Michael Levitt did not set out to make his mark on the world by parsing the secrets of noxious flatus. His fellowship advisor had the idea. The gas chromatograph had just come into use as a laboratory tool, and no one had yet had the ingenuity — or nerve — to apply the technology to human emissions. “He called me into his office,” Levitt recalls. “He said, ‘I think you ought to study gas.’ I said, ‘Why’s that?’ He said, ‘Because you’re pretty much of an incompetent, and this way if you discover anything, at least it’ll be new, and you’ll be able to publish something.’”

Levitt published thirty-four papers on flatus. He identified the three sulfur gases responsible for flatus odor. He showed that it is mainly trapped methane gas, not dietary fiber or fat, that makes the floater float. Most memorably, to this mind anyway, he invented the flatus-trapping Mylar “pantaloon.”

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“Even now,” he says of his flatus work, “it overshadows everything else I do.” Levitt and I are sitting in a conference room upstairs from his lab at the Minneapolis VA Medical Center. Levitt has a goofy, lopsided smile and a pale complexion. I couldn’t recall, while writing this, whether his hair was gray, so I typed his identifiers into Google Images. A photograph of a can of baked beans came up.

For the record, here are some of Michael Levitt’s other contributions to medicine: He invented the breath hydrogen test, which originated not as a flatulence assessment technique but to diagnose malabsorption of carbohydrates in the small intestine. He debunked a diet fad for foods made with “nonabsorbable” carbohydrates. He showed that the wriggling movements of the villi are the key to intestinal stirring and to healthy absorption of nutrients. “I wrote the book on intestinal stirring.”

After what I judge to be a sufficient number of follow-up questions on intestinal stirring, I ask whether it might be possible to see the Mylar pantaloons.

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Levitt designed the garment for a pair of studies that aimed both to identify the gases responsible for noxious flatus and to test devices claiming to adsorb — the formal term for binding to something’s surface — those gases. He doesn’t know where they are stored, but digs out a photograph of a woman standing in the lab, modeling them. Shown uninflated, they fit more snugly than I’d pictured them. The material is silver, crinkly, and reflective. They’re the sort of clothes baked potatoes wear.

I ask Levitt whether it was difficult to recruit volunteers for the flatus studies. It wasn’t, partly because the subjects were paid for their contributions. People who sell their flatus are more or less the same crowd who turn up to sell their blood.

“What was hard,” Levitt says, “was finding the judges.” Levitt needed a pair of odor judges to take “several sniffs” and rate the noxiousness — from “no odor” to “very offensive” — of each of the sixteen people’s flatal contributions. The hypothesis was that noxiousness would correlate with the combined concentrations of the three sulfur gases. And it did.

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Curious as to which olfactory notes the different sulfur gases contributed to the overall bouquet of flatus, Levitt purchased samples of the three gases from a chemical supply house. 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 like methylmercaptan 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.

The great variety of flatus smells — from person to person and from meal to meal — presented a quandary for the second phase of the study, the evaluation of various odor-eliminating products. Which — whose — wind should represent the average American’s? No one’s, as it turned out. 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. This “artificial flatus” was put to work testing a variety of activated charcoal products: underwear, adhesive-backed underwear pads, and chair cushions. (Activated charcoal is known to be effective at binding sulfur gases. The circulating air supply in NASA spacesuits is filtered with activated charcoal, lest astronauts’ flatus be blown across their face three times a minute for the remainder of the spacewalk.)

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In a separate study to simulate real-life gas-passing conditions, Levitt taped a tube beside the subject’s anus, beneath the charcoal pad or underpant and the subject’s pants. (Cushions were strapped in place.) The subject then pulled the Mylar pantaloons over whatever product was being tested, and an assistant duct-taped the cuffs and waistband to the skin. Levitt hit a switch, and just under a half cup (100 milliliters) of synthesized flatus shot through the tube for two seconds — Levitt’s best guess for the size and life span of a typical fart. “Immediately following gas instillation,” wrote Levitt in the final paper, “air inside the pantaloons was constantly mixed via vigorous palpation over a 30-second period.” Levitt claims to have no video footage. Last, a syringe was fitted into a port in the Mylar to withdraw the gas, and Levitt measured the sulfur gases the charcoal had failed to trap.

