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Down The Gullet: A Guided Tour Of Your Guts


This is SCIENCE FRIDAY. I'm Flora Lichtman. Most of us probably don't give much thought to what happens to our food after we swallow it. But my next guest certainly has. In her new book, "Gulp: Adventures on the Alimentary Canal," she investigates phenomena like intestinal stirring and paradoxical sphincter contraction.

Open to a random page and you might read: President James Garfield was the poster boy of rectal feeding. Or, quote, if you must spend time in a digestive organ, I recommend the penguin's stomach. And of course every good investigation includes a bit of experimentation. She's centrifuged her saliva, dropped gastric acid on her arm and even got a colonoscopy without painkillers just to see what it's like in there. We'll hear all about it.

Mary Roach is the author of "Gulp: Adventures on the Alimentary Canal." And you may also know her as the author of "Stiff." And you can read an excerpt from her new book at sciencefriday.com/saliva. She joins us today from Minnesota Public Radio. Welcome to SCIENCE FRIDAY, Mary.

MARY ROACH: Thank you, Flora.

LICHTMAN: So on your adventures through the alimentary canal, what was your favorite stop?


LICHTMAN: My favorite stop - I have to say I was pretty blown away by the interior of my own colon, I guess partly because it's mine and you don't really get to wander around inside yourself very often. And I was just - it's very pretty in there. For the organ that we kind of most despise and fear, I think, because of what's in there, it's beautifully pink, and the walls are kind of shiny and glistening, kind of like Saran Wrap.

It's a very - obviously it's been cleaned out. When I'm - when I was visiting, the janitors had been in there for some hours before I got a glimpse. But the colon, it's really a beautiful - and it's very different than the small intestine, which kind of looks velvety, kind of like terrycloth, which makes sense because it is almost - it's like terrycloth, it's very absorbent, a lot of surface area.

Right, that's - right, that's where a lot of the nutrients are absorbed, right?

ROACH: Yeah, the preponderance of nutrients are absorbed through these little finger-like sort of Dr. Seussy projections, little finger - villi, they're called villi, yeah.

LICHTMAN: So I've heard you do some other interviews, and often the interviewer will do this disclaimer that this is a little gross, but at SCIENCE FRIDAY, because, you know, fecal transplants are practically our bread and butter on this show...

ROACH: Me too.


LICHTMAN: We're not going to make any apologies. But I was curious what you think about this. Why do you think we're so grossed out by our own bodies?

ROACH: I think by and large humans prefer to think of themselves as minds, from the neck up. We don't really like to think of ourselves as another animal, another digesting, excreting, mating, snoring, sleeping kind of sack of guts. I don't think we like that. I think we'd rather not be reminded of it.

And we love food, but we don't like to think about what happens to it once it leaves the plate. And I'm here to encourage people to think about it a little bit because it's fascinating and really not that scary at all.

LICHTMAN: Did your ick factor change as you wrote this book, or were you always interested in this?

ROACH: I have always, well, at least since I'm - I don't really remember as a kid. But I've always been someone whose sense of curiosity trumps any feelings of revulsion or disgust, and this is true going back to "Stiff," you know, when I spent some time at the body farm, this is a book about cadavers.

And that was a situation where while it was overwhelming in terms of what you're seeing and smelling and just what I was learning and the kind of contagious enthusiasm and passion of the scientists who were showing me around completely distracted me. So I don't - I'm not troubled by the ick factor at all.

LICHTMAN: If you want to get in on this conversation, ask Mary Roach a question. Our number is 1-800-989-8255, 1-800-989-TALK. One of the scientists that seemed really passionate about her work was working on saliva. I didn't even know that we produce different kinds of saliva. Tell me about this.

ROACH: No, I didn't either. There are two kinds: stimulated and unstimulated. Stimulated saliva, when you chew, and it doesn't matter what you're chewing, you could chew on a cotton plug, which I did, which is a way to collect saliva for experimentation, for measuring it, you start to chew on something, and no matter what it is, your body starts producing this saliva, just readying the material for the big swallow.

