Interviews
11:33 am
Fri March 14, 2014

In Digestion: Mary Roach Explains What Happens To The Food We Eat

Originally published on Fri March 14, 2014 2:06 pm

This interview originally aired on March 26, 2013.

For all our talk about food, we don't like to think much about it after we put it in our mouths. But Mary Roach, author of Gulp: Adventures on the Alimentary Canal, did just that. Gulp takes a close look at the human digestive system, from the mouth on down, and Roach writes that she wants readers to say not, "This is gross," but instead, "I thought this would be gross, but it's really interesting. OK, and maybe a little gross."

Roach is known for her popular science books, like Stiff: The Curious Lives of Human Cadavers and Spook: Science Tackles the Afterlife, and she says she's somewhat surprised that she didn't tackle the human digestive system years ago.

"It's right up my alley," she tells Fresh Air's Terry Gross. "It's a little bit taboo. It has to do with the human body. It's bizarre. It's a very strange place, the human body. It's an alien planet that I love to come back to again and again, and the gastrointestinal tract and the mouth are really fascinatingly bizarre and kind of marvelous."

As she attempts to understand how it all works, Roach explores everything from the taste and texture of foods to some surprising uses of rectums. She tests olive oils in an attempt to understand the nuances of flavor, goes on location to a fecal transplant, and meets murderers who hide contraband like tobacco and cellphones in their rectums behind bars, in a practice known as "hooping."

She, herself, also underwent a sedation-free colonoscopy because she was curious as to what her own insides looked like. The occasion of researching this book, she thought, offered ample opportunity.

"I had come across this wonderful [article]," she says. "It was in maybe The Lancet — one of the major medical journals. It was someone who had put two photographs side by side. One of them was the interior of Gaudi's La Pedrera. ... It was this wonderful archways that look very much like the connections between segments of the colons, and [the author] had made this comparison between this work of art and the human colon and I thought, 'I want to see my own internal Gaudi. I want to see that.' "


Interview Highlights

On attending an olive oil tasting panel at University of California, Davis

"I was trying out to become someone who would be a sensory analyst who'd ... be able to make fine discriminations among different olive oils and be able to tell if it's fusty or rancid, or if it's got a stale walnut taste or sewer-dregs taste. There are all these off tastes that olive oil develops depending on how it was made and stored, and if you're really, really good at identifying these very specific odors [and] flavors, you can work as a sensory analyst in the industry. I, however, was unable to discriminate between five olive oils that were apparently in all differing degrees of bitterness. To me they a) weren't bitter, any of them, and b) tasted all the same."

On the antibacterial and antiviral elements of saliva

"It renders viruses noninfective. ... You think of kissing someone as a way of getting a cold or flu, but it's not a good way if you want to transfer a cold or flu. [Getting sick is] mostly you're touching something, then you touching your nose or your eye, and in that way it makes its way into where it can infect you. Also, in saliva there's these [proteins called] histatins which help wounds heal, so when someone kisses a baby's booboo — like a scrape — or when a pet licks its wounds, you think, 'Ewww. It's full of bacteria. Don't do that,' but there's these healing elements. Saliva was a home remedy for cuts and scrapes."

On scientists studying flatus and flatulence

"The man who's done the most work on flatus — and I counted 34 papers on flatus — Michael Levitt at the VA Medical Center in Minneapolis, he was testing a number of products that allegedly remedied noxious flatus — that noxious smell. He was testing pads you can put in your underwear. There's elasticized underwear; there are pills you can take; there is actually a remedy — there's something called Devrom, which is an internal deodorant. ... However, I spoke to one gastroenterologist [and] I asked him about this and he said, 'You know, when I get someone who comes in and is complaining about noxious flatus, I tell them: Just get a dog.' In other words, so you can blame the dog."

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

DAVID BIANCULLI, HOST:

This is FRESH AIR. I'm David Bianculli, editor of the website TV Worth Watching, sitting in for Terry Gross. Maybe you're the kind of person who doesn't want to know what happens to your food once you've swallowed, as it makes its way down the digestive tract until it comes out the other end. But if you want to know, Mary Roach has just written the book for you.

She says to her readers: I don't want you to say this is gross; I want you to say I thought this would be gross, but it's really interesting; OK, and maybe a little gross.

Her book is called "Gulp: Adventures on the Alimentary Canal." It comes out in paperback next month. Roach is known for her humorous books about science, including "Stiff: The Curious Lives of Human Cadavers," "Spook: Science Tackles the Afterlife," "Bonk: The Curious Coupling of Sex and Science," and "Packing for Mars: The Curious Science of Life in the Void." Mary Roach spoke with Terry Gross last year.

