How do you make a lion from a lamb? A good question for a zoo director. Lions are more popular with the public than lambs. Fortunately the solution is simple: put the lamb near the lion. In no time at all the lamb will have become a lion. That’s how you make cats from mice, mice from cheese, cheese from cows, and cows from grass. Carrot becomes rabbit, rabbit becomes human being. In the beginning you’re no more than a single fertilised egg. The rest of you consists of fish, peanut butter, meatballs, pork loin and potato salad. A cow’s ear becomes a human toe, a lamb chop becomes a human ear, the nose of a salmon becomes the pimple on your bum. The brains I used to write this book were once located in a pig’s backside, the fingers I used to type it consist partly of rabbit paws. That’s pretty clever, such a conversion, but it’s also scary. Imagine them getting stuck halfway. The fact that what you are now was first flounder and pig is still frightening. Molecule for molecule, an animal becomes a human. It can’t get any more intimate than that. Too close for comfort. That accounts for the flourishing business in organic steak and unsprayed hot dogs. Many people are becoming vegetarians. From now on their brains will come from turnip greens, their fingers from asparagus, which only makes the miracle greater. Have we found the philosopher’s stone? We still can’t extract gold from iron, but every day we make human beings and joie de vivre from chicken and apple sauce. No matter how clever it was of Zeus to turn into a swan, and for Jesus Christ to become incarnate, for an ordinary mortal metamorphosis is child’s play. Literally. To become an adult, a child has only to eat himself up. Innocent children’s tissue is transformed into a tremendous penis or massive breasts, brains full of innocence into a tub full of double entendre. Caterpillars eat themselves up in order to pupate into butterflies, amphibians eat themselves from fish to frog.
All that a living being needs, human or otherwise, in order to incorporate another type of creature into itself is a magic vessel: the gut. Within the intestines of all the animals in the woods and all the dwellers in the cities, plants and animals are constantly changing their identity, day in and day out. Living nature seems to resemble a box of Lego or a Meccano set. First the food is broken down into its building blocks, then the building blocks are transformed into a new creature. A very complicated business. It takes two days alone just to demolish the food. It all bears a suspiciously close resemblance to glass recycling, that labyrinthine process of crushing bottles to shards so that new bottles or maybe jam jars can be made. What a waste of energy.
It takes a lot of energy to make a bit of a human from a mouthful of food. Yet food provides energy. Most of it is not used for building but for heating, and that’s good. A human being eats a tonne of food a year. If you were to convert all those kilos into kilos of human being, you’d end up as heavy as a whale. In order to maintain your weight, it’s a good idea to burn up most of the food in due course. Fortunately there’s a big demand for fuel to keep your body going. Your whole body is alive with sputtering and bubbling; nerves flash, arteries beat, turds are eager to find an escape route. Every animal and plant is sizzling: it gurgles and hisses, hormones go screeching, juices are absorbed, colours and smells are dispersed. All of nature is convulsing from the living factories in the industrial zones borne by millions of tiny legs; factory smoke wafts from billions of anuses, big and small. In each of those countless organisms, substances are transformed into something else until they drop from exhaustion. What are they making there, anyway? And why are they working so urgently? The end product of all that effort, all that fuss and bother, is life. And life goes on. And on. And on. And on. For three billion years now.
The entire surface of our planet is seething, thanks to life. Sometimes it even threatens to boil over. The engine behind this global commotion is the sun. Every day the sun drenches the natural world in hundreds of times more energy than all our power stations put together are able to extract from coal and oil, and life gets a tiny share of that solar energy. Essentially, life is no more than a detour along which the energy from a glimmer of sunlight is able to play outdoors before being swallowed up in heat or disappearing into the depths of the universe. Crowing with pleasure, solar energy gurgles on through tree, human and worm. Animals, plants, people: we’re all the sun’s playthings. It’s a lively chaos for those who see the fun of it, and a vale of tears for those who can’t bear the idea of finitude.
Why are we here on earth? We are on earth to metabolise. Metabolism provides the energy that keeps life going. In the words of biologist D’Arcy Thompson:
Energy is life, and life’s currency. It unites and divides all living things; its flow from one place to another controls everything from cells to forests.
