The most prudent option, it seems, is to have a balanced and moderate diet. A sensible approach is, in short, the sensible approach.

  *1 There is a surprising lack of consensus on who actually invented the calorie with respect to diet. Some food historians say Nicolas Clément of France came up with the concept as far back as 1819. Others say it was a German, Julius von Mayer, in 1848, and still others credit two Frenchmen working together, P. A. Favre and J. T. Silbermann, in 1852. What is certain is that it was all the rage among European nutritionists by the 1860s, when Atwater first encountered it.

  *2 The eight are isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, threonine, and valine. The bacterium E. coli is unusual among living things in its ability to utilize a twenty-first amino acid, called selenocysteine.

  15 THE GUTS

  Happiness is a good bank account, a good cook and a good digestion.

  —JEAN-JACQUES ROUSSEAU

  INSIDE, YOU ARE enormous. Your alimentary canal is about forty feet long if you are an average-sized man, a bit less if you are a woman. The surface area of all that tubing is about half an acre.

  Bowel transit time, as it is known in the trade, is a very personal thing and varies widely between individuals, and in fact within individuals depending on how active they are on a given day and what and how much they have been eating. Men and women evince a surprising amount of difference in this regard. For a man, the average journey time from mouth to anus is fifty-five hours. For a woman, typically, it is more like seventy-two. Food lingers inside a woman for nearly a full day longer, with what consequences, if any, we do not know.

  Roughly speaking, however, each meal you eat spends about four to six hours in the stomach, a further six to eight hours in the small intestine, where all that is nutritious (or fattening) is stripped away and dispatched to the rest of the body to be used or, alas, stored, and up to three days in the colon, which is essentially a large fermentation tank where billions and billions of bacteria pick over whatever the rest of the intestines couldn’t manage—fiber mostly. That’s why you are constantly told to eat more fiber: because it keeps your gut microbes happy and at the same time, for reasons not well understood, reduces the risk of heart disease, diabetes, bowel cancer, and indeed death of all types.

  Nearly everyone equates the location of the stomach with the belly, but in fact it is much higher up and markedly off center to the left. It is about ten inches long and shaped like a boxing glove. The wrist end, where the food enters, is called the pylorus, and the fist part is the fundus. The stomach is less vital than you might think. We give it way too much credit in popular consciousness. It contributes a bit to digestion both chemically and physically, by squeezing its contents with muscular contractions and bathing them in hydrochloric acid, but its contribution to digestion is helpful rather than vital. Many people have had their stomachs removed without serious consequence. The real digestion and absorption—the feeding of the body—happens further down.

  The stomach holds about one and a half quarts, which is not very much compared with other animals. The stomach of a big dog will hold up to twice as much food as yours does. When food reaches the consistency of pea soup, it is known as chyme (pronounced “kime”). The rumblings of your gut, incidentally, come mostly from the large intestine, not the stomach. The technical term for gut rumblings is “borborygmi.”

  One thing the stomach does do is kill off many microbes, by soaking them in hydrochloric acid. “Without your stomach, a lot more of what you ate would make you ill,” Katie Rollins, a general surgeon and lecturer at the University of Nottingham, told me one day in the dissecting room there.

  It is a miracle that any microbes get through, but some do, as we all know to our cost. Part of the problem is that we bombard ourselves with a lot of tainted stuff. An investigation by the Food and Drug Administration in 2016 found that 84 percent of chicken breasts, nearly 70 percent of ground beef, and getting on for half of pork chops contained intestinal E. coli, which is not good news for anything but the coli.*1

  Foodborne illness is America’s secret epidemic. Every year three thousand people, the equivalent of a small town, die of food poisoning in the United States, and around 130,000 are hospitalized. It can be a decidedly horrible way to die. In December 1992, Lauren Beth Rudolph had a cheeseburger at a Jack in the Box restaurant in Carlsbad, California. Five days later, she was taken to the hospital suffering excruciating abdominal cramps and bloody diarrhea, and her condition was rapidly deteriorating. In the hospital, she suffered three massive cardiac arrests and died. She was six years old.