TERRY GROSS, HOST:

Mary Roch, welcome to FRESH AIR.

MARY ROACH: Thank you.

GROSS: So why did you want to write about the digestive system from start to finish?

ROACH: Well, I'm kind of surprised I haven't written about it years ago. It's kind of right up my alley. It's a little bit taboo, it has to do with the human body, and it's delightfully - it's not - it's bizarre. It's a very strange place, the human body. It's an alien planet that I love to come back to again and again. And the gastrointestinal tract and the mouth are really fascinatingly bizarre and kind of marvelous.

GROSS: I like that you think they're fascinatingly bizarre and marvelous because a lot of the subject matter you're writing about in this book a lot of people find really icky. And, you know, let's face it: A lot of people find their body and their bodily functions unpleasant.

ROACH: True. People - it's a surprising...

GROSS: I'm not arguing that excrement is pleasant, but I mean it's not...

ROACH: No, but not...

GROSS: It's not uninteresting.

(LAUGHTER)

ROACH: Right, no, the - but not just the bottom end of the tract but the mouth, saliva. People are surprisingly offput just by saliva, the substance that you carry around in your mouth. You swallow it. You have no objection to it. But then it leaves your body, and you're just revolted. So it - that - just that right there to me is a fascinating thing.

GROSS: So let's talk about saliva. I mean if you didn't have saliva, you would have a dry mouth. It would be hard to eat. It would be hard to talk. Your tongue would stick to your palate. It would be bad for your teeth. But we have such, as you pointed out earlier, we have this kind of dual relationship with saliva. You know, in our mouths it's very helpful, but once you spit it out, it's considered really quite disgusting.

And even if you can tolerate your own saliva, anybody else's saliva is going to be perceived as quite disgusting. And one of the biggest insults you can give in a casual way is to spit, to spit at them. So what does saliva do for us? For example, people think of saliva as spreading germs, but also saliva has a kind of neutralizing germ effect in your own mouth. So can you talk about that dichotomy?

ROACH: Yes, saliva - and this was all news to me. I went over to The Netherlands, and I visited this woman, this beautiful Italian woman, Erica Saletti(ph), who's devoted her career to the study of saliva, this most repellent of substances. And I learned a lot of things that day. I had no idea. I mean I like anyone else find saliva fairly repugnant once it's outside my own mouth.

But she did this amazing demonstration where she took a medicine dropper, and she took some vinegar, and she could've used wine or anything acid, orange juice. She could have used really anything in that low-pH acid zone. And she dropped a couple drops of vinegar on my tongue and said pay attention.

And immediately this flush of what felt like tepid water came into my mouth. And that was my body sensing that there was this acid in the mouth, which can break down tooth enamel, and rushing in like the cavalry to dilute that acid and bring the pH to a safe level. And it happens right - I mean, I would love - people need to do this. They need to take a gulp of wine, even soda, soda has acid as well, and pay attention.

And there this gush of fluid, and I'm 54 years old, and I never noticed this. So I went around for days taking sips of wine, going, oh, there it is, there's the saliva coming in. And that's critical. If you don't have that, your enamel softens. It just begins to - your teeth dissolve. So if you have a condition where you don't generate enough saliva, it's not just icky and dry mouth, it's - your teeth are in danger.

Honestly, someone should bottle this stuff. It's fairly miraculous. And we haven't even gotten into the anti-viral and the anti-bacterial elements of it. There's...

GROSS: Go there. Go there.

ROACH: Yeah, OK, it renders viruses non-infective. You think of kissing someone as a way of getting a cold or flu, but it's not a good way. If you want to transfer a cold or a flu, it's mostly you're touching something, then you touch your nose or your eye, and it that way makes its way into where it can infect you.

But also there's - in saliva there's these histatins which help wounds heal. So when someone kisses a baby's booboo, like a scrape, or when a pet licks its wounds, it's actually - because you think oh, oh, it's full of bacteria, don't do that. But there's these healing elements. Saliva was a home remedy for cuts and scrapes and shankers and things. People would apply the spittle of a - first-thing-in-the-morning spittle of an old man or something would be, like, the remedy. But there's some medical sense to it.

GROSS: I thought that saliva of some animals like cats would help them, you know, cleanse or heal themselves but not true for humans.

ROACH: I did, too. I found a study on human bites and the rate of infection after somebody's been bitten by a human. And sometimes this is in the form of somebody punching someone and their mouth passively opening a wound on their hand. So there are quite a few cases of that.