Long before the concept of ‘energy’ acquired its modern connotation in the nineteenth century, it was generally thought that something like it flowed through every living organism. Taoists called it ‘chi’, Hindus ‘prana’. Until the twentieth century there were also Christian biologists who believed that life could not be understood merely in terms of scientific formulas. There was also said to be evidence of a life force or vis vitalis, whether sent by God or not. But when it became possible to determine with greater and greater precision that a plant or animal uses up exactly as much energy as it takes in, these earlier voices were forced to admit defeat. Life, too, complies with the pure physical law stating that energy always remains constant. At the very most it changes form.
The art of living is all about nibbling as much as you can from the available energy before you die. Plants capture as much solar energy as they can with the help of leaves that are as plentiful, as big and as close to the sun as they can be. Humans churn the energy out of their food in their intestines, a place that not a beam of sunlight ever penetrates. They leave the capturing of sunlight to the green plants. That seems like an awkward arrangement. Why would you content yourself with secondhand solar energy? Why don’t we capture the sunlight ourselves? Why didn’t evolution give us green leaves so we could tap the sunlight directly? The answer can be found in every garden. There they are: the roses, the hydrangeas and the lilies-of-the-valley. They live from the air and the light. Maybe that makes them happy. But all they do is stand there. They can never check out the neighbour’s garden to see if it’s nicer, they can never search for a sunnier corner on their own initiative. Plants don’t have legs, true, but even if they did they wouldn’t get very far. They simply don’t have the get up and go to move themselves forward. With their leaves, plants can grow and blossom, they can attract insects and feed people, but even the biggest giant sequoia has to spend all its centuries in one and the same place. Plants lives their lives as plants.
If you want to have enough energy to live an active life, then your only choice is to turn to crime. Instead of laboriously extracting energy yourself from the sunlight, you rob the hard-working plants of the solar energy they’ve stored in their tissues. With just a few mouthfuls you get the harvest of energy that took months to glean. See how energetically the antelope leaps through the immobile grass, see the birds fly on the energy they derive from seeds under house arrest, hear the rutting buck bellowing from the strength that’s been churned up from static tree bark. But even their lives are not carefree. They have to be on guard against the super bad guys who see such plant plunderers as even more densely concentrated energy packets. With one neck bite, one gash of the beak, a lion or eagle handily relieves an antelope or hare of all its diligent labour; with one visit to the butcher, a human being takes home the result of a lifetime of chewing and re-chewing. The advantage of eating meat is that you need less of it: you get more energy from one chop than from a whole bale of hay.
You eat to live. But living still doesn’t use up all the energy you get from food. There’s plenty left over for a hefty pile of shit. The amount of energy you lose with your faeces is not precisely known. ‘In man there have been few investigations in which the excreta are collected and their heats of combustion determined,’ writes Scottish physiologist Kenneth Blaxter, not without understatement, ‘no doubt because such tasks are not pleasant ones.’ It has been estimated that 6 per cent of the energy in your food ends up in your shit, 3.5 per cent in your pee, and 0.5 per cent in your farts; all told that’s one-tenth of your energy budget.
If energy is the currency, we’re talking about 10 per cent of your life. The idea that you actually lose some of your vitality with your excretions was generally accepted for centuries. From the ancient Greeks to Christian folk religion, philosophers and healers believed that everything that leaves your body—sperm, menstrual blood, sweat, tears, breath and even your voice—is animated by the life force in your innermost being. Shit and urine were considered the most life-filled, since they briefly retained their warmth. Thanks to their vitality you could use them to make all sorts of medicines or even work magic with them. Something of this conviction lives on in the custom of criminals defecating at the site of their crime. With the help of its vis vitalis, the turd stays on the lookout while its owner makes himself scarce. There’s no time to lose, however, since a turd only works as long as it’s warm. A cold turd loses all its strength. So the crook, with his belief in the old superstitions, is properly observing the main laws of thermodynamics, which hold that all energy is ultimately reduced to heat.