The challenge, it turned out, lies in bringing the gas fully into contact with the charcoal — easy with an airtight spacesuit, less so a business suit. Seat cushions were relatively useless, most products trapping a scant 20 percent of the sulfur gases. The underwear pads delivered a 55 to 77 percent reduction, their efficacy compromised by “rectal gas blow-by”: the tendency of the wind to glance off the pad and out the sides rather than penetrate it. The seventy-dollar briefs performed best, adsorbing virtually all sulfur gases, though it was unclear how many wearings they were good for. And given the cost, in terms of both cash and self-esteem, they would seem to have a limited market.

*

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As an alternative to wearing activated charcoal or gluing it to your underpants, you could swallow some pills. But don’t bother, because Levitt has done a study on this too. Activated charcoal pills did not “appreciably influence the liberation of fecal gases.” Levitt surmised that the binding sites were saturated by the time the charcoal made it to the rectum.

Bismuth pills, on the other hand — and Levitt has tested these, too — reduce 100 percent of sulfur gas odor. Bismuth is the "bism" in Pepto-Bismol. Daily doses of Pepto-Bismol can irritate the gut, but not bismuth subgallate, the active ingredient in Devrom “internal deodorant” pills.

I had never before heard of Devrom. This may be because mainstream magazines often refuse to run the company’s ads. Devrom’s president, Jason Mihalopoulos, e-mailed me a full-page ad he had hoped to run in Reader’s Digest and AARP magazine. A smiling gray-haired couple stand arm in arm below the boldface headline “Smelly Flatulence? Not since we started using Devrom!” Mihalopoulos was told he could not use the phrases "smelly flatulence" and "stinky odor," or the word "stool." One of the magazines suggested changing the copy to say that the product “eliminates intestinal gas,” but that’s not what Devrom does. That’s what Beano does. So unless you read the Journal of Wound Ostomy & Continence Nursing or the International Journal of Obesity Surgery, you won’t see the happy, internally deodorized Devrom couple.

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The noxious-rectal-gas taboo in mainstream advertising has proved stronger and more lasting than that of condoms and even vibrators, which now turn up in brazenly suggestive ads on cable television (though still under the century-old euphemism “massager”). Mihalopoulos told me the editors of a CNBC feature on quirky businesses refused to air a segment about Parthenon, the family-run business that makes Devrom. “People don’t like to hear flatulence,” he said, quickly adding that he meant the word. Or anyway, people think people don’t.

Given the obvious strength of the taboo, I wondered who had posed for the Devrom ad. How much do you have to pay people to appear in a full-page ad in a national magazine, talking about their smelly flatulence?

“Oh, I’d be shocked if someone would be willing to pose in an ad we’d run,” Mihalopoulos said. “It’s a stock photo.” Meaning anyone, for a fee, can run the image for whatever purpose they choose. The couple probably have no idea. Think twice before you sign a model-release form.

Most Devrom customers are people with extenuating digestive circumstances. They’ve had their stomachs stapled or bypassed to shed weight, or they’ve had all or most of a diseased gut removed and they’re excreting into an ostomy pouch. Mihalopoulos explained that, depending on how high up the opening is, the pouch may need to be emptied every few hours. Less time in the colon means less water is absorbed. The runnier the waste, the more surface area is exposed to the air and the more volatiles escape to reach the nose. “If you were to use the restroom at the airport, say ...” Mihalopoulos paused to figure out where he was going with this. “You could tell right off that someone was emptying their pouch.”

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It seemed, then, that we were not even talking about passing gas. “No, that too,” said Mihalopoulos. He explained that some people with an ostomy pouch will open a corner of it to let a little gas out. “It’s like Tupperware.”

Mihalopoulos didn’t have data he could share regarding the number of people who were taking Devrom to defang garden-variety flatus odor, rather than because of a medical situation. I’m guessing there aren’t very many of them, and I think I know why. I think I know what’s keeping internal deodorant from charging ahead as a mainstream product. I’m going to let Beano inventor Alan Kligerman tell you what it is. “When I talk to people,” he told me, “when I really get them down to the nitty-gritty, I don’t know anybody, really, in their heart of hearts, who has any objection to the smell of their own.” And, unlike bad breath or stinky feet, “smelly flatulence” is everyone’s problem. And thus really, no one’s.