And the rate - I don't have the statistic in my head, but it was very low. It was like two in 52 or something. It was a very low rate of infection and again counterintuitive. You assume that the mouth is this sort of cesspool of bacteria. And while there is bacteria in there, anytime you put food in your mouth, or you stick your finger in your mouth, you're introducing bacteria into a warm, moist place, and initially those bacteria are going yee-haw.

But then saliva's got - it's also got an anti-clumping element so that colonies don't tend to build up on teeth the way they might otherwise. So it's a pretty miraculous substance, and it doesn't get any respect at all.

GROSS: How much saliva do we produce a day, about?

ROACH: Between two and three pints of - most of it, almost all of it, is parotid saliva, which is the nicer, the more - there's two kinds. There's the stimulated saliva, when you chew, you get the - or when you have acid in your mouth like I was talking about earlier, that's the watery. It's mostly water. And it's - that's most of it. But then there's background saliva, the kind of gooier, mucousy kind that's unstimulated saliva, and that's generated at a much slower rate all the time.

GROSS: So let's talk about taste. You write about how taste is really largely smell. So why is taste so dependent on our noses?

ROACH: Well, there's taste, which is on the tongue, and that's just the basic building blocks. That's salt, sweet, bitter, sour and umami, which umami is kind of a brothy - kind of a latecomer. I don't know who let umami in. But anyway, it's those five basics.

And everything else - chocolate, vanilla, lemon, cherry, wood smoke, all of these fabulous things that you pick up when you taste wine, all of that is happening in the nose. It's the upper reaches of the nasal cavity, the olfactory epithelium, all these volatile gases.

When you hold wine in your mouth or chocolate, and it begins to melt, it releases all of these gases that head up into the top of the nasal cavity, and you read them as flavor, but really they are smell-based. About 80 to 90 percent of what we call flavor is olfactory.

GROSS: As part of your research to understand taste and how it works and how smell figures into it, you participated in an olive oil tasting panel. Would you describe what your job was, what your mission was?

ROACH: Well, I was - I went to the tryouts for the olive oil tasting panel at the Olive Center at U.C. Davis. Unfortunately I got an email that night that said hi, Mary, thank you for trying out, unfortunately you didn't make the cut. I didn't even come close.

I was trying out to become someone who'd be a sensory analyst, who'd be able to discriminate, make fine discriminations among different olive oils and be able to tell if it's fusty or rancid or it's got like a walnut - stale walnut taste or a sewer dregs taste, all of - there's all these off tastes that olive oil develops depending on how it was made and stored.

And if you're really, really good at identifying these very specific odors, flavors, you can work as a sensory analyst in the industry. I, however, was unable to discriminate between five olive oils that were apparently in all differing degrees of bitterness. To me they: A, weren't bitter, any of them; and B, tasted all the same.

GROSS: Is the moral of that story that the finest olive oil, the subtleties of that finest olive oil you wouldn't be able to taste anyways because your palate isn't subtle enough?

ROACH: The moral of that story is that if you make olive oil, and you're a European olive oil maker, and you want to unload some not-so-great olive oil, the United States is a great place to do it because we don't know anything about olive oil. We're like hey, that tastes good, pass the bread.

(LAUGHTER)

ROACH: And there was one - one of the olive oils in the initial, like you had to ID some flavors, and not just olive oil, that was one of them, and you had to taste it and ID it. And I was like, hey, I got this, it's olive oil, no problem. And in fact it was rancid olive oil, but I had no idea. The people who knew olive oil were like oh, that's a rancid - that's a horrible olive oil. And I'm kind of going hey, this is good.

(LAUGHTER)

GROSS: If you're just joining us, my guest is Mary Roach, and she's the author of the book "Gulp: Adventures on the Alimentary Canal." That's all about your digestive system. Let's take a short break here. Then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is Mary Roach, and she writes a lot about the body and strange things that the body does, things that the body does in outer space. Her first book, "Stiff," was about corpses. Her new book is called "Gulp: Adventures on the Alimentary Canal." And it's all about the digestive system from start to finish.

So you mentioned that there are laundry detergents that use the same digestive enzymes that we have in our digestive system to help us break down food. What are they doing in laundry detergent?

ROACH: They are doing the very same thing that they're doing in your body. You've got - in the mouth you have amylase, which breaks down starches. You've also got some lipase. Lipase also comes into play further along down the line. Lipase breaks down fat, and then there's proteinase that breaks down protein.

So when you think about it, if you've got leftovers on your dirty dishes, or you've spilled something on your clothes, it's the same material that needs to be broken down. And so somebody had the bright idea to take these bodily digestive enzymes and put them into laundry detergent. Laundry detergent is essentially a digestive tract in a box.

GROSS: But there's other kinds of stains, I mean like dirt, you know, dirt from the ground or something.