Unfortunately for crooks and mystics, the vis vitalis does not exist. Even the best burglar or the most isolated hermit runs on the energy provided by chemical processes. Today you can buy all the necessary chemicals at your local supermarket. In every can of beans, every pork chop, every container of cottage cheese there’s enough power to cause an explosion. Energy galore. But how do extract it? Put the meat and the beans on a plate, pour the cottage cheese on top, and nothing happens. Not a whisper of a chemical reaction. It’s as if there were a brake somewhere. In order to get your food to break down chemically and thereby release its energy, you first have to supply a little energy. You have to lift the process over a threshold, as it were. Start it up. Give it a crank. To light a campfire you first have to make fire with a match. Only then will the twigs burn and the energy be released in the form of heat. It’s not all that different in your body. Beans, pork chops, and cottage cheese are actually burnt up in order to release their energy. It sounds risky. Fire has a way of quickly getting out of control. How do you burn meat in a body made of meat? If you’re a woodworm, how do you burn your wood without setting the whole antique on fire?
Good questions. But first you have to get the whole process going. How do you start a bean? Hold a match to it and you’re left with a lump of coal and a burnt finger. Put it in a nine-metre glass tube with the rest of the beans, the pork chop and the cottage cheese, and nothing happens—unless there’s a laboratory attached to the tube that will unleash a whole series of chemical processes. Stick the whole business in a living intestine seven metres long and enough energy will be produced for you to walk around all day, to laugh and to love. The secret of the intestine can be found in these extraordinary elixirs, the enzymes. A drop of this and two drops of that are enough to get the chemical processes going. Fortunately, there’s no fire involved. For combustion to take place, oxygen is necessary by definition, provided by means of respiration. But the process works just as well without fire as with it—or better even—provided that the combustion entails a large number of steps, each with specific catalysts that help each sub-reaction over its particular threshold. For every kind of food there’s a separate enzyme. These are the juices with names like lipase, amylase, and peptidase, which have robbed many a student of the desire to continue their study of biology. Lots of people know what goes in and what comes out, but what makes the sun shine again in the darkness is a mystery to them. Their intestines are the longest black box in their body. Yet you can easily get a glimpse of how the intestines work—at the table. Choose a dessert that has lots of kiwi fruit in it. The digestive process begins before you take your first bite. The enzyme in a kiwifruit is the same as that in your stomach, an enzyme that is simply wild about animal products like dairy or gelatine. Kiwifruit loves ice cream and gelatine desserts more than anything else. If you don’t move fast, the kiwifruit will eat the dessert into snot before you can even get started.
Leonardo da Vinci (1452–1519): The digestion machine.
It’s striking how little interest people show in the forces that keep them going. They have a vague notion about the combustion of food in the belly, but most of them let it go at that. Biology is something outside, not inside. The more distant it is, the more we know about it. Exotic animals fill the television screen, exotic palm trees entice us to exotic beaches. We have clear images in our minds of the Ganges, with its praying bathers and bathing prayers, burning corpses and sky full of vultures, but the intestinal river that winds through our bodies is something we rarely try to imagine. More expeditions down the Nile and the Niger have been broadcast than those down the small and large intestine. Towards the end of the era of the great scientific expeditions, the American writer George S. Chappell (1877–1946) decided to take his own voyage to the human interior. In Through the Alimentary Canal with Gun and Camera (1930) he describes his adventures in search of the sources of the Bile. The other three members of the expedition prefer to remain anonymous (‘one never feels quite the same toward a person who has looked one’s liver squarely in the eye’), but they show up promptly for the adventure (‘Be here at six-thirty sharp. And don’t forget your rubber boots’).
Before us lay who knew what terrors of attack! what onslaughts of infuriated bacilli, scissor-jawed microbes, fierce phagocytes and, most dreaded of all, wild-eyed heebie-jeebies which give nor take no quarter.
Travelling in a collapsible canoe called the Rubber Duck, the expedition crew enter the locks of the Oesophagus and descend to the intestines. Danger lurks at every turn:
Several times we seemed to be bearing down on wicked reefs or about to be dashed against the living wall, but a quick turn of the wheel swept us by in safety. One of the gravest elements of danger lay in the traffic, for as usual in this part of the trip the way was clogged with many heavy-laden food boluses southward bound. These clumsy craft took up most of the room. They are too heavy to manoeuvre for position, and we had to get by them as best we could.