As with the first bottle of Scope, the first bottle of Devrom, Mihalopoulos confirmed, is often left anonymously by a coworker or purchased by a spouse. “They themselves don’t object to it,” he said, “it” referring to the smell, not the purchase. Levitt said he is constantly approached at cocktail parties by women complaining about their husbands’ gas. He has never once heard a husband complain about a wife, despite this scientifically proven (by Levitt) fact: “the flatus of women has a significantly greater concentration of hydrogen sulfide and was deemed to have a significantly worse odour by both judges.” (However, this is likely balanced out by the male’s “greater volume of gas per passage.”)

The Devrom company is to be commended for not aggressively pushing internal deodorant on the public at large. Good for you, Jason Mihalopoulos, for not following in the springtime fresh footsteps of douche marketers and, most recently, the Fleet enema company. “Keep your backcountry clean,” says the Fleet Naturals ad copy, over an image of pristine mountain wilderness. “Created specifically for rectal cleansing ... Mild enough for daily use.” Really? On top of gargling, on top of powdering our feet and perfuming our armpits, now we should worry that our assholes smell?

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I later stumbled upon a “Tell Your Patients ...” press release that Fleet had sent out to physicians. (One of them had posted it on his blog.) It turns out that Fleet Naturals is a product “for before or after anal intimacy.” Well OK then.

The simplest strategy for bouts of noxious flatus is to not care. Or perhaps to take the advice of a gastroenterologist I know: get a dog. (To blame.) Barring that, a person might try to steer clear of certain foods, the ones that provide bacteria with the raw materials for making sulfur compounds. The main offender is red meat. Cruciferous vegetables (broccoli, cabbage, brussels sprouts, cauliflower) can also kick up a stink. As can garlic, dried and sulfured fruit (for example, apricots), certain aromatic spices, and, for reasons unclear, beer. In short, so many delightful things that a sane person would, I like to think, rather have the gas.

*

I traveled to Minnesota with a fantasy that Michael Levitt might be able to whip up a batch of artificial flatus. I’m curious to see how close Science can get to Nature. Levitt smiles one of those placeholder smiles that buy you a moment to phrase your no. He elects to fob me off on his research partner Julie Furne, who has the ingredients downstairs in the lab. I recognize Furne’s name from the pantaloon studies. It turns out she had been one of the odor judges.

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Things haven’t changed all that much for Julie Furne. We find her in the lab, syringing gas out of a plastic vial in which a raisin-sized rat turd has been incubating at ninety-nine degrees. (She and Levitt are investigating the relationship between intestinal hydrogen sulfide and colitis. More on this shortly.)

Furne recently arrived at her fifth decade, her brown hair beginning to silver at the hairline but a girlish humor still intact. Instead of a lab coat, she wears a muted orange heather cardigan, vintage from the fifties, I’m guessing. There was probably a time when you could have pressed this sweater to your face and smelled traces of hair spray or homemade pot roast. Probably you wouldn’t have that experience now.

“This is Mary,” says Levitt. “She’d like to sniff some gases. But don’t kill her.”

Hydrogen sulfide is as lethal, molecule for molecule, as cyanide. This may explain why humans evolved such exquisite sensitivity to its smell. Repellent odors are unpleasant but often helpful in terms of not dying. As with any poison, dosage makes the killer. The concentration of hydrogen sulfide in offensive human flatus is around 1 to 3 parts per million. Harmless. Ramp it up to 1,000 parts per million — as can exist in manure pits and sewage tanks — and a few breaths can cause respiratory paralysis and suffocation. Workers die this way often enough that a pair of physicians, writing in a medical journal, coined a name for it: dung lung. Hydrogen sulfide is so swiftly lethal that farm- and workplace-safety organizations urge anyone who enters a manure pit or attempts to clear a blocked sewage pipe to wear a self-contained breathing apparatus. Which may explain the man my husband and I once saw walking along a sidewalk in San Francisco in a wet suit with a toilet plunger over his shoulder. “Hell of a clog,” Ed said.