ROACH: Yeah, well, OK, then there are soaps too. It's not just the enzymes. But for food stains, for your food stains, your fats and your starches and your proteins, that adds an extra stain-fighting component that wouldn't otherwise be there.

GROSS: Let's swallow.

(LAUGHTER)

ROACH: OK.

GROSS: So we've tasted the food, mmm, good, we've chewed it, we swallow it. What happens next?

ROACH: In the stomach, the stomach is - I had always thought that the main function of the stomach and its acids and enzymes was to break down the food. And while it does do that, the other thing it's doing is it's killing bacteria. Gastric acid is - it's another one of the body's defenses against bacteria that you've taken in.

The whole alimentary canal, this, you know, from mouth to the other end, that's a zone where bacteria are coming in. So the body has all these defense mechanisms. And gastric acid is one of those. It's very effective. It doesn't kill 100 percent of the - there are bacteria that can survive, but a whole lot of them get killed right in there in the stomach.

But the stomach's also - it's reducing what you eat to this material, kind of a slurry called chyme, C-H-Y-M-E. And that is then, when it's of the proper consistency, spurted along to the small intestine, where those wonderful fingers, the villi, absorb whatever nutrients they can.

GROSS: So we're eating, we're following the food down the digestive system. How does our body tell us you've eaten enough, you can stop now?

ROACH: The human stomach, the stomach goes to great lengths not to burst because the stomach resides inside a creature that really, really loves to eat and frequently overeat. So it needs to be able to take emergency measures if its owner is eating to excess. And one of the simplest things your stomach can do to relieve pressure is something called the transient lower esophageal sphincter relaxation, which means burp. That's what that is. That's a burp. But that's the medical term.

So - just ditching the gas. Whatever - if you've got gas building up, that's causing pressure. And if you've eaten something fermenty, or you've eaten bicarbonate of soda to make yourself feel better, and you took too much, sometimes the gas can build up very quickly, so quickly that it outpaces the body's ability to react. And in the very, very, very rare cases of stomach rupture, it's frequently bicarbonate of soda that's been the culprit, not the food itself.

GROSS: What does the bicarbonate do?

ROACH: It produces gas. It hits the stomach and it - there's a reaction. A chemical reaction happens and it produces gas. And the idea is you want to be able to burp, and it makes you burp, and you feel better because burping is a release of gas. So a little bit is good; a lot not so good.

But the next thing that your body does if you persist in overeating is it simply is a regurgitation reflex; you would throw up. And I interviewed professional, competitive eaters about this. And I said, well, how do you deal - this is automatic reflex that happens. If you stretch the stomach beyond a certain point, you'll throw up. And how do you guys not throw up?

And he said, well, we do, but the rules are it can come up, it just can't come out. So you swallow it back down. And that was probably the most revolting thing I learned in the entire book.

GROSS: I can't comprehend competitive eating.

ROACH: And now I've shared it with millions of Americans, but anyway.

GROSS: If you're a competitive eater, do you stretch your stomach so it can accommodate more food? Like do you gradually stretch it like you were exercising your stomach or something?

ROACH: There was a gastroenterologist who brought a competitive eater into the hospital where he worked and took a look at what was going on, set him in front of a fluoroscope and gave him hot dogs with kind of a side of barium so they'd show up. And he watched as the competitive eater and an ordinary person ate hot dogs.

A competitive eater was to eat this prodigious amount of hot dogs. And he doesn't have a larger stomach. Their stomachs are no larger. But they're what's called compliant. They're able to stretch and accommodate more food. So they start - very often these are people with highly compliant stomachs. They're kind of like lions, you know, a feast or famine type of animal that has a huge capacity to take it in and then live off of it for a week.

So they've got these compliant stomachs. But then on top of that they're training themselves. They're doing water-loading, where they drink a gallon of water - and that's something nobody should do because that can kill you if you're not used to it. And so they do both. They have a genetic advantage, and then they're practicing.

And it's largely mental, I'm told, that you're getting used to a really - just a really unpleasant, bloated feeling and forcing yourself to keep going and kind of getting into this Zen state where you - it isn't anything like eating. They don't even - you ask them, oh, what were you eating at that contest. Oh, I don't know. Because they don't even - the flavor doesn't even enter into it. It's just stuff that they need to get down as fast as they can.

GROSS: What an odd sport. So you write about probiotics in your book. And, you know, we've talked on FRESH AIR about one of the ways doctors are - you know, scientists are experimenting now with creating the right bacterial balance in your gut is the transplantation of fecal material.

And of course it's - you know, it's like dried and sterilized or whatever, I guess not sterilized, but so the idea is to take the flora from a healthy person's gut and transport some of that in somebody's gut that doesn't have the healthy flora. What's the most interesting thing you learned about probiotic research?