  Over the next few weeks, seven hundred customers who had visited seventy-three Jack in the Box restaurants in four states grew ill. Three of them died. Others suffered permanent organ failure. The source was E. coli in undercooked meat. According to Food Safety News, the Jack in the Box company knew that its hamburgers were being undercooked “but had decided that cooking them to the required 155 degrees made them too tough.”

  * * *

  —

  Equally pernicious is salmonella, which has been called “the most ubiquitous pathogen in nature.” According to a USDA study, about a quarter of all chicken pieces sold in stores are contaminated with salmonella. Some 40,000 cases of salmonella infection are reported in the United States each year, but the real number is thought to be much higher. By one estimate, for every reported case a further 28 go unreported. That works out to 1,120,000 cases a year. There is no treatment for salmonella poisoning.

  Salmonella has nothing to do with spawning fish. It is named for Daniel Elmer Salmon, a U.S. Department of Agriculture scientist, though it was actually discovered by his assistant Theobald Smith, yet another of those forgotten heroes of medical history. Smith, born in 1859, was the son of German immigrants (the family name was Schmitt) in upstate New York and grew up speaking German, so was able to follow and appreciate the experiments of Robert Koch more quickly than most of his American contemporaries. He taught himself Koch’s methods for culturing bacteria and was thus able to isolate salmonella in 1885, long before any other American could do so. Daniel Salmon was head of the Bureau of Animal Husbandry at the U.S. Department of Agriculture and was primarily an administrator, but the convention of the day was to list the bureau head as lead author on the department’s papers, and that was the name that got attached to the microbe. Smith was also robbed of credit for the discovery of the infectious protozoa Babesia, which is wrongly named for a Romanian bacteriologist, Victor Babeş. In a long and distinguished career, Smith also did important work on yellow fever, diphtheria, African sleeping sickness, and fecal contamination of drinking water, and showed that tuberculosis in humans and in livestock was caused by different microorganisms, proving Koch wrong on two vital points. Koch also believed that TB could not jump from animals to humans, and Smith showed that that was wrong, too. It was thanks to this discovery that pasteurization of milk became a standard practice. Smith was, in short, the most important American bacteriologist during what was the golden age of bacteriology and yet is almost completely forgotten now.

  Responsibility for food safety is split among a raft of federal agencies in America in a way that rather defies logic. The Food and Drug Administration is responsible for the skin of sausages, but the Food Safety and Inspection Service is responsible for what goes inside them. Cheese pizzas are looked after by one agency, but meat pizzas by another. And so it goes through a whole range of foodstuffs. Altogether fifteen agencies have a regulatory role in some aspect or other of American food safety. No one agency has overall control.

  Incidentally, most nausea-inducing microbes need time to proliferate inside you before they make you sick. A few, like Staphylococcus aureus, can make you ill in as little as an hour, but most take at least twenty-four hours. As Dr. Deborah Fisher of Duke University told The New York Times, “People tend to blame the last thing they ate, but it’s probably the thing before the last thing they ate.” Actually, many infestations take far longer than that to manifest. Listeriosis, which kills about three hundred people a year in America, can take up to seventy days to show symptoms, which makes tracking down a source of infection a nightmare. In 2011, thirty-three people died of listeriosis before the source—cantaloupe from Colorado—was identified.

  The largest source of foodborne illness is not meat or eggs or mayonnaise, as commonly supposed, but green leafy vegetables. They account for one in five of all food illnesses.

