February 7, 2014

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

We’ve come to the end today! Literally and figuratively! Ha! You can tell right from the start here this post is going to be full of colon-related double entendre. (You're welcome.)

Yes, here we are, finally, at the last stop on our long train journey north to south through digestion and the gastrointestinal (GI) tract. If you’re just tuning in, you might want to check out the previous entries in this series. We’ve covered the major organs and glands involved in breaking down the foods we eat into the amino acids, fatty acids, and simple sugars our bodies can absorb and do something with, and some of the snags that can happen along the way when those organs and glands aren’t performing at their best.

I ended the most recent post by saying that the colon, or large intestine, is large and in charge. And I said I would explain. So here goes. (Note: I will use the terms colon and large intestine interchangeably.)

Remember way back to when we talked about the role of the brain in digestion? We explained that being stressed out, anxious, worried, or otherwise in a “bad” frame of mind is a recipe for digestive disaster. (Nutshell: a stressed out brain keeps us in “fight or flight” mode and prevents the “rest and digest” part of the nervous system from kicking in.) So a whacked-out brain can cause bad digestive juju. We also talked about a leaky gut (the layman’s term for increased small intestinal permeability), and how components of partially digested food can pass through the lining of the small intestine, get into places they’re not supposed to be, and cause all kinds of health havoc—everything from mild food allergies to conditions that are downright debilitating, like multiple sclerosis. In case it wasn’t clear in the posts about leaky gut, let me emphasize that this tells us that while the brain can influence digestion for better or worse, digestion can influence the brain, also for better or worse. So things like anxiety and worry can compromise digestive function, but compromised digestive function can bring about anxiety and other psychological disturbances. Talk about a vicious circle. So far, we’ve talked about this only as it relates to gluten and leaky gut up in the small intestine. But what about being constipated? If you think sub-optimal bowel function could lead to sup-optimal psychological function, you’re right. The influence of compromised bowel function on mood is a pretty big deal—or maybe I should say large deal, as in large intestine!

You tawkin' to ME?!
I’m going to let you in on a little secret: Nutritionists love talking about poop. (Good nutritionists do, that is. If yours isn’t asking you about your bowel habits, find a new one.) This thing that we don’t talk about in polite company can tell us a lot about what’s going on inside us. The way it looks, smells, the ease and frequency with which it does or does not come out—all of these offer clues that, by themselves, might not mean much, but when combined with other niggling little signs and symptoms we experience, can give us a pretty good picture of how things are working (or not working) elsewhere in the body.

When we think about the colon, we tend to think of just one thing: number two. And when it comes to number two, there are generally three problems that come up: going too often, not going often enough, and the thing we all dread, especially when trapped in a meeting at work, or maybe while having an intimate moment with our significant other: flatulence.

I’m going to spend most of this post and the next addressing not going often enough, so lets address going too often and flatulence first and get ‘em out of the way. Going too often doesn’t necessarily imply diarrhea, but it usually does. (Diarrhea is more a consistency issue than a frequency issue, but let’s not get hung up on the details.) Diarrhea is typically less a large intestine thing and more a result of goings on in the small intestine.

Diarrhea is simply the body’s way of quickly getting rid of something it doesn’t want inside it, whether it’s because it can’t digest it well (for example, milk or wheat), or it’s something downright toxic (e.g., a food-borne pathogen or the microorganisms that cause dysentery). If things are moving too fast, it’s likely either because you’re consuming foods that don’t agree with you, or things are passing through too quickly because your digestive function is compromised. Your body wants to get rid of whatever’s coming through pronto and gives your large intestine no choice in the matter.

As for flatulence, gas is typically the result of colonic bacteria feeding on undigested food particles. Say what? Y’know how you often hear about “gut flora” or “gut bacteria?” Those are the “good bugs” that live in the large intestine and help us in the very final stage of digestion. (More on this in the next post, or, should I refer to it as #2 on #2? *Ba-dump-tsch!*) Long story short, some foods—mostly certain fibrous carbohydrates—are very hard to digest. So hard, in fact, that sometimes they make it all the way to the large intestine partially intact. Inside the large intestine, the good bugs feast on them via fermentation, which produces gas. (As I said back in the post about gluten, think about how grains are fermented to make beer – beer ends up bubbly with gas, right? Similar deal happens right there in your colon.) 