ROACH: Well, just on the topic of fecal transplants, which is a form of probiotics, a very extreme form of probiotics, and in fact I observed one, and it isn't - at the time it was not - it's not dried. It's certainly not sterilized.

It's basically put into a blender and turned into a liquid that's then put into the body as quickly as possible because, you know, you take it from the donor and there's - you don't know how long these bacteria can live outside the body because they're anaerobic. They don't want to be in oxygen.

So it's very rush-rush. The donor comes in, hands off the bag, rush it over to the lab, make the preparation, drive it back to where the patient is waiting and use the same equipment you'd use for a colonoscopy and introduce that. And it's a substantial amount of fluid. It's not just a few drops. It's a - you know, a cup of - I don't know if it's a cup or two cups, but it looks like coffee with a little bit of skim milk in it.

And it's done under a fume hood. So there's no - you know, it's not - it sounds like a really revolting thing, but it really wasn't. And it was so kind of inspiring because the person who was receiving this material, it was somebody who had a chronic C. diff infection.

C. difficile is a bacteria that can set up housekeeping inside the human gut. It's very hard to get rid of. It gets inside little pockets, and it creates inflammation, it causes diarrhea, and it makes - it ranges from the person being miserable to ultimately dying.

And the fecal transplant has a 90-something percent cure rate. It's not expensive. You know, feces is cheap. It's not - there's no downside to it. And so it's rare to me that - it's just rare that medicine comes up with such an effective cure that is affordable and has no side effects. So it's fairly inspiring. For something that sounds just intuitively really wrong and gross, it's really - it's miraculous.

GROSS: Yeah, and I think my attempt to make it less gross-sounding in my description, I totally sanitized the process in a way that you couldn't do it without killing the good-guy bacteria. So thank you for giving a far more accurate description of the transplant. And...

ROACH: Well, eventually they - I was going to say that eventually the hope is to make a - somehow make a pill form. I mean you can freeze bacteria. You can - eventually it will be a more standardized and less gross technique. But right now it's very simple: Take it out of Person A and put it into Person B.

BIANCULLI: Mary Roach, speaking to Terry Gross. We'll continue their conversation in the second half of the show. I'm David Bianculli, and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

BIANCULLI: This is FRESH AIR. I'm David Bianculli, in for Terry Gross. Let's get back to Terry's interview from last year with Mary Roach. She's known for her witty science books including, "Stiff" and "Bonk." Her latest book "Gulp," is a tour of the human digestive system.

GROSS: At this point, I think I ought to say, like, this isn't going to be the most appetizing part of the conversation. If you're eating, you know, a meal now or a snack, I know you might find part of this conversation a little off-putting. At the same time, think of it this way, when you're done eating that snack or that meal, what's happening to your body is going to be exactly what we're going to talk about.

(LAUGHTER)

GROSS: So, so let's talk deeper into digestion. And one of the most embarrassing things about the digestive process is digestive gas when it is accidentally released. What is digestive gas and why does it smell so bad?

ROACH: The technical term is flatus. Flatulence is the condition of having flatus. Flatus is mostly hydrogen, in about a third of the population, it's also methane. And both of them are odorless. The owner is provided by mostly sulfur compounds. Hydrogen sulfide is the main culprit in what is the technical term here again, noxious flatus. So it's your sulfur compounds. The foods that you want to avoid if you're troubled by noxious flatus would be any of the sulfur containing cruciferous vegetables like broccoli, cabbage, kale. Those are notorious. Protein. Red meat is also makes for a smelly or more noxious flatus.

GROSS: You know, you write that the amount of intestinal gas in your system piques five hours after a meal. Can you explain why?

ROACH: Yes. Because its formed in the colon, which is the last stop before the rectum. That's where the food that your body couldn't absorb, the stuff that it couldn't absorb and use, the waste material, insoluble fibers and, you know, bits and chunks of corn or whatever, you know, the woody stuff, it gets all passed into the colon and there are bacteria in there that can make use of that. It's food for your bacteria, in essence, and when bacteria breaks this material down they produce gas. So in essence, your gas is the bacteria's gas. It's bacteria farts, the buildup. And at a certain point your body makes the decision to expel it and if - it's because it gets painful after a while. If you have a tremendous amount of gas and often older people have a flabbier colon and it stretches more and the only way you feel pain in the colon is from stretching and the stretch receptors read their activation as pain, so you get that gas pain and you want to get rid of it and that's your passage of flatus.

GROSS: Now I think you spoke to several scientists who were studying flatus. What are they looking for?

