the appendix.
So then we travel through that entire colon. And there are about a hundred trillion different microbes in here. It's pretty significant. And that whole fermentation production of the short-chain fatty acids and absorptive process creates about 10% of our daily calories.
So it's not insignificant. And that again comes from all that fiber you're eating. And I know sometimes when you say calories or fat people get afraid to like, "oh my gosh, I don't want that", or Like, "take out my large intestine" No, It's good.
We need that healthy fat to support the body and calories. This is a whole different topic, but they matter much less than food quality and listening to our hunger appetite cues, etc. when it comes to weight. So weight, I see as a symptom, if you're struggling and want to lose weight, that's a symptom.
We have to start by getting the body back into balance. And then you'll probably notice that it's much easier to get to a weight that feels healthy and normal for you. So in this mentation process, some gas is natural. So it's natural for humans to have a little gas.
It's when we have excessive gas that we want to dig deeper and see what's going on, and these bacteria could even help us to metabolize some drugs and foreign substances from the body. You could call that toxin too. There's some really interesting research there.
And interestingly, we have a hundred times the number of bacterial genes as human genes. So it begs the question. What are we? Are we more actually bacteria than we are human? I mean, that's another topic.
And then also the types of foods that we eat can change the type of bacteria in our large intestine within just a day. It's amazing. So if you're eating like a more standardized American Diet or eating plan, like a lot of this, like ultra-processed, low color foods, and then switch to whole real foods, like more so foods from the farmer's market.
Your bugs can take a drastic turn in just a day. So if you do this and you're like, "oh my God, is this worth the effort?" Just think like, those bugs are changing and yes that's significant in such a short amount of time.
Now we're getting to the end, the nerves of the rectum will feel the food coming and it will stimulate the expulsion into a stool. And we have another topic about the Bristol stool chart and what our stool tells us so you can continue your education about the digestive tract and ultimately it takes. 30 to 40 hours, this entire journey of the food from starting to chew it to bring it out into the stool.
And then last but not least, and maybe the most important one we have this Mesentery that keeps all the organs in place. This tissue keeps all where it should be so that it can function together properly in the way that I just described.
I'll see you all on the next topic. Have a great day.
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