Sometimes intestinal gas is odorless. But sometimes flatulence comes with an especially foul aroma. (Even that of the beautiful, slender women normal women like me love to hate. Their you-know-what doesn’t smell like roses any more than yours or mine.) The odor is largely determined by the type of food eaten, with sulfur-rich cruciferous vegetables near the top of the list (cabbage, broccoli, Brussels sprouts), neck-and-neck with alliums (onions, garlic, leeks). Without going into the level of detail that will make this post even longer than it’s already going to be, I’ll point you toward the FODMAP list from Aglaee Jacob, “the Paleo Dietitian.” (For an explanation of this, listen to this episode of the Ask the RD Podcast or read the transcript. The gas question starts at 18:50.)

Okay. Back to the main event.

Barring any unfortunate occurrences, such as vomiting, we should only see our food twice: once when it’s going in, and once when it’s coming out. (Bear in mind, though, that when it comes out, it should look nothing like it did when it went in. More on this in a minute.) So do take a look once in a while. If you’re not taking a gander at what comes out now and then, you could be missing some key clues about what’s going on in your GI tract. (Remember what Ferris Bueller said: “Life moves pretty fast. If you don’t stop and look around once in a while, you could miss it.”

To be more accurate, let me say we should see the food only once in our digestive process: when it goes in (i.e., when we eat it). When it comes out, if we’re seeing actual, recognizable bits of food, there’s trouble. More on this later on. I’m jumping the gun a little in talking about what’s coming out. Let’s rewind and go back up to where the mostly digested food moves from the last part of the small intestine (called the ileum) into the large intestine. Specifically, it enters the cecum, and the little flap that separates them is called the ileocecal valve. (In case you’re ever on Jeopardy!) Did I mean to say mostly digested food? Yes, that wasn’t a mistake. Ideally, nearly everything we eat would be fully digested by the time it reaches the large intestine, but this is very rarely the case. And that’s okay. I’ll explain why in a bit. First, let’s finish the anatomy lesson.  

The large intestine is divided into three main sections, which makes it look kind of like an upside-down U. 
Image courtesy of Wikipedia.

There’s the ascending colon on your right side, inside of which the detritus of digestion moves up, or ascends in the body. Then there’s the transverse colon, which transverses or moves across the anterior abdominal cavity. Next is the descending colon on your left side, where—you guessed it—the “stuff” descends and eventually ends up in the rectum, where, on a good day, it exits via the anus.

The large intestine is only “large” because its diameter is larger than that of the small intestine—about 3 inches, compared to small intestine’s 1 inch. The length is much shorter—about 5 feet compared to 20 for the small intestine. (Not that size matters, hehheh.) It’s totally fine that the colon is shorter. See, the small intestine has to be long, because that’s where the vast majority of actual digestion of food and absorption of nutrients happens. So there needs to be a very large area over which this can take place. In contrast, almost no further digestion occurs in the large intestine. The main role of the LI is to absorb water from the waste products as they move through. Yes, you read that correctly: we absorb water from what is essentially our poop. Don’t try to outsmart mother nature; this happens no matter how much “clean” water you drink, so you can’t prevent reabsorption of this water by chugging down three gallons of Poland Spring a day. Like I’ve said before: the human body is the ultimate reuse and recycle machine, including recycling some of its own wastewater. (So all you homesteaders, off-gridders, and others who collect and repurpose your “grey water,” take heart. Your body’s doing it for you as we speak!)