So, you know, it's kind of a - like whatever you're going to try and swallow, we're here to help you. So your parotid glands are - and it's very - it's not just to help you swallow, it's also stimulated by - if you take in something acidic - wine, citrus juice, vinegar, even cola, that is in the range of, the acid range, pH, which is going to - it can dissolve tooth enamel, can actually literally soften it.

So if you take a sip of wine, and I was doing this for a while, going wow, you take a sip, and you feel this gush of saliva in your mouth. And if you're paying attention, which I never did before, but when I had visited Dr. Saletti(ph), I became sort of a crusader for the miraculousness of saliva, but you get this gush of saliva coming in to dilute the acid and bring the pH to a safer level and protect your teeth.

The other kind of saliva, unstimulated saliva, is kind of a background. That's the more traditionally gross mucoid, kind of like what you see in "Alien," the stuff hanging out of the mouth, that ropey - it's an unpleasant looking substance. But...

LICHTMAN: That the loogie, right?

ROACH: The loogie, yeah, the unpleasant kind. Stimulated saliva, you centrifuge that out of the little plug - sort of cotton wad you're chewing, it just looks - it is essentially water. It has a few other, you know, it's got minerals and some other enzymes and things in it, but it's basically water.

But try to get anybody to drink that water, once it's outside the mouth, and not even Erica Saletti(ph) will drink it.


LICHTMAN: Well, why would it be a good detergent?

ROACH: Detergents contain enzymes, and they break down - in the mouth you have amylase, which breaks down starch, a little bit of lipase, babies have a lot more lipase, it breaks down fat, and because babies can't chew their food, they're doing some digesting in the mouth, as it were. So there's higher levels of lipase in baby drool. But we have a little as well.

So when you think about it, the same foods that you're putting into your mouth, you're also dropping in your lap onto your clothes. So you - and I do this...

LICHTMAN: If you're me, yeah.

ROACH: If you're me, too, and I kind of use saliva as a laundry presoak. And in fact laundry detergents, when they say enzymatic action, what they're talking about are digestive enzymes. There's amylase, lipase, proteinase, not taken from human digestive tracts, they're manufactured, they get bacteria to produce them, but they're basically - laundry detergent is a digestive tract in a box in some ways.

LICHTMAN: Have you tried this out? I'm sure I have stains on my clothes right now, maybe now...


LICHTMAN: Have you tried it?

ROACH: I spilled coffee. The first day of my book tour I dribbled coffee onto, you know, one of the few skirts that I have. And so yes, I think I looked a little strange, but I was dabbing saliva. I wasn't actually spitting in my lap.


ROACH: I was just taking - you know, wetting my finger and dabbing it. And I think it works well, perhaps not as well as a Tide stain pen, but my belief is that it works pretty well.

LICHTMAN: Well, you're always carrying it, so that...

ROACH: You always have it with you, yes.

LICHTMAN: Does saliva have antibacterial properties?

ROACH: Saliva has antibacterial properties. It also has things called nerve growth factor, skin growth factor, histatins which help with wound closure. So when you see an animal licking a wound or even a mom kissing a child's boo-boo, there's some, there's some good science behind why one might do this.

I mean, it's an old, old sort of old wives' remedy. Saliva, you see it in medical texts from, you know, the 1500s and 1600s that saliva would be a therapeutic agent. And there have been studies on rodents where they've - one rodent was prevented from licking the wounds, and the wounds took longer to heal. So it seems to - yes, it - that's a very long-winded way of saying yes, it has antibacterial properties.

LICHTMAN: That doesn't surprise me because I was - you see little kids all the time (unintelligible) in their mouth. I was in a café the other day, and a little kid was just sort of vacuum cleaning the floor. I think a man's shoe ended up in her mouth. And I was thinking you must have - there must be protection in there.