  * * *

  —

  For a very long time, nearly everything we knew about the stomach was thanks to an unfortunate accident in 1822. In the summer of that year, on Mackinac Island in Lake Huron in upper Michigan, a customer was examining a rifle in the island general store when it suddenly went off. A young Canadian fur trapper named Alexis St. Martin had the misfortune to be standing just three feet away and directly in the line of fire.*2 The shot tore a hole in his chest just below the left breast and gave him something he really didn’t want: the most famous stomach in medical history. St. Martin miraculously survived, but the wound never entirely healed. His doctor, a U.S. Army surgeon named William Beaumont, realized that the inch-wide hole gave him an unusual window into the trapper’s interior and direct access to his stomach. He took St. Martin into his home and looked after him, but with the understanding (sealed with a formal contract) that Beaumont would be free to perform experiments on his guest. For Beaumont, this was a peerless opportunity. In 1822, no one knew quite what happened to food once it disappeared down one’s throat. St. Martin had the only stomach on Earth that could be studied directly.

  Beaumont’s experiments principally consisted in suspending different foods on lengths of silken thread into St. Martin’s stomach, leaving them for a measured interval, then pulling them out to see what had happened. Sometimes, in the interests of science, he tasted the contents to judge their tartness and acidity, and by so doing deduced that the principal digestive agent of the stomach is hydrochloric acid. This was a breakthrough that caused great excitement in gastric circles and made Beaumont famous.

  St. Martin was not the most cooperative of subjects. Often he disappeared, once for four years before Beaumont was able to track him down. Despite these interruptions, Beaumont eventually published a landmark book, Experiments and Observations on the Gastric Juice and the Physiology of Digestion. For about a century, almost all medical knowledge of the process of digestion was thanks to St. Martin’s stomach.

  Ironically, St. Martin outlived Beaumont by twenty-seven years. After drifting around for a few years, he returned to his hometown of St. Thomas, Quebec, married, raised a family of six children, and died aged eighty-six in 1880, nearly sixty years after the accident that made him famous.

  * * *

  —

  The heart of the digestive tract is the small intestine, twenty-five feet or so of coiled tubing where most of the body’s digestion takes place. The small intestine is traditionally divided into three sections: the duodenum (meaning “twelve,” because that is how many finger widths of space it was deemed to take up in the average man in ancient Rome); the jejunum (meaning “without food” because the jejunum was often found to be empty in corpses); and the ileum (meaning “groin” on account of its proximity to same). In fact, however, the divisions are entirely notional. If you took your intestines out and laid them on the ground, you wouldn’t be able to tell where one part began and the other ended.

  The small intestine is lined with tiny hairlike projections called villi, which add enormously to its surface area. Food is passed along by a process of contraction known as peristalsis—a kind of stadium wave for the gut. It advances at the rate of about one inch per minute. A natural question is, Why don’t all our ferocious digestive juices eat through our own gut lining? The answer is that the alimentary canal is lined with a single layer of protective cells called the epithelium. These vigilant cells, and the gooey mucus they produce, are all that stand between you and digesting your own flesh. If that tissue is breached and the gut contents get into another part of the body, death without immediate medical treatment inevitably follows. Yet that only rarely happens. So battered is this front line of cells that each is replaced after just three or four days, just about the highest turnover rate for the whole body.

  Wrapped around the outside of it, like a wall around a garden, is six feet of broader-gauge plumbing known variously as the large intestine, bowel, or colon. Where the small and large intestines join (just above the belt line on the right-hand side of your body), there is a pouch called the cecum, which is important in herbivores but of no particular consequence in humans, and jutting off the cecum is the fingerlike protrusion known as the appendix, which has no certain purpose but kills about 80,000 people around the world every year when it ruptures or grows infected. In the United States, nearly 400,000 people are hospitalized with appendicitis annually and about 300 die, according to the U.S. National Library of Medicine.

  The appendix is strictly the vermiform appendix, in recognition of its wormlike shape. For a long time, all that could be said about the appendix was that you could remove it and not miss it, which strongly suggested that it had no purpose at all. Now the best thinking is that the appendix serves as a reservoir for gut bacteria. About one person in every sixteen in the developed world will suffer appendicitis at some point, enough to make it the most common cause of emergency surgery. Without surgery, many appendicitis victims would die. Once it was quite a common cause of death. The incidence of acute appendicitis in the rich world is about half today what it was in the 1970s, and no one is quite sure why. It remains more common in wealthy countries than in developing ones, though the rates in developing countries have been rising rapidly, presumably because of changing dietary habits, but again no one knows for sure.