ROACH: Well, the man who's done the most work - and I counted 34 papers on flatus - Michael Levitt at the VA Medical Center in Minneapolis - he was testing a number of products that allegedly remedied noxious flatus - that sulfur smell. He was testing there's pads you can put in your underwear. There's elasticized underwear; there are pills you can take; there is actually a remedy - there's something called Devrom, which is an internal deodorant. It's an internal deodorant and the main ingredient is bismuth subsalicylate, and that binds up the sulfur compounds and it reduces the odor. However, I spoke to one gastroenterologist, I asked him about this, and he said, you know, when I get someone who comes in and is complaining about noxious flatus, I tell them: Just get a dog. In other words, so you can blame the dog.

(LAUGHTER)

GROSS: Is there a difference between male and female flatus?

ROACH: Well, that's a controversial area we've just wandered into, Terry.

GROSS: Is it?

ROACH: Yes it is. Women supposedly have a more noxious smell, this is what I've been told. But men make up for it with the larger on average volume per passage. The other thing going on with women, and this I have to get into a little bit of how you measure flatulence. It's not done with a tube in the rectum. It used to be. But now it's done, it's a breath hydrogen test because, you know, when you have these things in your large bowel and your colon that are being broken down, they're producing this gas, a certain amount is absorbed into the blood and then exhaled out the lungs. So if you want to see how much hydrogen someone's producing you can just take it off their breath. Now if you have a woman who's suppressing her gas and not ever - like my mother-in-law, you will not hear that women pass gas. OK, so she must be suppressing, and in her case, is coming out her mouth. So if she were doing bad breath hydrogen test she would appear to be highly flatulent, but I don't believe that she is. It's skewing the results. So it's a very complicated subject.

GROSS: So since you've been doing this research are you anymore or less embarrassed if you're in the room with somebody and you accidentally, accidentally release gas?

ROACH: It depends entirely on who's in the room with me. There are some people, this is just really way more information than anyone wants about Mary Roach, but there are some people who I feel completely free to let it fly and other people I just, you know, like anyone else I would be horrified. I remember I was at the TED conference when I was in the bookstore and one sneaked out, and I was like this hasn't happened to me in a long time or ever. I don't know. And I was horrified because people like ooh, who did - what was that? So I'm no more - although I talk about it constantly, I'm not immune to that very unique embarrassment.

GROSS: Now I can easily imagine you going well, there's a funny and interesting story about this.

(LAUGHTER)

ROACH: Let me tell you how you collect...

GROSS: You might be wondering about the science behind this.

(LAUGHTER)

ROACH: Let me tell you how you collect that stuff.

(LAUGHTER)

GROSS: So now that you can visualize what's going on in your colon, do you feel your digestion more powerfully? Like do you visualize the food going down? Do you visualize the twist and turns in your intestines?

ROACH: No. I'm one of those fortunate people that goes around largely ignorant of their intestinal goings-on. I don't really pay much attention to it. But what happened, I did, as I was working on the mouth side of it, I became unpleasantly aware of things happening in my mouth. Things I had never really given any thought to when you chew food what is going on in there. And really, you're taking something apart by, grinding it up and then you're reassembling it for the swallow and you're creating this thing called a bolus, which is a cylindrical moistened cohesive mass. And every time you take a bite of something and chew it up you're building a bolus in your mouth. And I didn't, I just. I don't want to know about my bolus building. I don't want to think about it. And I think because the food has just been in front of you as this wonderful, sensuous, delicious thing and now it's in your mouth and it's becoming this unpleasant, not sensuous, revolting thing.

GROSS: Right.

(LAUGHTER)

ROACH: Does that clear things up?

GROSS: Yes.

(LAUGHTER)

GROSS: And speaking of revolting, so as we make our way down the digestive system, again, I want to say to our listeners this is subject matter you might find very unappetizing. My apologies if you're eating at the moment. But again, I just want to mention this is what's going to happen in your system after you're done eating.

So, let's go down toward the end of the digestive process. And I learned from your book that - something that should've been obvious, so I can't say I ever thought about it at all, is that the rectum has a lot of nerve endings in it because?

ROACH: Well, the anus, specifically, is the champion here. The human anus is this ring of muscle and it's highly enervate. It's got tons of nerves. And the reason is that it needs to be able to discriminate, by feel, between solid, liquid and gas and be able to selectively release one or maybe all of those. And thank heavens for the anus because, you know, really a lot of gratitude, ladies and gentlemen, to the human anus. It's kind of an amazing thing that it does. It's very impressive.

GROSS: If you joining us, my guest is Mary Roach. She's the author of the new book called "Adventures on the Alimentary Canal." It's all about the digestive system.

