Gulp: Adventures on the Alimentary Canal

by Mary Roach

Humans have taste receptor cells in the gut, the voice box, the upper esophagus, but only the tongue’s receptors report to the brain. “Which is something to be thankful for,” says Danielle Reed, Rawson’s former colleague at Monell. Otherwise you’d be tasting things like bile and pancreatic enzymes. (Intestinal taste receptors are thought to trigger hormonal responses to molecules, such as salt and sugar, and defensive reactions—vomiting, diarrhea—to dangerous bitter items.)

Rawson has an idea of what it is like to eat without perceiving tastes, because she has talked to cancer patients whose taste receptors have been destroyed by radiation treatments. The situation is well beyond unpleasant. “Your body is saying, ‘It’s not food, it’s cardboard,’ and it won’t let you swallow. No matter how much you tell your brain that you need to eat to survive, you’ll gag. These people can actually die of starvation.”

Higher-end detergents contain at least three digestive enzymes: amylase to break down starchy stains, protease for proteins, and lipase for greasy stains (not just edible fats but body oils like sebum). Laundry detergent is essentially a digestive tract in a box. Ditto dishwashing detergent: protease and lipase eat the food your dinner guests didn’t.

We are large, mobile vessels of the very substances we find most repulsive. Provided they stay within the boundaries of the self, we feel no disgust. They’re part of the whole, the thing we cherish most.

(Colds and flus aren’t spread by drinking from a sick person’s glass. They’re spread by touching it. One person’s finger leaves virus particles on the glass; the next person’s picks them up and transfers them to the respiratory tract via an eye-rub or nose-pick.)

Human teeth can detect a grain of sand or grit ten microns in diameter. A micron is 1/25,000 of an inch. If you shrank a Coke can until it was the diameter of a human hair, the letter O in the product name would be about ten microns across.

If you must spend time in a digestive organ, I recommend the penguin stomach. Penguins can shut down digestion by lowering the temperature inside their stomach to the point where the gastric juices are no longer active. The stomach becomes a kind of cooler to carry home the fish they’ve caught for their young. Penguins’ hunting grounds may be several days’ journey from the nest. Without this handy refrigerated mode, the swallowed fish would be completely digested by the time the adults get back—“like

Meaning “by way of the anus.” “Per annum,” with two n’s, means “yearly.” The correct answer to the question, “What is the birth rate per anum?” is zero (one hopes). The Internet provides many fine examples of the perils of confusing the two. The investment firm that offers “10% interest per anum” is likely to have about as many takers as the Nigerian screenwriter who describes himself as “capable of writing 6 movies per anum” or the Sri Lankan importer whose classified ad declares, “3600 metric tonnes of garlic wanted per anum.”

A study in the journal Obesity Surgery reported no significant differences in the size of the stomachs of morbidly obese people as compared with non-obese control subjects. It is hormones and metabolism, calories consumed and calories burned, that determine one’s weight, not holding capacity.

The nether distances of the gastrointestinal tract are a holding chamber for what remains of a meal once the intestine has absorbed what it can of the nutrients. Water is absorbed from the digesta as it travels along, and if all goes optimally, it leaves the body around the time it’s reached a manageable water content: somewhere between 2 on the Bristol Stool Scale* (“sausage-shaped but lumpy”) and 5 (“soft blobs with clearcut edges”). The lovely upshot is that one need only attend to the emptying once or twice a day.