It is fitting that the Devil is said to smell of sulfur. Hydrogen sulfide is a diabolical killer. Its telltale rotten-egg smell, screamingly obvious at 10 parts per million, disappears at concentrations above 150 parts per million; the olfactory nerves become paralyzed. Without the odor to warn them, coworkers and family members may rush into a manure pit to rescue the fallen. Whole families have been taken out in a catastrophic “chain of death.” One case report included a police photo taken after the victims had been pulled from the mire and laid on the ground. It was a wrenching play on the family portrait, the four adults arranged in a row in matching knee-high muck boots, black bars over their eyes. The farmer had gone in to unclog a pipe. Both he and the worker who tried to rescue him collapsed and died. The farmer’s mother found the two, hurried down the ladder and also succumbed. Then the son came along. And on it went, all the way to a team of pathologists nearly overcome in a poorly ventilated autopsy room.

Hydrogen sulfide is a reliable way to kill oneself — as well as the people who try to save you. In 80 percent of the hydrogen sulfide suicides in this country, emergency personnel or good Samaritans have been sickened by the fumes when they tried to help. One suicide in Japan caused the evacuation of 350 neighbors.

“Ask Julie how she felt,” says Levitt over his shoulder as he leaves. The day of her training as a flatus odor judge, Furne worried that she’d poisoned herself. She was “sick as a dog” and had a headache all that evening. Vegetarian activist John Harvey Kellogg wrote that he had “known vigorous young men” who suffered “violent attacks” of headache from working in a lab with “the bowel discharges of a meat-eater.”

The hydrogen sulfide pulled from the tube that held the fermenting feces of rat E2 clocks in at a concentration of 1,000 parts per million. “You don’t want to smell that straight up,” says Furne. She glances off to the side, reading out an imaginary headline: “AUTHOR KILLED BY FECAL ODOR.” Furne has a homey northern Midwest accent, the voice of "Fargo’s" Margie, diluted to a nonlethal concentration.

But this is hydrogen sulfide off-gassing in a vial smaller than a lipstick. Are there circumstances in which ordinary concentrations can harm you? Are flatulent people a public health risk? The author of "Inner Hygiene," James Whorton, quoted a nineteenth-century physician who thought so. He admonished the flatulent to hold in their gas for the sake of family and friends, saying, “It is as much a crime to poison a neighbor with gas, as with a poison more tangible.” I wondered if there might be a grain of truth to this — inside a confined space, say. When it’s cold, I tell Furne, I sometimes sleep with my head under the covers. Winter is brussels-sprout season, and they’re Ed’s favorite side dish.

Furne assures me there’s enough air under a comforter to dilute a spouse’s hydrogen sulfide and render it harmless. When I followed up with Levitt by e-mail, he concurred that the “passive inhaler” has no cause to worry.

Especially compared with the perpetrator. He who dealt it incurs a “relatively enormous exposure to hydrogen sulfide via absorption through the colonic mucosa.” Or as John Harvey Kellogg rather more excitably put it, “If the mere breathing of the greatly diluted volatile poisons arising from such putrescent matter will produce highly unpleasant effects, how much more grave must be the effects when through the retention within the body ... all of their poisonous contents are absorbed and sucked up into the blood and circulated throughout the body?” Levitt had been quick to add that no research has shown that absorption — of hydrogen sulfide or any other colon-residing breakdown component — into the bloodstream is harmful.

In matters of health, however, the public rarely requires proof. Most people trust intuition more than they trust studies. And the theory behind fecal self-poisoning — aka autointoxication — makes strong intuitive sense. “[People] reasoned that if feces are foul, then the body must be in the best condition when freest from such material,” wrote Walter Alvarez in his wise and tide-turning 1919 essay in the Journal of the American Medical Association. The less time “feculent” poisons reside in our colons, the thinking went, the less we absorb into our blood, and the healthier we’ll be. Autointoxication was one of the most pervasive and enduring concepts in the long, bloated history of medical pseudoscience.