This cutie knows what 
she’s doing!
This is a normal and natural process, but that doesn’t mean it can’t get out of hand. The human body is about 60% water. This water is found in the endless list of fluids inside and outside our cells: the blood, cerebrospinal fluid, synovial fluid (lubricating the joints), vitreous humor (in the eyeballs), intracellular fluid, pericardial fluid (cushions the heart), pleural fluid (cushions the lungs) – you get the point. The body needs a lot of water. Needs it. It’s not negotiable. (Your body might want to be twenty pounds lighter or four inches taller, but it can get by just fine being heavier and shorter. It needs the water, no ifs, ands, or buts about it.) So if you don’t drink enough water on a regular basis, your body will get it from somewhere, and by any means necessary. If you’re not providing sufficient water, your body will take it from the foods you eat, by reabsorbing some from the colon. Again, this will happen even if you do drink a lot of water, but you can certainly make the colon’s job a little easier by giving your body enough water that your colon isn’t left with the task of wrenching every last drop it can from whatever you tossed down the piehole. Some foods contain more water than others, which can also help matters here. Think of things like lettuce, watermelon, apples, bell peppers, celery—these are examples of “high water content” foods. Even rare-ish pieces of meat and moist roast chicken have a lot of water. Pretzels? Bread? Bran flakes (without milk)? Pretty dry. All of this is the long way of saying dehydration is a common cause of constipation.

Another common cause of things moving too slowly through the colon is insufficient dietary fiber. Well, actually, this is what some experts think is a common cause of constipation. I’m not saying they’re wrong, but I have to mention that plenty of other experts think this whole fiber thing is bunk or darn close to it, and I'm inclined to agree. (Check out what Chris Kresser has to say here.)

The relatively low amount of fruits and vegetables in low-carb diets is often cited as a potential cause of constipation, due to a lack of fiber (and water). The thing is, plenty of people on very low carbohydrate diets (with almost no plant material [fiber] whatsoever) poop just fine. How did we ever get along before Fiber One cereal, Metamucil, and oat bran? Um, maybe we just ate real food and our GI tracts worked the way they’re supposed to. (Obligate carnivores like lions and tigers don’t spend their days on the savannah chomping down on endless piles of kale, broccoli, or bran muffins, and they seem to defecate pretty well without pharmaceutical intervention. I’m not suggesting the anatomy and physiology of these big cats are the same as humans, so therefore we don’t need any fiber, either. Of course not. I’m simply pointing out that somewhere in nature there exists a mechanism for allowing healthy animals to excrete without the need for copious amounts of plant fiber.)

For someone experiencing constipation who’s not drinking a good bit of water every day (about half their body weight [in pounds] in ounces of water), or someone who’s not consuming fiber-rich vegetables, fruits, or grains (if grains are on the menu), I would say it’s worth increasing water and fiber intake to see what happens. For some people, these are easy, low-level solutions.

Leave me alone for a while, kids. 
And no playing with matches 
while dad’s on the pot.
However, constipation is not always a fiber or water issue. If you’re well hydrated, eating a lot of fiber, and are still experiencing constipation, then those aren’t the problem and drinking more water and eating more fiber aren’t likely to help. Over-the-counter laxatives and stool softeners can help in the short term, but they are just that: short-term fixes. They do nothing to identify and address the underlying cause.

So what, then, are some underlying causes of a colon that’s gone to sleep? One is a dairy intolerance. We much more frequently hear about gas, bloating, and diarrhea being associated with lactose intolerance, but that’s exactly that: lactose intolerance. In people who produce insufficient lactase enzyme to break down the lactose (“milk sugar”), the bacteria in their colon feed off of it, which results in the gas and bloating. And we already said that diarrhea is the body's way of quickly ridding itself of something it doesn't want.

The other side of the coin is that dairy seems to have the opposite effect on some other people. It’s well-recognized in Ayurvedic medicine that dairy—cold, creamy dairy, in particular—is constipating for many people. (It's said to aggravate Kapha, if you must know.) So if you’ve got fiber and water covered but are still constipated, try eliminating dairy for a while and see what happens. Butter and ghee are probably okay; it’s the cheese, yogurt, cream cheese, and cottage cheese-type stuff you’d want to stay away from.

