In her latest book exploring the science that surrounds life's unmentionables, Mary Roach goes for the gut. Literally.

Roach has already taken on sex ("Bonk"), death ("Stiff"), the afterlife ("Spook") and the final frontier ("Packing for Mars"). In "Gulp: Adventures on the Alimentary Canal," she surveys centuries' worth of weird and wonderful discoveries about our digestive system, from the lips all the way down to the anus (which Roach says has some of the most densely innervated tissue on the human body).

In the course of exploring the alimentary canal, Roach addresses questions about our body's oddities (What keeps our stomach from digesting itself out of existence?) as well as the chemistry of digestion (How does Beano fight flatulence? How does Devrom stop the stink?).

One of the most fascinating tales has to do with the curse of Elvis Presley's colon: He died in 1977, while straining on the stool — and through the years, experts have pointed to drug abuse as well as a bad heart as contributing causes. But Roach concentrates instead on constipation, a problem that apparently plagued Presley for much of his life. The autopsy showed Presley had an enlarged "megacolon," horribly impacted with claylike material from a barium X-ray procedure that the King went through four months earlier.

It turns out that other folks have suffered fatal cases of constipation, but there's so much ickyness surrounding the subject that you don't hear much about it. "I doubt you'll be seeing bus posters about defecation-associated sudden death any time soon," Roach writes.

There's a similar ick factor about many of the topics touched upon in "Gulp" — but fortunately, Roach has a knack for turning the "ick" into "ooh!" "wow!" and "really!?" In an interview last week, Roach discussed the ick factor and listed some of her favorite "Gulp" moments. Here's an edited transcript of the Q&A:

"Gulp" answers questions ranging from Elvis Presley's cause of death to the frontier of fecal transplantation. W.W. Norton

Cosmic Log: Tell me how the book got started. How did you get into "Gulp"?

Mary Roach: Well, a couple of things: One of them was something I stumbled onto when I was writing "Packing for Mars." I came upon a rather bizarre space nutrition study at the University of California at Berkeley back in the '60s, where they were testing bacteria as an entree. Dead bacteria. They actually had subjects go into a metabolic chamber and they sat them down, and they served them a slurry of bacteria of different varieties. And it was a terrible fiasco, of course.

That got me thinking about eating, and how it's a sensual thing and something that involves the mind, something we look forward to. But underneath all that, it's a basic biological need, and a process. We have a food processor, but we don't like to think about that. So I thought, maybe I'll think about that. Maybe I'll go down the alimentary canal and have a look.

Q: You talk a lot about the taboos that are associated with eating and digestion. Could you put your finger on the silliest taboo you came across? Is there some attitude toward eating that really makes no sense?

A: The first one that comes to mind is saliva. Saliva is something that's a highly taboo substance. Once it's outside your body, your own saliva is a source of disgust. Which is quite bizarre, because you're swallowing it all the time. You generate two to three pints of it, right there in your mouth. And yet, once it leaves the body, it's an object of revulsion. It's fascinating — something that has to do with the boundaries of the self.

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Q: You debunk a lot of myths in the book, too. Is there particular bit of accepted wisdom that you're proudest to show is not really true?

A: The myth that I had the most fun with was the Jonah myth. Some people take the Bible literally, and try to make the case that a human being could survive in a whale's stomach. So I looked into this and tried to figure out which whale. A sperm whale would be the most likely candidate, because it's got a big enough gullet, and it doesn't have gastric acid. What it does have, though, is a very powerful stomach that crushes whatever is in its gut. You would be tumbled around and probably have some broken bones if you were inside a sperm whale.

Q: Is there something in the book that people really should know, that they probably don't know? For example, if I ever feel like my stomach is full to bursting, I'm definitely not going to load up on bicarbonate of soda.