*

As a diagnosis, a health buzzword, autointoxication peaked in the early 1900s. It was a natural offshoot of “miasma” theory. From the early to the late 1800s, before physicians figured out the role of microorganisms and insects in causing and spreading disease, much of the blame was placed on clouds of nonspecific toxic gases — or miasmas — emanating from open sewer flows, garbage dumps, even graves.

If one bought into the dangers of miasmas, it wasn’t much of a leap to buy into the dangers of one’s own internal sewage. Purveyors of laxatives and enema devices played up the connection, referring to the colon as “the human privy,” “an obstructed sewer,” “this cesspool of death and contagion.” Whorton’s book reproduces a magazine ad for the French laxative Jubol, showing tiny uniformed men on their hands and knees with scrub brushes and buckets inside a colon, like workers in the Paris sewers.

It made no difference that neither the specific poisons nor the mechanisms by which they might be causing harm were known or named. In the realm of quackery, vague is better. “It met a need,” wrote Whorton, “that medicine has felt in every age, providing an explanation and diagnosis for all those exasperating patients who insist they are sick, but are unable to present the physician with any clear organic pathology to prove it.” Autointoxication was the gluten of the early 1900s.

Bogus diagnoses beget bogus cures. Around the turn of the last century, hosing the colon was big business, far bigger than it is today, and nowhere bigger than at 134 West Sixty-Fifth Street, home of Tyrrell’s Hygienic Institute, a three-story New York brownstone dedicated to the manufacture and flatulent hyping of the J.B.L. Cascade colonic irrigator. J.B.L. stood for “Joy Beauty Life,” suggesting that your $12.30 was purchasing something loftier than a nozzle-topped whoopee cushion.

“The Internal Bath is taken by sitting on the J.B.L. Cascade,” states Charles Tyrrell in the 1936 promotional pamphlet "Why We Should Bathe Internally." Tyrrell’s prior business had been in rubber medical goods. Aside from the rectal nozzle protruding from its flank, the Cascade looked little different from one of Tyrrell’s old water bottles.

Between businesses, Tyrrell had dabbled in small-press publishing. The experience served him well. He printed up thousands of thinly disguised promotional booklets that he distributed to pharmacists to hand out to patients. The gospel of autointoxication and internal putrescence was laid on thick and spiked with testimonials: from customers, doctors, clergy, all wordily professing their satisfaction and gratitude. Gone was their insomnia, their fatigue, their melancholia. Here was the fix for acne, for bad breath, for lack of appetite and “loss of vim and snap.” An internal bath would rid you of irritability, “outrageous cantankerousness,” “the inability to hold down a job of lumber grading for over six months without quitting or getting fired.” One set of before and after photos seemed to imply that a high colonic could transform an unkempt, drooping moustache into a vigorous, curlicued handlebar.

It seemed there was no medical condition so dire that an internal bath would not fix it. Mr. H. J. Wells of 342 Lincoln Avenue, Detroit, credited the Cascade with relieving his wife of “an accumulation of effete mucous tissue ... in strips about half an inch wide and from four to six inches long.” Mrs. Cora Ewing of Long Beach, California, waved good-bye to “a sack of pus above the left ovary.” People thanked Tyrrell for curing their asthma, their rheumatism, their typhoid fever, and their jaundice. Paralysis even! Epilepsy! The medical claims were sufficiently far-fetched that Tyrrell felt a need to point out that the “disorders may be due to factors other than ... autointoxication.”

The American Medical Association’s Bureau of Investigation received so many letters from outraged physicians that it drafted a form letter to send in response. “We plan to get around to this institution after a while,” it promised. The first such letter in the Tyrrell Hygienic Institute file at the AMA archives is dated 1894, and the last, 1931, suggesting that a little more vim and snap might have been applied.

One member rose on his own to the task. In 1922, physician and autointoxication doubter Arthur Donaldson artificially and incontrovertibly constipated three dogs by temporarily sewing shut their anus. After four days, all the while eating regular meals of meat, milk, and bread, the dogs showed no physical symptoms beyond a mild loss of appetite — nothing to suggest a poisoning from within. All three, impressively, “seemed to be in fair spirits.”