ROACH: Yes, yeah, and also the stomach acid, one of its main roles is to kill bacteria. People just think of stomach acid as helping you digest, but it's also, it's also there to keep bacteria - to kill potentially harmful bacteria.

LICHTMAN: We have a lot of callers who want to talk to you and ask questions. Let's go to the phone. Rianne(ph) in Dover, you're on the air.

RIANNE: Yes. Hi. Thank you.


RIANNE: I was wondering if your poo smells the same as when you're a baby as when you're an adult.

ROACH: If your poo smells the same when you're a baby as when you're an adult. No, poo is very much diet-dependent. So you're going to have a very different - if you're on a 100 percent dairy diet, it's going to be different from vegetarian, versus omnivore. So there's really a - just a direct correlation between the smell and what you eat. In fact, Dr. Alan Kligerman, who was at the Beano company and has a digestive disease research center, had this wonderful quote. He said - I'm going to - I know I'm going to botch his lovely quote, but he said: "A gas smell is as unique - an individual's gas smell is as unique as a fingerprint."

RIANNE: There you go.

ROACH: Yeah.

LICHTMAN: Thank you for calling.

RIANNE: Thank you. Thank you. Bye-bye.

LICHTMAN: On that topic, I'm hoping that Bart Simpson can introduce my next question.


NANCY CARTWRIGHT: (as voice of Bart Simpson) Beans, beans, the musical fruit, the more you eat, the more you toot.

LICHTMAN: That's Bart getting kicked out of school for that.


LICHTMAN: Why is this so? Does science have an answer for why beans are the musical fruit?

ROACH: Yes. Beans contain a lot of - it's a complex carbohydrate called oligosaccharides, and your small intestine is not so good at breaking that down, and you don't absorb it in the small intestine. It makes its way into the colon, where lots of bacteria have a go. And they break it down, and in the process, produce a lot of hydrogen, which is one of the main components of flatus. So your legumes and beans are quite efficient at producing large volumes - not necessarily smelly, that's coming from - that's a different component. But in terms of the volume of flatulence, beans are way up there.

LICHTMAN: I was wondering if you have a few chapters - I think three chapters in the book on digestion - yes...


LICHTMAN: ...on flatus, we use the clinical term.


LICHTMAN: I was wondering if you might read a little for us from one of those chapters. Do you have your book with you?

ROACH: I do. Sure. Yes.

LICHTMAN: And so I'll give a little - and you can add to my intro, here. But one of the scientists you write about is a real character, Michael Levitt. And there's a passage in the book where you described his quest to discover the ingredients of passed gas.

ROACH: Yes. Specifically, he was trying to figure out which ingredients are responsible for noxious flatus, the unpleasant smell that we all know so well. So I'll start reading here.

(Reading) Curious as to which olfactory notes the different sulfur gases contributed to the overall bouquet of flatus, Levitt purchased samples of the three gases from a chemical supply house.

And he had already - I should say he had already determined that sulfur gases were the culprits, that those correlated most highly with what was judged to be very offensive flatus. So...

LICHTMAN: Before you go on...


LICHTMAN: ...I'm going to give an ID break, just so we get it done. This is Flora on - SCIENCE - on SCIENCE FRIDAY, from NPR, talking with Mary Roach.


LICHTMAN: Go ahead.

ROACH: All right. Sure.

(Reading) Curious as to which olfactory notes the different sulfur gases contributed to the overall bouquet of flatus, Levitt purchased samples of the three gases from a chemical supply house. The judges agreed on the following descriptors: rotten eggs for hydrogen sulfide - the gas with the strongest correlation to stink - decomposing vegetables for methanethiol, and sweet for dimethyl sulfide. Though lesser players, like methylmercaptan, contribute as well, it is for the most part these three notes, in subtly shifting combinations and percentages, that create the intimate - the infinite olfactory variety of human flatus. To quote Alan Kligerman: "A gas smell is as characteristic of a person as a fingerprint."