The fourth—and probably the most under-recognized—potential culprit in constipation, particularly long-term, chronic constipation, is low thyroid function. When the thyroid slows down, everything slows down. The thyroid, courtesy of the hypothalamus and pituitary glands, is the master regulator of basal metabolic rate. We tend to think of metabolic rate in terms of burning calories or losing our spare tires and saddlebags. But metabolic rate really means the rate at which the body does just about everything. People with low thyroid tend to experience sluggishness/slowness all over their body: low energy levels, low resting heart rate, low blood pressure. They’re usually cold, especially in their hands and feet. The body’s so sluggish that it fails to heat itself efficiently. (This is usually less a thyroid thing and more a hypothalamus thing. The hypothalamus (HPT) is kind of like the body’s thermostat, so when it’s on the fritz, you tend to feel cold. The poor thyroid gets the blame when the fault actually lies with the HPT and its failure to send signals to the pituitary gland, which then tells the thyroid to get crackin’ and warm things up. The thyroid ain’t doin’ squat unless the HPT and pituitary tell it to, yet it’s the thyroid that ends up with the bad rap. Pitiful.) But I digress...

With a sluggish thyroid, one of the things that slows down is the movement of waste through the colon. Since the problem has nothing to do with fiber or water, again, more fiber or water won’t help. (OTC laxatives will “get things moving,” which is important, but the underlying issue still needs to be corrected.) I will neither confirm nor deny that I have personal experience with this. Let’s just say I know of what I speak.

What else can slow the ol’ colon down? How about stress? Our old digestive nemesis rears its ugly head again. (Rears! Get it? Am I delivering on the colon jokes or what?) Think about what happens when you’re stressed out: Your whole body is tight, tense, and poised to pounce on Lumbergh when he comes by your cube to tell you you have to come in on Sunday, too. Well, you already know your outsides are tense, so what about your insides? The muscles lining the colon that contract and relax in order to propel waste along tense up, too, as does the anal sphincter. (Case in point: at basic training for the Air Force, none of the girls in my flight did "number two" for at least the first week. We were way too on edge. Things got back to normal once we got used to the routine and were a little more relaxed.) 

If you’re a giant, constipated stressball, you may have noticed—or you will notice, after reading this and you start paying more attention—that when you do finally have a nice, easy bowel movement, it’s most likely on a Saturday or Sunday morning when you’ve slept in and you’ve got nothing on the agenda and aren’t hurrying to get anywhere (especially not to work, if you’re a Monday-to-Friday type). You probably also “go” more easily on vacation, for the same reason—your whole body (and MIND!) is just more relaxed, and the colon can do its thing. (Unless, of course, you're on vacation with a new or potentially new significant other [or possibly in-laws] and you can't possibly have them knowing your body has this unavoidable, ugly, terrible thing called excretion. The sounds, the smells...Oh, the humanity! There's no way around it. You're just going to have to hold it until you get home, even if your vacation is two weeks long. <--Just kidding, but come on, you totally know what I'm talking about!) 

Stress: bad for digestion and excretion. This guy does not look like
he could have a nice, easy BM anytime soon.

This post is ridiculously long, so let’s call it a day for now. Now that we’ve addressed some of the causes of constipation, next time, we’ll get into
why chronic infrequent bowel movements can mess with physical and psychological health. (Remember when I mentioned that kids on the autism spectrum often have severe bowel dysfunction? I wasn’t kidding when I said the large intestine is “large and in charge.” In charge of mood, mental outlook, and behavior, that is—in children and adults.) Tune in next time and it’ll all come out all will be made clear. (That last one was too much, even for me.)  ;-)

P.S. There's a chance all this constipation stuff applies solely to women. Men seem to be able to go anywhere, anytime. (And then brag about it, to boot.)

P.P.S. Before we adjourn, I’ll leave you with a couple of notes about effective digestion. If you’ve ever taken a good look at your poop (and remember, I unashamedly encourage this), you may have noticed undigested food particles in it. Usually these are little leafy green bits, or sometimes entire kernels of corn. A little undigested food in the stool is nothing to be alarmed about, but if you frequently see undigested food particles there, that’s a sure sign of less than optimal digestive function. Let’s take the corn example, because it’s pretty common. If you’re seeing whole pieces of corn, not only did your digestive enzymes fail to break them down, but let’s go back to the start: you didn’t even chew them! I’d say that was step one along the route to your digestive FAIL.