Donaldson didn’t rest his case there. He withdrew small amounts of blood from his surgically constipated charges, once at the end of fifty-five hours, again at seventy-two hours, and finally at ninety-six hours. This he injected into the bloodstream of two normal, unconstipated dogs to see whether symptoms suggestive of “fecal poisoning” would develop. They did not.

Donaldson contended that the symptoms people and doctors were so quick to blame on autointoxication were in fact caused by the simple mechanics of constipation: rectal distention and irritation. To test the theory, he packed four men with turd-sized wads of cotton. After three hours, the men began to exhibit the sorts of symptoms commonly blamed on autointoxication. The moment the wads were removed, relief ensued. If fecal blood poisoning had been the culprit, relief would have taken far longer. It takes several hours for the liver and kidneys to clear chemicals from the system. The reek of asparagus pee, Walter Alvarez pointed out, though not in those exact words, doesn’t abate the moment you set down your fork. It lingers through the following morning. The very swiftness with which the enema brings relief itself refutes the premise of autointoxication.

In the incomparable phrasings of gastroenterologist Mike Jones, “Everybody who’s bound up feels a whole lot better after a big dump. From where I sit, you don’t need to invoke anything else.”

*

The alternative approach to ridding the body of “faeculent poisons” was to eat so much fiber that digesta sped through the colon too quickly to generate them. Insoluble dietary fiber, or roughage, is the indigestible, nonfermentable parts of plants — internal yard trimmings that the gut cannot break down. This fiber sponges up water, contributing dramatically to fecal “bulk.” The bulkier the trash, the sooner you need to empty the bin.

John Harvey Kellogg was the archbishop of roughage. The healthy colon, he maintained, empties itself three or four times a day. This was “Nature’s Plan.” As evidence, he cited the estimable bowel frequency of “wild animals, wild men, ... infants and idiots.” Kellogg’s sources included the staff at “well-managed idiot asylums” and ape keepers at the London Zoo. Kellogg paid several visits to the latter “for the express purpose” of discussing the toilet habits of their charges. The chimpanzees, noted Kellogg, “move their bowels four to six times daily.” All the more to throw at zoo visitors. Kellogg effected a habit of dressing in immaculate white suits, but probably not on the second and third visits.

Kellogg didn’t gather data on the regularity of “wild men,” but someone else sure did. In the early 1970s, epidemiologist A. R. P. Walker held a post at the South African Institute for Medical Research, affording easy access to Bantu people and others “pursuing a primitive manner of life.” In his travels through South African villages, Walker noted that “unformed stools are frequently encountered among rural Bantu.” One man’s ruined footwear is another’s "eureka!" moment. The Bantu, Walker knew, were almost never diagnosed with Western digestive diseases. Was it because they ate so much fiber? Did their woody digesta exit the colon too quickly to inflict harm?

Walker got busy clocking stool: British versus Bantu. Subjects swallowed radio-opaque pellets and then “voided” into plastic bags that they labeled with the date and time. The bags were X-rayed so researchers could see exactly how long it took the pellets to complete their journey. As with foot races, so with digestion: the slowest third of the Bantu were quicker than the fastest third of the Caucasians. This was because, Walker assumed, the Bantu ate a shitload of insoluble fiber in the form of millet and corn porridge.

Walker was the man behind bran. Papers published by him and, more recently, his research partner Denis Burkitt, fueled a decade-long fiber craze. Americans were forcing down unprecedented amounts of bran muffins, oatmeal, and high-fiber breakfast cereals. Whorton cited a 1984 survey that found a third of Americans eating more fiber to stay healthy.

You don’t hear so much about fiber these days. Curious, I ran a PubMed search on cancer and dietary fiber. The most recent study, published in the American Journal of Epidemiology in 2010, followed three thousand Dutch men for thirteen years. Get a load: “Frequent bowel movements were associated with an increased risk of rectal cancer in men, and constipation was associated with a decreased risk.” Mike Jones wasn’t surprised. The medical community was never completely on board Burkitt’s fiber train. “He was comparing the Bantu to, like, British naval recruits, guys who ate practically no fiber and they all smoked.” Many other factors also set the British apart from rural black Africans — how do you control for them all? “It was correlation, not causality, and you really couldn’t take it any further.”