See, I didn't botch it too bad.


ROACH: (Reading) The great variety of flatus smells from person to person and from meal to meal presented a quandary for the second phase of Levitt's study, the evaluation of various odor-eliminating products. Which and whose wind should represent the average American's? No one's, as it turned out. Using mean amounts from chromatograph readouts as his recipe and commercially synthesized gases as the raw ingredients, Levitt concocted a lab mixture deemed by the judges to have, quote, "a distinctly objectionable odor resembling that of flatus." He had reverse-engineered a fart.

LICHTMAN: I hope that every child in the backseat of the car is thanking SCIENCE FRIDAY for that passage.


LICHTMAN: So you actually write that hydrogen sulfide, which is an ingredient in flatus, is...


LICHTMAN: ...as lethal as cyanide.

ROACH: Yes. Hydrogen sulfide, it's all in the concentration. Hydrogen sulfide, the human nose is exquisitely sensitive, able to detect less than one part per million. And a noxious fart would have in the neighborhood of two or three parts per million. But when you take it up to about 1,000 parts per million, hydrogen sulfide is fatal. It causes respiratory paralysis, and it kills people quite swiftly.

LICHTMAN: This is related to dung lung, isn't it?

ROACH: Yeah. Dung lung is a - yeah. I don't think it's used widely.


ROACH: I saw this in a paper. And what amazed me is what - with the dung lung, this term for people who work with - say, on pig farms where there's large concentrations of hydrogen sulfide, and occasionally, they inhale it and are killed. Dung lung - the person who wrote this paper was Dr. C-R-A-P-O, Crapo.


LICHTMAN: That is amazing.

ROACH: You think he would cease to find this sort of thing amusing. But, anyway, yeah, dung lung is a - yeah. This is a - it's an occupational hazard, if you work around - if you're in a sewer business, or you work with - on pig farms, somewhere the concentrations of hydrogen sulfide can reach dangerous levels.

LICHTMAN: But I read in your book that cows don't burp. So there's no sort of flammable danger there.

ROACH: Yeah, cows. That was interesting to me. You hear a lot about methane produced by cows and its effect in the environment, et cetera. And I wondered why there's methane. About a third of us produced methane when we - in our flatus. And I wondered about cows and how they - because they produce a lot of gas, because they're doing - they're fermenting. They've got big composters inside them. And I thought, well, why do you never hear them belch? You never - because it's the rumen. It's much higher up the digestive tract. And an agriculture professor at UC Davis explained to me that cows - what they do - they're in - they're not - it's not coming out their rear. They have a way of shifting around their internal tubes so that they simply exhale. They don't belch it. They - it just becomes exhaled, the methane. So they sort of quietly off gas through the mouth, yeah.

LICHTMAN: Well, lots more to come with Mary Roach. Stay with us.




LICHTMAN: This is SCIENCE FRIDAY. I'm Flora Lichtman. We're taking a tour of our guts this hour with my guest, Mary Roach. Mary is the author of "Gulp: Adventures on the Alimentary Canal." And you may know here also as the author of "Stiff." And we have tons of people calling, so I'm going to the phones. Dean in South Field, Michigan. He might not be there, it sounds like. We also had a tweet that I thought you might have something to say about. Susan Deary(ph) asks: Why isn't corn digested, and why is it visible in fecal matter?

ROACH: Oh, great question. I love this question. Corn - the corn kernel has a very tough envelope, exterior, and the body doesn't manage to break it down. So it just travels along largely intact. The kernels sometimes are broken, you know, into bits and pieces, but it is so hardy that corn can be used as a marker food. And that means you can - and this is a cool thing you can do. You can time your own intestinal transit time. You can, you know, make note of when you ingest the corn, and then keep an eye out for it when it emerges on the other end, and the amount of time that has elapsed will be your own personal intestinal transit time. So peanuts are also marker foods. Peanuts have a very tough sort of - the matrix of the peanut doesn't completely break down, so you have little chunks of it. You could use dye, also, but that's not as much fun.