  The most extraordinary story of appendectomy survival that I know of occurred aboard the U.S. submarine Seadragon in Japanese-controlled waters in the South China Sea during World War II when a sailor named Dean Rector from Kansas developed an acute and obvious case of appendicitis. With no qualified medical personnel on board, the commander ordered the ship’s pharmacist’s assistant, one Wheeler Bryson Lipes (of no known relation to the present author), to perform the surgery. Lipes protested that he had no medical training, did not know what an appendix looked like or where it was to be found, and had no surgical equipment to work with. The commander instructed him to do what he could anyway as the senior medical person aboard.

  Lipes’s bedside manner was not perhaps the most reassuring. His pep talk to Rector was this: “Look, Dean, I never did anything like this before, but you don’t have much chance to pull through anyhow, so what do you say?”

  Lipes succeeded in anesthetizing Rector—in itself an achievement because he had no instructions on the dosage to give—then, wearing a tea strainer lined with gauze as a surgical mask and guided by little more than a first aid manual, he cut into Rector with a galley knife and somehow managed to find and remove the inflamed appendix and to sew up the wound. Rector miraculously survived and enjoyed a full and healthy recovery. Unfortunately, he did not get to enjoy a full and healthy life. Three years after his appendectomy, he was killed in action on another submarine in nearly the same location. Lipes served in the navy until 1962 and lived to the ripe age of eighty-four but never performed surgery again, which is of course just as well.

  * * *

  —

  The small intestine empties into the large intestine via a connection called the ileocecal sphincter. The large intestine really is a kind of fermentation tank, home of feces, flatus, and all our microbial flora, and a place where nothing much happens in a hurry. In the early twentieth century, Sir William Arbuthnot Lane, an otherwise distinguished British surgeon, became convinced that all that sluggish muck promoted a buildup of morbid toxins, leading to a condition he called autointoxication. He identified an abnormality that became known as Lane’s kinks and began surgically excising lengths of large intestine from sufferers. Gradually, he extended the practice until he was performing total colectomies—a procedure that was entirely unnecessary. People flocked to him from all around the world to be parted from their bowels. After his death, it was shown that Lane’s kinks were entirely imaginary.

  In America, Henry Cotton, superintendent of Trenton State Hospital in New Jersey, also took an unfortunate interest in the large intestine. Cotton became convinced that psychiatric disorders were due not to disturbances in the brain but to congenitally misshapen bowels and embarked on a program of surgery for which he had no apparent aptitude. He managed to kill 30 percent of his patients and cure none—but then none had any conditions that needed curing. Cotton also became an enthusiast for tooth extraction, taking out almost sixty-five hundred teeth (an average of ten per patient) in a single year, 1921, without the use of anesthetic.

  The large intestine is in fact engaged in a lot of important work. It reabsorbs large volumes of water, which it returns to the body. It also provides a warm home for vast colonies of microbes that chew away at whatever the smaller intestine hasn’t taken already, in the process capturing lots of useful vitamins like B1, B2, B6, B12, and K, which are also returned to the body. What’s left is dispatched for evacuation as feces.

  Adults in the West produce about 200 grams of feces a day—a little under half a pound, about 180 pounds a year, 14,000 pounds in a lifetime. Stools consist in large part of dead bacteria, undigested fiber, sloughed-off intestinal cells, and the residues of dead red blood cells. Every gram of feces you produce contains 40 billion bacteria and 100 million archaea. Analysis of stool samples also finds many fungi, amoebas, bacteriophages, alveolates, ascomycetes, basidiomycetes, and a great deal else, though whether some of these things are permanently resident or just passing through is rarely certain. Stool samples taken two days apart can give strikingly different results. Even samples taken from two ends of the same stool can seem to come from two different people. We are a long way from understanding it all.