Synopsis: Most people have little idea what goes on in the digestive system. A science writer explains everything she found during an exhaustive investigation.

Host: Reed Pence. Guest: Mary Roach, author, Gulp: Adventures On the Alimentary Canal

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Alimentary Adventures


Reed Pence: Science writer Mary Roach is accustomed to subjects that are slightly off kilter. She’s written about the odd things that happen to human bodies donated to science, and looked at the strange and sometimes unmentionable side of what goes on in space travel. But Roach says she wasn’t completely prepared when she decided to investigate the human digestive system.

Mary Roach: This book was full of surprises for me I didn’t have any sort of a background in physiology of eating, digesting, excreting, so it was all surprising to me. I just, a lot of assumptions we have about how things work. For example, the teeth. I thought of the jaw and the teeth as sort of like these he-man powerful muscles. You do read about, oh it’s the strongest muscle in the human body, etcetera, but in fact, what’s really interesting is how sensitive the jaw muscles are and the teeth. Who knew? I didn’t know that the human body has two sets of nostrils. I didn’t know that when you blush, your stomach blushes. I had no idea that there are people out there who spend their time researching oral processing or flatulence or how competitive eaters do what they do. So, the whole thing was a big surprise.

Reed Pence: The result of all those surprises is Roach’s new book, Gulp: Adventures On The Alimentary Canal. She follows your breakfast from one end to the other, so you might not want to be having breakfast when you hear about it. But digestion is an essential part of life, and when it comes to the digestive tract, no two of us are exactly alike. It starts the moment we take a bite and start chewing.

Mary Roach: You can’t say, oh this person is eating peanuts or this person is eating fruitcake or custard, and therefore we know the precise range of chews, the number of chews, because it’s all over the map. It’s like a personal trait. Some people are long chewers, meaning they chew for a long time, a hundred and seventeen chews per bite versus ten chews per bite. Other people give it a few chews and sort of swallow it in big chunks. There are people that are right chewed and left chewed, like left-handed and right handed, they favor one side. Some people kinda go side to side, some are straight up and down, some have their mouth open, some shut, so, yea, it’s almost like a fingerprint.

Reed Pence: What we sense as taste is unique, too. Some of that’s genetic, but training and experience can also play a role. Roach found that out when she tried out for an olive oil tasting panel at the Robert Mondavi Institute for Wine And Food Science at the University of California, Davis.

Mary Roach: I failed. I didn’t make the cut, not even the first cut. I think there were probably three levels, and I didn’t make even the first one. And it isn’t that I have genetically inferior nose or tongue, it’s more to do with experience. When people, (whether it’s the olive oil industry or wine or beer) when you’re in that industry, you begin to learn a language of smell. Because most of what’s happening with flavor is happening in the nose. It’s these volatile aromas that, when you have the food or wine in your mouth, they waft up into the nose, and that’s all of those wonderful flavors that you see on the wine bottle. Black cherry and vanilla and oaky and all of that is in the nose, so when you’re exposed to these different olive oils, you begin to be able to discriminate among them, and it’s kind of like learning a vocabulary.

Reed Pence: Lacking that background, Roach says all of the olive oils she tried tasted exactly the same. Even one that experienced tasters said was rancid. It turns out that most of us are exactly the same way, so she says we may as well buy the cheap stuff rather than spending too much.

Mary Roach: Yeah, pretty much, and in fact, the European olive oil makers know this. America is known as a dumping ground for inferior olive oil. There was a whole book (I think it might be even called Extra Virgin,) anyway, it was kind of an expose about the olive oil industry and how cheap olive oil is passed off on Americans because we don’t know any better. Until you’ve been to tastings, and you really spend some time learning about olive oil, you can’t really make those fine discriminations.

(Editor’s Note: the book Roach is referring to is called Extra Virginity: The Sublime and Scandalous World of Olive Oil by Tom Mueller)

Reed Pence: However, apparently good taste can be learned.

Mary Roach: You can buy these kits, like Lenne Devev, a nose of wine which is twenty, thirty, forty, I forget how many, little bottles that have the reference molecules that you smell, and you learn say, okay, pear, this is what pear smells like. This is what melon smells like. This is what black berry smells like. And then when you drink a wine, if it’s wine that you’re working on, then you are able to recognize and put a name to that aroma, and if you don’t do that, you have a sense that there’s a complex thing going on in your nose, but you can’t really put words to it. You can’t really identify what you’re experiencing. Like learning a foreign language or learning music, first you just hear the symphony, and then the more you’re exposed to it, the more you learn about it, you can pick out the different instruments, the different themes that are coming into play, and you just have a different experience of the music than somebody who’s coming to it without having studied, without having experience.

Reed Pence: You might say that part of what we’re tasting is our own saliva. But then we’re used to that, so we ignore it. However, Roach says if you pay close attention, saliva is a lot more than just wet and icky.

Mary Roach: People tend to think saliva is just a moistening agent for the food that we’ve broken down with our teeth. And it does do that, obviously, it fits together the particles, you know, and helps you make that lovely item called a bolus. The oral processing people call it bolus. But it does a lot more than that. One of the main things it’s doing every day is, if you eat or drink something acidic like wine or vinegar or citrus juice, cola, things that are down in the 1, 2, 3 range of the PH scale, those things will literally dissolve your tooth enamel, and when you consume them you feel (if you’re paying attention) you feel this kind of astounding… it feels like tepid water, but it’s saliva. It’s parotid saliva, stimulated saliva coming in to dilute the acid and protect your teeth.

Reed Pence: Roach says this was demonstrated to her rather vividly by a professional saliva researcher in the Netherlands.