LICHTMAN: There you go: a citizen science project for the weekend.


ROACH: Yeah.

LICHTMAN: One of the...

ROACH: Parents everywhere will go like, oh, no, no.



LICHTMAN: One of the parts I love about your book is that you dive into these fascinating and bizarre histories of how we learned about our guts, and one was this guy William Beaumont. Tell us more about him.

ROACH: Yes. William Beaumont was a physician who - well, he was an army surgeon, and he was posted on Mackinac Island, where there was a fur trading company, a trading post. So trappers would come in with the pelts and, you know, get supplies and things. And he was - there was also a military outpost there. And one of the trappers was accidentally shot in the side, blew a hole open on his side, which opened up the stomach. And so Beaumont came to help him, thought that he would die. The man, Alexis St. Martin, recovered. The wound healed as a fistula, which is an unintended passageway. So it went from the stomach, straight to the outside. A gastrocutaneous fistula, I believe, is the terminology. Anyway, it's a hole that never healed properly.

And Beaumont quickly realized what this - what opportunity this presented for him to study human digestion, because no one had really ever seen human digestion in action. And he commented that he could - that when St. Martin lay on his side, he could peer through the hole and actually see what was going on and watch and observe digestion. And he began doing a series of experiments, hundreds of experiments, putting foods, different foods in a little mesh bag on a silk string, putting it into the stomach. And then after intervals, pulling it back out, seeing what was digested, how quickly, and then took some of the gastric juice out of the stomach to see would it work on its own, did it need to be at a center temperature. He'd have Alexis St. Martin walking around the house with a test tube of his own gastric juices strapped under his armpit to see, you know, would this work outside the body? Lo and behold, it did. So - and then he wanted to figure out what it was.

And so he did a series of - he was really the first American physician to study human digestion, the - and specifically looking at, was it chemical or was it mechanical. There had been a debate as to whether the contractions of the stomach were responsible for breaking foods down, or was there some chemical process. And so he - and this spanned - Beaumont was a bit of an obsessive. This went on for 30 years. And St. Martin would sometimes tire of it, take off, disappear. Someone else - someone would run into him, send a note to Beaumont and say, hey, I found your patent digester out here. And how much do you want to offer to get him back? And so Beaumont would offer more and more money. It was kind of game of Coyote and Roadrunner back and forth over the years, but a fascinating and kind of odd relationship the two had.

LICHTMAN: It did seem pretty intimate, with all the stomachs there.

ROACH: It was very...


ROACH: Yeah, there was a moment when Beaumont was describing, you know, putting his tongue to the mucus layer and tasting it and just a very - and St. Martin - his letters were always - he didn't - we were right that he would dictate to someone. And they were always very polite and with love to the family and that kind of thing. But they were a kind of a - it was a bit of an adversarial relationship. But a mutual dependency on and off over the years.

LICHTMAN: Let's go to the phones. David in Arlington, Virginia, welcome to SCIENCE FRIDAY.

DAVID: Hi. I listen to your show when I can on Friday afternoons.


DAVID: I've wanted to share a piece of digestive trivia. I read a review of Ms. Roach's book the other day. Didn't see anything about it, but when I was in college studying - I took a physiology course, and in the chapter about the digestive tract, it touched on the subject of whether having fecal matter in your body for an extended period of time was harmful. And it mentioned an anecdote about a guy who hadn't been able to defecate for about a year.

And he had a hundred extra pounds of poop in his body that didn't go away. And I would mention this to friends and they would say, you got to be crazy. It can't possibly be true. And I would show them in the book and as long as I had the book, it always impressed them. So I thought you might be interested in that.