Let's take a short break here, then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you joining us, my guest is Mary Roach. She's written several books about the human body. And her new book called "Gulp: Adventures on the Alimentary Canal." And it's all about the digestive system.

So one of the things you did as research for your book is to swallow a pill cam - like a little pill-size camera - to see your digestive tract. You did it in your own body?

ROACH: Actually, I watched footage from an unknown person's pill cam. I didn't actually...

GROSS: Oh, OK.

ROACH: I watched my own - I had a colonoscopy done without drugs so that I could see my insides, so I did an even more heroic thing, Terry. I didn't swallow a pill cam, but I did do that.

(LAUGHTER)

GROSS: So why don't we start with the colonoscopy. Why did you do that? I know that there was sedation standing by should you need it - which you didn't - but why did you want to endure a sedation-free colonoscopy in order to bear witness to what you're insides looked like?

ROACH: Well, that is precisely why. Well, I don't know that I would have done it were I not writing this book. But this was an opportunity - probably the only opportunity - hopefully, the only opportunity - I'll ever get to witness to see the inside of my large intestine. And, OK, my rectum, also interesting. And it's your guts and I don't know how people can kind of be curious about what their own insides look like. So I had this curiosity and also I had come across this wonderful, it was then maybe the "Lancet," one of the major medical journals, it was someone who had put two photographs side-by-side. One of them was the interior of Guadi's "La Pradera" - I believe this is how you pronounce it. It was this wonderful archways that looked very much like the connections between segments of the colon. And he had made this comparison between these, you know, this work of art and the human colon. And I thought I want to see my own internal Gaudi; I want to see that.

GROSS: What did it look like? What did you learn from seeing it?

ROACH: I failed to see the Gaudi in there. For me it was just kind of surprisingly pink and glistening - a kind of a bubblegum pink and pretty. I mean that sounds - because it's been cleaned out very, very, very, very thoroughly, it's a surprisingly - it looks like, you know, the interior of a very clean spaceship or something, not at all what you would expect.

GROSS: And it was healthy.

ROACH: And it was healthy, thankfully. No strange growths, yeah.

(LAUGHTER)

GROSS: So let's get back to the pill cam.

ROACH: Yes.

GROSS: And you watched an ominous person's inside through this pill-size camera. But you compare what happens to the stomach - what you saw happening to the stomach - with the Titanic. Why?

ROACH: Yes. It was this dark, murky kind of watery scene with little bits kind of floating around, and that's immediately what came to mind. And what was striking is as soon as you pass through that sphincter into the small intestine, everything changes and now it's a pale pink, kind of the color of that white tuna sushi, that lovely pale pink, that's the color and it's got these little sort of Dr. Seuss projections, little fingers. It looks like velvet or terrycloth. And that's what it is essentially, by terrycloth, it's increasing the surface area for absorption. Obviously, with a towel you are absorbing more water and with the small intestine you're absorbing nutrients. But it really is this kind of velvety undulating surface, it's like that, what is that film where people take a trip down the Elementary Canal and they see these sites? It really, you do have this sense of being able to fly over this bizarre alien landscape. I thought it was absolutely fascinating and not at all disgusting.

GROSS: So one of odder things that you learned for your book is about hooping. And this relates to stashing things in your body...

ROACH: Yes.

GROSS: ...so it will go undetected. Say a little bit more about like what hooping is and what are some of the things you learn have been stored at the bottom end of the digestive tract?

ROACH: Well, this is the rectum chapter and I wanted to provide a narrative that would be appropriate and interesting setting for the reader. And I thought well, what is the rectum? Well, it's a storage facility. The rectum enables us to hold on to waste and that means we don't need to stop what we're doing and find a bathroom. We can hold it. I mean we have a little closet, we have a little suitcase - if you will - and we can hold it there. And that, you know, it's a force for civilized behavior and again, thank you to the rectum well. OK, so I thought all right, well, who would be someone who'll would be interesting to talk to. And people who smuggle contraband. For people who smuggle contraband, the human body, the human alimentary system, is a handy pocket - if you will - particularly the rectum. In prisons, there's a, I was at Avenel State Prison where they have a problem with cell phone smuggling, people smuggling cell phones in and conducting drug deals and hits and things like that that they shouldn't be doing. And so I spent some time with a man who is - it's called hooping - as in through the hoop. And hooping is a way of life in prison. It's not - 'cause I thought well, this would be a little strange to sit down with the man and talk about the things that he puts in his rectum. But, in fact, he was just like yeah, well, what do you want to know? So it was an interesting afternoon.

GROSS: So what are some of the things that he's put there?