Mary Roach: She put a couple drops of vinegar on my tongue and said, “Pay attention to what happens.” And it’s remarkable! So I’ve now become this horrible dinner companion for people because they take a sip of wine and I say “Okay, pay attention, pay attention, you’re gonna get this gush of saliva coming in and that’s diluting the acid and it’s protecting your tooth enamel. And, you know, people really don’t want to hear that when their eating dinner.

Reed Pence: Despite the ick factor, saliva is our friend. For example, if an infant’s pacifier falls on the floor, many parents will clean it by sticking it in their own mouths before giving it back to the child. Now a study in the journal pediatrics suggests that may be extremely beneficial to the baby. The bacteria in Mom or Dad’s saliva, passed on to the child, reduces by nearly 90 percent the chance that the child will later develop asthma.

Mary Roach: Saliva has elements in it that help close wounds. They help skin grow and nerves grow. Animals lick their wounds apparently for a reason. People tend to just focus on the fact that there’s bacteria in the mouth, and it’s true, the mouth is a warm moist place. It’s a place bacteria like, but because of that, and because your body is trying to keep you healthy, saliva does have these bacteria fighting properties and also wound healing properties.

Reed Pence: By the time you get down to the stomach, things get a little more nasty. There’s hydrochloric acid in there, for example, a source of more misconceptions.

Mary Roach: Most of us, certainly me when I began this book, assumed that the only thing that that was there for was to break down food, and that is one thing that it does, but it’s also a line of defense. It’s there to kill bacteria. Because when you eat foods, you’re ushering in a lot of bacteria into your system. So, gastric acid is an anti-bacterial system as well. I had a snake digestion researcher, in fact, I had him put a drop of hydrochloric acid on my wrist, and in fact, it doesn’t burn. You don’t feel anything for a moment. It takes some time, and it takes a lot of it. So, your stomach is generally, it’s just sort of itched a little bit. It’s not the case where, like in the Wizard Of Oz, the witch melting or something like that, you need a lot of acid and you need some time for it to do it’s thing. There’s also enzymes that are breaking things down, so there’s a lot of things going on in there.

Reed Pence: Some people have even more going on than average, because they eat so much. Competitive eaters, for example… Men who can eat as many as 70 hot dogs and women as many as 45 in just 10 minutes.

Mary Roach: I had thought perhaps that competitive eaters were people who had larger than average stomachs but it isn’t that their stomachs tend to be more compliant that is they stretch out more. And this was demonstrated by a there was a gastroenterologist Dr. Metz who published a paper on this he brought a competitive eater into his lab and a regular eater and set them up with a lot of hot dogs and a side of barium so that it would show up on a fluoroscope. So they could actually watch what was going on with these two men’s stomachs and the competitive eater didn’t have a larger stomach but his was able to stretch tremendously just like the difference between regular gum and bubble gum sort of.

Reed Pence: Roach went to talk to some competitive eaters… And asked how they can eat so much without throwing up.

Mary Roach: Regurgitation is a protective reflex. If your stomach is about to burst your body will just empty it reflexively. And I said, well, how did you get around this? Because that’s physiology. How are you able?” He says, “Well, this will sound gross, but in fact that does happen to us, and we just have to swallow it back down.” Because the rules say that food can come up but it can’t come out. So they have to swallow it back down. It really is unimaginable how they’re able to do this. A lot of this is mental. If you put it, I imagine it’s almost like a Zen state, where you just keep going despite the discomfort and the fact that your body is doing everything it can to tell you to stop.

Reed Pence: Next stop after the stomach is the intestines. The small intestine, where nutrients are mostly pulled out, and the colon, which processes what’s left. Scientists are learning that the bacteria community living in your colon is pretty important.

Mary Roach: There are nine micro… (this is a statistic that was surprising to me) nine microorganisms, nine cells of them to one cell of you. There are trillions of bacteria in the colon. They are breaking down the things that you don’t absorb, the nutrients you don’t absorb, in the small intestine, make their way down to the large intestine, and there are bacteria breaking them down, and creating gas in the process–that’s intestinal gas.

Reed Pence: But with so many bacteria living down there, there’s always the chance you’ll have bad ones along with the good. In fact, those can even be life threatening.

Mary Roach: If you have certain bacteria that can get a foothold in the colon, that can wreak havoc and cause a lot of diarrhea and misery, one of those is C difficile. There is an interesting way to treat that which is to take a healthy person’s microbiome, in the form of their fecal material, and process it– not a lot, you don’t want to kill the bacteria, and just put them into the person who is ailing after you’ve given them antibiotics to kind of start with a clean slate. C difficile tend to get set up in little pockets called diverticulita. They tend to get a foothold. They’re very hard to get out of all the nooks and crannies, so you kill them with an onslaught of someone else’s bacteria, a healthy person’s community of bacteria, and it’s a very fast and effective cure– ninety something percent cure rate.

Reed Pence: Doctors say C difficile infections are becoming more common, more severe, and more difficult to treat by other means. So doctors are enthusiastic about finding a cheap and effective treatment. However, they’d like to isolate just which microbes are the ones that are helpful, so they can remove some of the “ick factor” and quit relying on a small number of donors.

Mary Roach: Nobody knows exactly which bacteria are the good guys, but it is, there are folks who seem to have a good success rate, and those donors tend to get used over and over, just because it works. Nobody knows exactly why their waste works, but it does, and so they’re sort of these unsung heroes saving peoples lives. Thirty thousand people die each year from chronic C Difficile Infection, so there’s clearly something special about these people, but nobody is able to quantify rather qualify exactly what it is.

Reed Pence: It’s unfortunate that the digestive system isn’t pretty, but we all have to interact with it every day. You can find out more about the book, Gulp, at mary Roach-dot-net. You can find out about all of our guests on our website… I’m Reed Pence.

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