Digestion for (not-so) Dummies: Small Intestine Pt.1

The main event! Finally!

I’ve been saying for weeks that we’d get to the small intestine, and we’re finally here.

In our north-to-south journey through digestion, we started in the mind, then went south to the mouth, the esophagus & stomach, and also talked about the importance of sufficient stomach acid, and what to do if you don’t have sufficient stomach acid. Last time, we did reach the small intestine, but before we could talk about the incredibleness that is this very long, convoluted tube, we had to cover the roles of the gallbladder and pancreas.

So today we’ll start tackling the role of the small intestine (SI) itself. It’s gonna take some time, though—and definitely more than one post—because we’ve got a lot of ground to cover here, and I mean that figuratively and literally. Some sources say that if you unfolded the SI and laid it flat, its surface area would cover a tennis court! This isn’t only because it’s long—about 20 feet long, in fact. (It fits inside your abdominal cavity because it’s got lots of turns and sort of folds up on itself. See the illustration above or another good one here.) The reason the surface area of the SI is so huge is because it is exquisitely designed to suit its function. The SI is the site of absorption of almost all nutrients into the bloodstream. Remember: the GI tract is a hollow tube that connects to the outside world at both ends (mouth at the top and anus at the bottom). Until food is actually absorbed from the intestines into the bloodstream, some people consider it to still be outside the body. So the SI is really the point of connection between outside and inside, “us” and “them.” This is an extremely important point, and we’ll come back to it for sure.

For having the astounding task of absorbing all the useful stuff from our food and passing along to the large intestine anything not useful, the SI is shaped just right. First, there’s its length. Things get absorbed as they move along, and since there’s a lot of stuff that needs to get absorbed, it’s a good thing there’s a long distance over which this can happen. Second, just like the stomach, the SI is divided into sections, and different things happen in them. The first part of the SI is called the duodenum, and it’s mostly the site of even more mechanical and chemical breakdown of food. Recall from this post that the muscles surrounding the stomach contract and relax to create a “churning” action, which is basically the stomach’s way of chewing. It helps mix the food with gastric juices the same way the swishing and turning of your washing machine mixes clothing with water and detergent. The same thing happens here in the duodenum. There are muscles wrapped around several layers of your SI, and they squeeze and relax, too, giving the food its final chance to get as small as possible and mix thoroughly with the digestive enzymes secreted by the pancreas. (By the way, since I haven’t mentioned it in a while, the million-dollar science-speak word for these involuntary muscle contractions that help propel food along the entire GI tract is peristalsis. [Y’know, just in case you’re ever on Jeopardy! someday. Might come in handy.])

So the food is getting very small and highly mixed with enzymes. At this point, it’s not really “food” anymore. In fact, by the time it left the stomach, it was called chyme, and now, assuming digestion has gone well so far, proteins have been broken down into individual amino acids and small peptides (chains of amino acids linked together), fats have been broken down into mono- and diglycerides and free fatty acids, and carbohydrates have been broken down into mono- and disaccharides (one or two sugar molecules linked together). These are the smallest or just a few steps away from the smallest particles that make up food. And good thing they’re so small now, because they need to be small to be absorbed through the lining of the SI and passed into the bloodstream to reach the rest of the body. 

After the duodenum’s done its job of breaking things down, it passes the nutrient baton to the next two sections of the SI, called the jejunum and the ileum. These next two are where the nutrients are absorbed. Whew! It’s taken us a while, but we’ve finally arrived at the point where the food we eat becomes part of our body, or at least starts on its way toward that goal. To be physiologically accurate, I have to specify here that after passing through the cells in the SI, amino acids & small peptides, and mono- and disaccharides can enter the bloodstream directly. Monoglycerides and free fatty acids, however, have to take a detour first. See, they’re not water soluble, so they can’t travel through the blood without help. (Kind of like mixing oil and water...as in, they don't mix.) These breakdown products from fats have to go through the lymphatic system first and then into the blood. (Note: the fat-soluble vitamins, like A, D, E, and K, have to travel this way, too, because they tag along with the fat breakdown products.)  Not a huge issue for our purposes here, but I don’t want the biochem police banging on my door at three in the morning to question me about fallacies in my blog posts. And as long as I’m trying to be accurate, there’s one more thing I should say about fat digestion before we move on. Most fats have to pass through the SI and lymphatic system before they’re usable in the body, but there are a couple of classes of fats that do pass directly into the blood. These are the short and medium-chain fatty acids—fatty acids less than 12 carbon atoms long. (Most commonly found in butter, coconut oil, and palm kernel oil.) They’re very easy to digest, since they don’t have to go all the way through the intestine – and therefore also don’t require bile to be broken down. In fact, coconut oil is so quickly digested and such a good source of energy that some low-carbers and Paleo eaters have found out the hard way that eating too much coconut milk or oil before bed will keep them wide awake half the night. Also, because these shorter chain fats don't require bile for digestion, they're a good choice for people who've had their gallbladders removed. Better than longer chain fats, like, say, olive oil or the fat found in beef and pork. (Note: the longer fats are still okay for people minus gallbladders to eat; they just might require a little supplemental biliary support is all.)

