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TMI Science How much do you want to know about your gut?

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It is one of the paradoxes of our culture that while food itself is an object of desire, the mechanics of eatingin the abstract, anywayreally gross us out. Chewing, salivating, and digesting, never mind excreting, are aspects of a meal we do our best to forget as we pore over photos of toast with ramp butter and quail’s eggs or slow-braised veal shank. We are in collective denial about what ingesting a meal really entails.

To a certain extent, this has always been true. Peter Greenaway’s 1989 film, The Cook, The Thief, His Wife and Her Lover, played on the fact that the comfort of middle-class life depends on keeping the pleasures of dinner well apart from its inescapable upshot in the bathroom. Thinking about digestion is a nasty appetite killer. But we are arguably more coy about the stomach than any previous generation. Our grandparents openly spoke of foods being “indigestible” or making them “bilious” if they ate too much of them. Now, we tend talk of the consequences of food only in terms of whether it will make us fat, ignoring the more pressing business of digestion. Public health officials perkily try to sell children on eating more vegetables “because of the vitamins” rather than telling them what really happens when they don’t eat their greens (around 30 percent of all American kids are constipated, going on estimates from dietetic pediatricians).

Mary Roach wants to redirect our attention away from the “clamor of cuisine” to the weird landscape of the alimentary canal. Her book is a bit like the old sci-fi classic Fantastic Voyage, where a miniaturized submarine goes on a mission through the inside of a human body. Roach has a fine line in quirky science writing, having previously looked at cadavers in Stiff and sex in Bonk. Her starting point in Gulp is cheekily to invert many of the assumptions of modern food writing. Eating, we have been loftily told by other writers, is about so much more than its chemical components. It is about culture, about tradition, about meals. Roach wittily flips this around. “Yes, men and women eat meals. But they also ingest nutrients. They grind and sculpt them into a moistened bolus that is delivered. … into a self-kneading sack of hydrochloric acid and then dumped into a tubular leach field, where it is converted into the most powerful taboo in history.”

Far from avoiding the grossness of digestion, Roach revels in it. Squeamish readers, be warned: There is TMI on every page. Roach leads us through the esophagus, the intestines, and the colon, giving us countless revolting but fascinating factoids along the way. You will discover that delicious food does not in fact make our mouths water (though nausea may) and that obese people surprisingly have the same stomach capacity as everyone else (they just eat more). Roach parses the “olfactory notes” of “noxious flatus,” the technical term for gas. If a fart smells of rotten eggs, this indicates the presence of hydrogen sulfide; if of rotten vegetables, it’s dimethyl sulfide. If you’ve ever wondered why some people complain of gassiness after beans, while others eat them with impunity, Roach has the answer. If you’ve never wondered, too bad; Roach is going to tell you anyway. Apparently, half of the population lack a certain enzyme in the colon that is needed to break down the complex carbohydrates in legumes. As a result, they are “troubled by beans.” When the colon inflates, releasing gas, it is a “warning system”: “Because stretching can be a prelude to bursting, your brain is highly motivated to let you know what’s happening down there.”

We are in collective denial about what ingesting a meal really entails.

Roach, who is relentlessly fun to readshe even sees the humorous side in eating yourself to deathgives a graphic sense of what a brilliant and complex system human digestion really is. Take saliva. It is remarkable stuff, of which we generate two or three pints every day. Apart from its healing and cleaning propertieslicking your wounds really is a good ideasaliva is part of the chemical engineering that enables us to eat. Spit dilutes acids, keeps the mouth clean and gets food into a state where we can swallow it. Roach speaks to a saliva expert, Erika Silletti, who marvels at the speed with which the brain tells the mouth to produce saliva when something is eaten. Because it contains amylase, an enzyme, saliva helps break down starchessuch as breadinto digestible energy. “Add a drop of saliva to a spoonful of custard, and within seconds it pours like water.”

It turns out that while all saliva is pretty cool, it varies in potency and quantity from person to person. Art conservators have found that saliva is a great cleaning product for fragile paintwork and gold leaf, but not just anyone’s dribble will do. When Roach does a simple saliva test using a cotton wad, she is comic-dismayed to find that she is “a dried-up husk” in saliva terms. This theme of individual variation in our digestive powers recurs throughout the book. “The way you chew,” for example, “is as unique and consistent as the way you walk or fold your shirts. There are fast chewers and slow chewers, long chewers and short chewers, right-chewed people and left-chewed people.”

Maybe the ancients were on to something when they categorized human personality in terms of bodily fluids or “humours.” Our health and happinessand consequently our behavioris largely a function of our intestinal microflora. “Depending on who’s living in your gut,” writes Roach, you “may or may not benefit from what you eat.” Variation in colonic bacteria mean that some people derive goodness from the cancer-fighting properties of polyphenolsin some fruits and vegetables as well as coffee and red winewhile others do not. More dramatically, recent researchnot mentioned in the book, perhaps because it is so newindicates that the right gut microflora can protect against malnutrition. When given identical aid rations, one child flourishes and another starves.

A new medical treatment pumps a sick person with vials of a healthy person's poop.

Gulp’s compelling final chapter looks at the new medical intervention of the “fecal transplant” to treat patients with chronic gastrointestinal problems, pioneered by Dr. Alexander Khoruts in Minnesota. This is exactly what it sounds like. The sick patient is pumped with vials of a healthy person’s poop through a colonoscope. The procedureunusual, as Roach notes, in being “effective, inexpensive and free of side effects”has proved remarkably efficacious in curing severe cases of infection with Clostridium difficile. Yet at present “no US insurance company formally recognizes the procedure.” Roach suggests that this is partly because there are no pharmaceutical profits to be made from poop; but also the “ick factor.”

Roach finds it strange that “most of us pass our lives never once laying eyes on our organs, the most precious and amazing things we own.” But is it really so odd that we prefer to think of things other than colons? Gulp eloquently lays bare the secrets of the alimentary canal, but Roach’s obsession with the lavatorial (Can constipation offer sexual pleasure? And did it kill Elvis?) often seems as if it is missing the bigger picture about the human relationship with food. She quotes Khoruts on the subject of the gorillas, whose life, like that of so many animals, is dominated by digestion: “He’s processing leaves all day. ... There’s no room for great thoughts.” The great luxury of being a human being is that we have digestive systems so complex and efficient that most of the time we don’t have to think about them. So long as you don’t bring Gulp to the table, you are free to eat your dinner in a state of blissful denial.

Bee Wilson is the author of Consider the Fork: A History of How We Cook and Eat. Follow @KitchenBee.

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