A: Yes, the human stomach is surprisingly resistant to bursting. It has a couple of emergency ditching maneuvers. You burp, or you regurgitate. This is your stomach's way of saying, "OK, we don't want to burst, that would be fatal. So let's get rid of some stuff." The only time a human being suffers a case of a burst stomach tends to be somebody who ate a huge meal, and then felt uncomfortable and took a whole bunch of bicarbonate of soda. A little bit of gas makes you burp, and then you feel better. But a lot of gas, generated quickly, can outpace the body's safety mechanisms and burst your stomach. So after eating a huge meal, I don't recommend a large dose of bicarbonate of soda. Proceed with caution.

Q: "Gulp" includes lots of historical tales about those who have studied the alimentary canal. Is there one story you'd point to as deserving of more attention than it usually gets?

A: One of the people that impressed me was the very first experimenter to study and document human intestinal gas. This was in 1816. A Parisian doctor, Francois Magendie, had the opportunity to dissect a couple of guillotined prisoners. Because the prisoners had a last meal, and he knew what the last meal was, he could run a controlled experiment, if you will. He knew how long they'd been digesting. So he looked at what types of gas were in what part of the alimentary canal. He even figured out the hydrogen sulfide component, which is usually only 0.2 to 0.3 parts per million. It's a trace gas, but the human nose is quite sensitive to it, so it's possible he just used, uh, his nose. That was a novel approach to studying human intestinal gas. For originality, I give Magendie a lot of points.

Q: And when it comes to the scientific frontiers for studying the alimentary canal, a lot of people talk about fecal transplants. That's something that you address in the book.

A: Yes, if you have a certain type of bacteria called C. difficile, C. diff for short, it tends to set up camp in little pockets along the intestine, and it can be difficult to get rid of. It can be a kind of lingering infection that leads to inflammation and diarrhea. It's a quite serious condition, sometimes fatal.

If you take someone else's waste, and you use a colonoscope, you can put that material in and basically "seed" the patient's bacteria with a whole different set of bacteria that takes over. You take it from a healthy person, obviously, not from someone else who has C. diff. You take it from the waste material, which is one-third bacteria by dry weight. There's a lot of bacteria in human waste. Tons! That was a surprise to me. You don't really know what that stuff is, but a lot of it is bacteria.

This has about a 90 percent cure rate for chronic C. diff infection, and there's no real down side. It's rare that medicine comes up with something that simple, that effective, and with no side effects. The problem with it is just the ick factor. It's been slow to catch on, probably because there's no device maker or drug company to push a drug through. It has to be the hard work of M.D.'s who are just trying to get it into the system. They don't even know how to bill for it, so they bill for a colonoscopy.

Now people are starting to look at bacterial transplants of different kinds, as possible treatments for everything from weight loss to chronic ear infections. There's someone looking into it as a treatment for gum disease, by taking someone else's oral bacteria and giving them a dose of that. There's not a lot of down side, other than the ick factor.

Q: It strikes me that the ick factor, and how to deal with that, is a theme that runs through the book. Have you drawn any lessons about how to get over the ick factor when it hurts us rather than helps us?

A: This is one of those rare and wonderful cases where the media's fascination has been helpful. There have been a lot of articles written about fecal transplants, and that's partly because it's headline-grabbing. "Yeah, they put someone's crap in somebody else!" It gets people's attention, and they read it. But it's gotten so much coverage that now people are used to the notion of doing it, and they know that it's effective, and they know that it's useful. It's not such an intuitively horrific thing. The more people talk about it, the more they'll get used to it, and the more the ick factor dissolves. Then people with a problem feel free to go to their doctor and say, "Hey, I heard about this fecal transplant, and I wonder if maybe we can try that."

The fact that it's getting a lot of coverage, and a lot of people are talking about it, is making it OK to speak about it. And that's always a good thing.

Q: Do you feel as if "Gulp" actually serves that purpose? I realize every author feels as if his or her book is a boon to humanity, but is this a special case?

A: [Laughter] With my books, it's a little hard to make the case. But if I were to make the case, it would simply be that: I am encouraging people to talk about what's going on in the whole human food processor, from mouth to anus. It's a miraculous machine, and we owe it a little respect, instead of shame and embarrassment. I would love to see people having dialogues about it without feeling funny.

More cool facts about our food processor:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.