So why did we hear so much about fiber back then? Because, Jones said, there was money to be made: “things to go out and buy and eat more of.” Walker and Burkitt wrote the tune, but it was the cereal companies that kept on playing it. Jones said that when he sat down and looked at the studies on dietary factors and colon cancer, the thing that stood out as a determinant of risk wasn’t how much fiber you ate, but how many calories. The fewer calories, the lower the risk. No easy profits there.

*

And get this. The newest research suggests that slower transit time — that is, longer exposure to your nasty stuff, may in fact be of benefit. Hydrogen sulfide appears to prevent inflammation and its sometime consequences, ulcerative colitis and cancer. In rodent studies, anyway, the gas has a significant anti-inflammatory effect on the walls of the digestive tract: the opposite of what aspirin does in there. Aspirin and ibuprofen combat inflammation everywhere but the stomach and bowel; there they create inflammation. Used in tandem with hydrogen sulfide, says Ken Olson, a professor of physiology at Indiana University School of Medicine and author of multiple papers on the topic, aspirin or ibuprofen may be thousands of times as potent at preventing tumor growth — at least, in mice and in laboratory-grown tumor cells. Human trials have not yet begun.

Hydrogen sulfide is not the devil. Beneath the danger and stench is a molecule as basic and indispensable as sodium chloride. The gas is produced in all of the body’s tissues, all the time, regardless of what was for dinner. (Some recent thinking disagrees.) “It’s a gasotransmitter, a signaling molecule, it has tremendous therapeutic potential,” says Olson. “This is the hottest area in biomedicine right now.”

The moral of the story is this: It takes an ill-advised mix of ignorance, arrogance, and profit motive to dismiss the wisdom of the human body in favor of some random notion you’ve hatched or heard and branded as true. By wisdom I mean the collective improvements of millions of years of evolution. The mind objects strongly to shit, but the body has no idea what we’re on about.

*

Here’s the other hitch with autointoxication. Absorbing things is primarily the business of the small intestine, not the colon. That’s what the smaller tube, with its millions of villi, is for: Delivering nutrients to the blood. The autointoxication zealots would counter that, as John Harvey Kellogg put it, “the foul fecal matters in the colon pass back into the small intestine.” But, in fact, they don’t. The ileocecal valve, the anatomical portal between the small intestine and the colon, opens in one direction only.

It is possible to force open the ileocecal valve from the wrong direction, but it does not happen naturally, in the course of day-to-day living. It has tended to happen unnaturally, while dead, on a slab in a nineteenth-century anatomy amphitheater with one end of a flexible tube disappearing up the rectum and the other attached to a pump. No less than five experimenters, representing Britain, France, Germany, and the United States, from 1878 to 1885, tested the competence of the ileocecal valve. “Heschl made a number of experiments on the cadaver and satisfied himself that the ileocecal valve serves as a safe and perfect barrier against the entrance of fluids from below,” wrote the author of one review. W. W. Dawson of the Medical College of Ohio put the ileocecal through its paces on thirteen cadavers; in twelve, the valve held strong. The transcript of the thirteenth cadaver demonstration is printed in an 1885 issue of the Cincinnati Lancet and Clinic. (“From your seats, ... you see the colon expanding as the fluid enters.”) This one, he concludes, was an anomaly. “The valve was doubtless imperfect.” But the showmanship flawless.

It seems fair to say that it takes an unnatural volume of liquid, under unnatural pressure, to breach the heroic ileocecal valve and enter the small intestine from the rear. It takes, perhaps, a Joy-Beauty-Life colonic irrigator. In their fervor to rid the body of fecal residues, devotees of internal bathing were flushing the dread residues higher up into the gut, away from the colon — a region of the anatomy that does relatively little absorbing — and right on into the one that evolved specifically for the job, the small intestine.

You may be wondering why the minds of medicine so assiduously concerned themselves with the matter. Were they drawn to it simply as lecture hall spectacle? Not entirely. The experiments aimed to resolve a lingering medical debate over the value of “feeding per rectum.”

Reprinted from "Gulp: Adventures on the Alimentary Canal" by Mary Roach. Copyright © 2013 by Mary Roach. With permission of the publisher, W.W. Norton & Company, Inc.