Six to eight times a day, unbeknownst to your thinking, feeling self, a peristaltic muscle contraction called a mass movement squeezes the contents of the colon farther along. Eating reliably triggers this, via something called the gastrocolic reflex. The bigger the meal, the more vigorous the push. Any older detritus that had been parked outside the rectum now gets loaded inside. In with the new, out with the old. “It’s a defensive reflex,” explains William Whitehead,† co-director of the Center for Functional Gastrointestinal and Motility Disorders at the University of North Carolina. It prevents the colon from bursting. When a load pushes against the rectum walls with sufficient pressure—as measured by stretch receptors—the defecation reflex is triggered. (You can trigger it prematurely by bearing down; this raises the pressure on the rectum walls to the requisite level.) The defecation reflex causes the rectal wall muscles to contract—that is, squeeze—at the same time the anal sphincter muscles relax. To the conscious mind this registers as urgency—somewhere between “Hello” and “Drop what you’re doing.” The larger or more liquid the load, the more pressing the urge and the tougher it is to hold back. Water will leak out a very small opening. As one gut expert put it, “Not even the sphincter of Hercules can hold back water.” Take this to its end point and you have the simple saline enema—and an urgency that is not easily, if at all, overridden.

Anxiety causes a mild contraction of the muscles of the rectum walls. This reduces the receptacle’s volume, which means it takes less filling to activate the stretch receptors and confer ye olde sense of urgency.

(Even mild anxiety has this effect. Using rectal balloons and regretful volunteers, motility researcher William Whitehead found that anxious people tend to have, on average, smaller rectal volumes.)

In a more perfect world, Whitehead would be a dermatologist, just as my gastroenterologist is Dr. Terdiman, and the author of the journal article “Gastrointestinal Gas” is J. Fardy, and the headquarters of the International Academy of Proctology was Flushing, New York.

In the early days of the space program, NASA fretted about flammable astronaut flatus building up inside the tiny, hermetically sealed space capsules. A researcher presenting at the 1960s conference “Nutrition in Space and Related Waste Problems” was concerned enough to suggest that astronauts be selected from “that part of our population producing little or no methane or hydrogen.”

Flammable is a safety-conscious version of inflammable. In the 1920s, the National Fire Protection Association urged the change out of concern that people were interpreting the prefix in to mean “not”—as it does in insane. Though surely those same people must have wondered why it was necessary to warn of the presence of gas that will not burst into flame.

Meaning “clotted or lumpy.” Grumous is one of many evocative words that deserve to break free from medical dictionaries and join the ranks of day-to-day vocabulary. Likewise, glabrous (“smooth and hairless”), periblepsis (“the wild look of delirium”), and maculate (“spotted”).

Pylorus is Greek for “gatekeeper.” That’s all. As you were.

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 contribute as well, it is for the most part these three notes, in subtly shifting combinations and percentages, that create the infinite olfactory variety of human flatus. To quote Alan Kligerman, “A gas smell is as characteristic of a person as a fingerprint is.” But harder to dust for.

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 a distinctly objectionable odour resembling that of flatus.” He reverse-engineered a fart.

Some people have more of the sulfur-producing bacteria. The sulfur-spewers, by the way, prefer to colonize the descending colon, the part nearest the rectum. This is why noxious flatus tends to have heat. The composting happens right near the exit, so the flatus is, as gastroenterologist Mike Jones put it, “hot off the press.”

Making matters riskier: bed pans! “The notorious frequency of sudden and unexpected deaths of patients while using bed pans in hospitals has been commented upon for many years,” wrote the Cincinnati doctors. Notorious enough for a term to be coined: “bed pan death.”

You are what you eat, but more than that, you are how you eat. Be thankful you’re not a sea anemone, disgorging lunch through the same hole that dinner goes in. Be glad you’re not a grazer or a cud chewer, spending your life stoking the furnace. Be thankful for digestive juices and enzymes, for villi, for fire and cooking, all the miracles that have made us what we are. Khoruts gave the example of the gorilla, a fellow ape held back by the energy demands of a less streamlined gut. Like the cow, the gorilla lives by fermenting vast quantities of crude vegetation. “He’s processing leaves all day. Just sitting and chewing, and cooking inside. There’s no room for great thoughts.”

You may be thinking, Wow, that Mary Roach has her head up her ass. To which I say: Only briefly, and with the utmost respect.