ROACH: The most common thing is bindles of tobacco, because you're not supposed to have tobacco. And so people put these plugs of tobacco wrapped in a condom or a piece of latex and that's often - he said if you're going into solitary confinement, you're going into the hole, the other hole, you want to bring in your smoking material. So they'll put up the tobacco and the lighter and so they'll smuggle it into solitary because you're not allowed to have it there.

And this shed light on something I had come upon when I was writing "Bonk," which was a collection of things that people had put in rectums, they showed up in the emergency room, and here was this one that just made no sense to me at the time. It was a magazine, tobacco pouch and spectacles.

And I thought who puts that in their rectum? And now I realize this was probably a guy going into solitary, and here was his reading material and his smoking material and his glasses.

GROSS: So was the person who you interviewed in prison there because he had smuggled things this way? Like how did you know to talk with him?

ROACH: No. No. He was there for murder. When I spoke to the public relations people at the California Department of Corrections and Rehabilitation, they said well, we can let you speak to the staff who deal with this, but we can also put you in touch with somebody who is known for his talents in this area.

So he was famous for having, on a bet, I was told, hooped something quite large. And so they thought that he would be the most appropriate person for me to speak with.

GROSS: That's so interesting that the prison itself recommended him. I guess, you know...

ROACH: Yeah. I know.

GROSS: It's legitimate - it's like legitimate research. It's very interesting.

ROACH: Yeah. I sent an email to the public relations guy explaining what I was doing, and I wanted to write about the alimentary canal as a criminal accomplice, essentially, and I wanted to speak to someone. And, you know, I expected to get no reply and I got this reply back that said well, absolutely. We have a big problem with cell phone smuggling and would four hours be enough for you? I'm like to speak to a stranger about his rectum? I think so.

(LAUGHTER)

GROSS: Have you ever had a kind of growth or rash or something that your body has produced or done that is so incorrect looking and just so, like, fascinating and revolting at the same time that you just, it just kind of stopped you?

(LAUGHTER)

ROACH: I had a bike accident a few years ago and I went to the emergency room and I had to have a gash sewn up. And I am the kind of person that I was sitting up fascinated watching to the extent that the doctor said, do you want to do a couple of stitches? You seem to be very interested.

(LAUGHTER)

ROACH: I'm really, I've been in operating rooms at UCSF here, I've for a couple of my books where I've been in the operating room and they've really had to stop and say Ms. Roach, could you step back? Your head is actually inside the body cavity.

I'm like - but I don't think that I'm strange. I could be very, very wrong about that, but I think people underestimate their ability to cope with kind of gross and upsetting things because a lot of it is fear of the unknown.

GROSS: Do you have a favorite place that you've visited for research for your book that you haven't told us about yet?

ROACH: Oh yeah, Mutter Museum in Philadelphia, the Mutter Museum.

GROSS: Oh, that place is so great.

ROACH: Yeah. Mutter is - yes.

GROSS: Yes, you write about the mega colon there. So you can talk about that.

ROACH: Yes.

GROSS: That's probably the most relevant thing for your book.

ROACH: Yes. The mega-colon is on display and it's this big glass vitrine with kind of perfectly placed museum lighting. And it's this colon that is as big around as my waist, literally. And it's a sad thing. The man who had it was a very unhappy man who had a lot of problem, you know, emptying his colon and eventually died from it. But it is a very - it's a strange place because a lot of the exhibits are upsetting.

But it's beautiful, like any well-curated museum. And you go in and there's the beautiful lighting and the professional people who - I just - the Mutter Museum is - I got to go into the basement where they have all kinds of weird goodies. Although...

GROSS: And I should explain it was a teaching museum for medical oddities like conjoined twins and the mega-colon that you mentioned. So it's just - it's a fascinating collection of things that can go wrong with the body, unusual things.

ROACH: Yes. And also some...

GROSS: Not typical things.

ROACH: ...some very unusual things that individuals have donated over the years. For example, a bracelet made of hemorrhoids, I think it was. Or maybe it was a necklace. Anyway, sometimes doctors come upon these things or they make them - I don't know - and they just drop them off at the Mutter Museum. So if you work at the Mutter Museum every day brings a new and strange surprise.

GROSS: Mary Roach, it's been a pleasure. Thank you so much. And happy digesting.

ROACH: Oh, same to you, Terry. Thank you so much for having me on.

BIANCULLI: Mary Roach, speaking with Terry Gross last year. Her book about the human digestive system is called "Gulp." It comes out in paperback next month. You can read an excerpt on our website, freshair.npr.org. Coming up, I review the new NBC series "Crisis," which premieres Sunday. This is FRESH AIR. Transcript provided by NPR, Copyright NPR.

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