Okay, back on track. Earlier I said the small intestine is stunningly well adapted to its function. Let’s talk about that for a minute. Last time, I mentioned that we would talk about how the SI is like a sweet shag carpet straight from the 1970s. Because it has such an enormous job to do, it’s got to have an enormous surface area over which to do it. And the way it achieves this is with villi and microvilli. If your SI is a shag carpet, then the villi themselves are like the individual shags of yarn or whatever those things are made out of—the single pieces of fabric that, when all put together, make a massive rug. As for the microvilli, picture each individual shag of that carpet being covered in a shag carpet of its own. (Man, I hope there aren't any British people reading this. We all know I'm talking about fabric shags, right?) So there are shags of material everywhere, coating every possible inch of this thing. This is the inside of your small intestine, and it is nothing short of an anatomical miracle. These microvilli are so tiny that even when viewed magnified a zillion times in special microscopes, they look like a layer of fuzzy haze. For this reason, it's often referred to (even in medical textbooks) as the brush border.

The villi and microvilli that make up the brush border are the actual sites where nutrients pass from outside the body to inside. Lining the brush border are the enzymes that do the final breakdown of anything useful we’ve ingested. Disaccharidases (sucrase, maltase, and lactase) break down carbohydrates, and peptidases break down any remaining long-ish proteins into single amino acids and di- and tri-peptides (2 or 3 amino acids strung together). So now we have these teeny, tiny molecules hanging out in the inner tube of the small intestine (called the lumen). How do they get into the blood, where we need them?

First, they enter the cells lining the intestinal lumen. These cells work their magic and then send the nutrients in bits and pieces into the bloodstream. Specifically, they send them to the portal circulation, which brings everything to the liver for a final spot-check, cleaning, and toxin removal. After that, the good stuff passes into the rest of the body’s blood supply, and the bad stuff gets excreted. (We hope. More on that when we get to the large intestine.) The liver is kind of like our body’s waste processing plant, recycling facility, and garbage disposal all rolled into one. Have you seen those “Don’t mess with Texas” T-shirts? The heck with Texas; don’t mess with your liver.

The cells that line the small intestine have a special name. They’re called enterocytes. And there’s something called the enteric nervous system—a nervous system associated with our intestines. The lining of the SI is loaded with nerve cells (“zillions,” to be exact), some of which communicate with the brain, and some of which function entirely on their own. This is big, big stuff, folks. Have you ever heard the phrase “butterflies in your stomach”? Why is it that when we get nervous or anxious—emotions that should be entirely in our minds—we sometimes feel queasy or nauseated? When we’re sad or upset, we sometimes lose our appetite. In cases of severe, debilitating stage fright (public speaking, for some), some people even throw up or have diarrhea. The enteric nervous system explains why we feel digestive symptoms when we experience certain emotions. This is so well recognized by the scientific community that the man who’s sort of “the father” of this idea wrote a book about its discovery and called it The Second Brain. The second brain. That ought to give us some idea of how strong the connection is between our emotions and our digestion. (Yet another reason not to eat when we’re angry, stressed out, anxious, or nervous.) Also -- this works both ways -- when you're anxious, nervous, worried, or in some other "not good" state of mind, it can affect your GI tract, but when your GI tract is not working so well, it can affect your state of mind. We'll talk about this in detail in upcoming posts. For now, let me just say it is not a coincidence that we're experiencing tandem epidemics of depression, anxiety, and other unpleasant mood disturbances and indigestion, impaired small intestinal function, and bowel dysfunction. They are absolutely related, and we'll get to why in the next few posts.

Worried or nervous? Stomach doing flip-flops? It’s not all in your head!

We’ve got plenty more to cover with the small intestine, but it seems like that’s enough for now. If you or anyone you know suffers from an autoimmune condition (like fibromyalgia, lupus, type 1 diabetes, rheumatoid arthritis, multiple sclerosis, Hashimoto’s thyroiditis, or psoriasis, to name just a few), allergies, chronic pain, or multiple food sensitivities, you’ll definitely want to catch the next part. If you’ve ever heard the term “leaky gut” and have no idea what it means, you’re about to find out. And you’ll learn why healing a leaky gut can spell relief for everything from chronic pain to depression to infertility. (Bonus: we’ll also talk about gluten and find out why going gluten-free is not just a fad. Hint: it has to do with leaky gut!)

P.S. I told you it looks like a shag carpet!

P.P.S. Ever heard of “enteric coated” medications? Well, now you know that “enteric” refers to the small intestine. So if you see “enteric coated” on a drug or supplement bottle, that means the pills inside are designed to survive chemical breakdown in the stomach so they can reach the small intestine intact, to be absorbed into the blood and do their job. 



Continue to the next post: Small intestine, part 2 - leaky gut.

Remember: Amy Berger, M.S., NTP, is not a physician and Tuit Nutrition, LLC, is not a medical practice. The information contained on this site is not intended to diagnose, treat, cure, or prevent any medical condition.