Brocc: delicious, but tough to digest.
(Especially when raw! Cook those suckers!)
Another reason we see these bits and pieces of non-digested plant matter in our stool is because plant matter (i.e., vegetables) is actually pretty darn hard to digest! There’s a great line from John Durant in his book, The Paleo Manifesto. (At least, I think this is where I saw it. If I’m wrong, someone please let me know.) He said, “Killing an animal is hard; digesting one is easy. Killing a plant is easy; digesting one is hard.” He’s right! All those fibrous, green, leafy things that are supposed to be so good for us are actually fairly difficult to extract nutrients from. There’s a reason cows have four stomachs and basically have to chew, regurgitate, and chew again, all day longIt takes a whole lot of digestive machinery to turn grass into protein. Don’t believe me? Check out this account from a man who had a jejunostomy—basically, he had a large portion of his small intestine removed and he was rigged up to a system that could “collect” his waste. Vegetable matter was common in the collected contents, but never did he see undigested meat or fat. (Now, granted, if he’d been left with a larger portion of small intestine, the vegetables probably would have been broken down a bit more by the pancreatic and brush border enzymes. Nevertheless, humans can not digest cellulose—the stuff that makes up the cell walls of all plants. We can’t. No way, no how. We don’t have the enzymes. And this isn’t like the lactase enzyme, where some people produce it throughout their lifetimes and some people don’t. No humans produce cellulase.) So as long as we have good, strong stomach acid and a functioning gallbladder, animal flesh protein and fat are fairly easy for our bodies to handle. Plant matter is another story. (And is possibly why some people experience gas and bloating when consuming a lot of vegetables.)

For more on the myth that humans aren’t designed to eat meat and that we can digest and thrive on plants, check out this post by J. Stanton at Gnolls. (And it IS a myth--i.e., NOT TRUE.) You can't go wrong with a post titled Does Meat Rot In Your Colon? No. What Does? Beans, Grains, and Vegetables!

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.


  1. Thanks again for such an informative series...you make it all so interesting and easy to "digest".....

  2. This series on digestion, in addition to other content here, is a valuable tool for the layperson to get a handle on one of the core fundamentals of health.

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  4. Love this post- and I can relate as well!

  5. Beckysue low thyroidOctober 22, 2014 at 7:13 AM

    I am so frustrated. Developed rosacea in February from eating special chocolate with probiotics in it. Should have known better since probiotics always gave me a headache. In spring, had pustules and large red veins on face. A doc put me on vegan diet, pustules continued but red veins calmed. Lost 5 lbs I didn't want to and reacted to many foods. I would eat something, face would itch and a watery pustule would burst out. Had diarrhea and couldn't tolerate legumes. Coffee enemas helped headaches. Reacted to yeast additives in food. Leaky gut? Heard of GAPS diet last month. Skin calmed somewhat on broth, but lost more weight because of low carb. Still had pustules, so added sub clinical doxycycline. Has helped pustules, but still have headaches. I am lactose intolerant, need B12, low thyroid. Trying to heal from what I thought was SIBO, but now I read a 2014 study that my ileocecal valve may be insufficient pressure wise and gut transit time slow. Related to thyroid and i don't know what to do. I don't look good. I feel very tired.

    1. Hey Beckysue, I'm sorry you're struggling so much. Are you working with someone now, or troubleshooting on your own? You've got a lot of things going on, but they probably have a connected underlying cause (or a few different causes), and you shouldn't have to drive yourself crazy tackling everything separately.

  6. I love your informative blog posts! So helpful :)

    You mentioned at the end, that we can't digest cellulose. I noticed that almost all vitamins have cellulose in them. What do I do about that?

    1. The same thing you do with the cellulose you eat from *every single vegetable and fruit* you consume -- let it pass through you and get pooped out. ;-) (Cellulose is part of what makes up plant cell walls, so if you eat any plant foods at all, you're already getting far more cellulose than you would ever get from the minuscule amounts in supplements, where I think it's mainly used as a filler or binder.)