ROACH: I'm mighty impressed by that. But - and that leads me to make mention of one of the body's many protective mechanisms, which is if you have a blockage and you're not able to evacuate your bowels, as they say, if you - if there's a blockage somewhere down the line, at a certain point everything shuts down. I mean, the hundred - that story, the hundred guy with a hundred pounds backing up, is very hard for me to believe.

But I do believe that you saw this - the article and that somebody seems to have documented it. But they'll - the whole - the process, the peristalsis by which the waste and the things are moved along, everything will sort of shut down. And this happens - drug mules will sometimes get pulled out of line and detained for suspicion of alimentary canal smuggling.

And they have - there was a case of one man who for - I think it was 20-some days - did not empty his bowels. Because they were - and the people at the airport, or wherever - I think it was in an airport - they're waiting. They are just - it's like, well, we're going to keep you here until we see what you've got. And he's like, well, I'm not going to let it go.

And it went on for a remarkable number of days. And to keep the intestines from bursting, the body will just short of shut - stop, sending it further on down the line. So that you don't have such a big log jam that you have a rupture. So the hundred pounds guy, that would seem to be - I don't know how it is that his - he must have had a mega colon, that's a genetic problem.

LICHTMAN: Didn't Elvis have a mega colon?

ROACH: Elvis did have a mega colon. And he probably had an undiagnosed case of Hirschsprung, which is where the end of the - the colon doesn't have nerves. The nerves that create the peristaltic contractions that move the material along, so you get this log jam. And he had a - and the colon stretches out, it becomes - in the case of the one specimen in the Mutter Museum, eight - 28 inches around. That's my pants' size.

So I would imagine this dude with a hundred pounds of material was one of those cases. The one - the mega colon in the Mutter Museum, I believe, 40 pounds of fecal matter, old, dried, hard, was - sorry. I hope no one's...

LICHTMAN: No. It's okay. You can see it in our website. We've actually covered this in the past, Mary.

ROACH: Oh, you have.

LICHTMAN: Yes. We have a video about the Mutter Museum which features an image of the...

ROACH: Okay. Yeah. So...

LICHTMAN: So we promised in the billboard that we would talk about eating through the other mouth, as you write in the book. And we only have a few minutes left, but I just wanted to make sure that we delivered on that. Tell us about this.

ROACH: Yes. Well, rectal alimentation is the term that was used. It's not done so much anymore now that you can see directly into the stomach or through an IV. But in the 1800s and 1700s, I think, even, but mostly in the 1800s, they're - doctors realized if you - that you could do something that's called a nutrient enema.

And that is you take - it was often like a beef broth with some beef peptonoids to sort of - to do the digesting that would be happening further on up the alimentary canal and the small, you know, sort of the small intestines. So there would be some digestive materials actually in the substance that you were putting in.

But if there was a - if there was - a person couldn't keep the food down or there was a blockage - President Garfield was sort of the - as you - I think you read earlier, the poster boy for that. And he was fed that way for some time. You know, he ultimately died. It was an assassination attempt and his - he'd been shot in the abdomen. And - but the - his doctor, Dr. Bliss, was quite impressed with the body's ability to take in nutrients that way.

Though it's very limited. You know, there are a lot of nutrients that the large intestine, the colon, can't absorb. Most of the absorption goes on in the small intestine. And you can't pass - the ileocecal valve is a one-way valve. It's very tough to get material up through the large intestine, into the small. So it's not a very efficient way to eat. I don't really recommend it to anybody.


LICHTMAN: There you have it, folks. That's about all we have time for. But do you have your next book picked out? Do you know - what will we...

ROACH: No. I'm not sure. I'm not sure what that's going to be yet. But I'm open to ideas.

LICHTMAN: Well, we're looking forward to it. Mary Roach is the author of "Gulp: Adventures on the Alimentary Canal," and you may also know her as the author of "Stiff." Thanks for joining us today.

ROACH: Thanks so much, Flora, for having me on.

LICHTMAN: It was fun. Transcript provided by NPR, Copyright NPR.