Along the banks of the upper Bile, Chappell hears a faint, curiously rhythmic sound:
Haemoglobins!
My hair rose as I thought of being enveloped by one of these ghastly wet-blankets. A headline in the Livermore Leader flashed through my mind, ‘Explorer Found in Bile Valley. Asphyxiation Indicated.’ I saw my body being discovered…the subsequent obsequies… weeping relatives…you know how it is.
This cannot end well. Strikes break out in Gastritis (‘they’re all reds’), colic spreads, and there are head winds all the way up to Colon-by-the-Sea. The crew members are just able to purchase postcards of the sights in the south that they hadn’t seen yet—the Appendix from Caecum, Caecum from the Appendix, the Topless Towers of Ileum, the Everglades of the upper Colon, the Elks Club at Colon-sur-Mer—before a coppery cloud eats up the landscape. The Bile has overflowed its banks! With a powerful eruption from the interior the entire expedition is washed to the north on a tidal wave, through the lock at the Gizzard, and back into the outside world.
Seventy-five years later, this pastiche of an adventure novel met its medical match on Dutch TV. This time the visitor was obviously quite capable of fitting into someone else’s intestines, since he was a gnome. Prikkeprak the Gnome (played by Arjan Ederveen) works for the Academic Hospital in utrecht, where Professor Tineke Poortvliet (Tosca Niterink) has developed a revolutionary method for removing intestinal polyps. As the hairy buttocks of the patient, Mr Struikebos, come into view, the professor explains:
Prikkeprak the Gnome is at the entrance and is waiting to begin. In just a moment we’re going to insert him into the rectum. The gnome is equipped with an oxygen tank. He’s carrying a video camera so we can watch the entire procedure, and he has a field telephone so we can communicate with each other.
All right, there he goes. First he’s going to examine the anus a bit. Can you feel that? Now he’s going to stick his little hand in. And now he’s going to try to get his cap in as well. Shouldn’t you put a little vaseline on that cap? He’s moistening it with some spit.
With the tip of his cap leading the way like an awl, the gnome disappears into the hindmost part of the intestines that were closed off to Chappell. On the monitor we see Prikkeprak get to work deep underground. When he finally finds the polyp (‘Mr Struikebos, are you sure you haven’t eaten anything? It’s really hard to see in here!’), the gnome takes a little hatchet and starts chopping away with the enthusiasm of a happy worker, accompanied by suitable songs (‘Hi-ho! Hi-ho! Hi-ho! Lalalalalala’). This takes quite a while, because ‘this thing’s a real brute’. His joy is all the greater when the job is done.
Prikkeprak the Gnome at the entrance to the rectum.
Intestinal tourism is no longer popular. Today the human adventurer has been replaced by a single eye in the form of a camera on a string: an endoscope. With the string, actually a flexible tube, you can inspect almost the entire intestine from within. Little robot hands come back out with the souvenirs they’ve harvested. This makes it possible to examine any growths for malignancy. But even an eye with hands can involve inconvenience. First the patient must empty his intestines. If he doesn’t, the eye won’t be able to see anything and the little hands won’t know what to grab. Here a laxative is as effective as it is disgusting. Fortunately, as an amateur you don’t have to go inside to get an impression of the state of your entrails. You can also wait until something comes out. Call it the volcano method. As a volcanologist you’re wise not to descend straight into the sulphurous hell of the crater. It’s best to wait until the volcano has erupted spontaneously so you can examine the innermost recesses of the earth somewhere on the outside at your leisure.
A cautious gastroenterologist will wait until a person has to vomit before studying the stomach. In one brief but powerful eruption the person’s abdominal contents come to meet the light of day. It’s rather startling. What until only recently had smelled and looked so appetising on the plate has now become revolting mush—not shit by any means, but just as nasty. With the exception of gastroenterologists, very few people are fascinated by vomit. An anatomical pathologist might be intrigued, but then there would have to be an interesting murder to investigate. I myself once tried to make fake vomit. On TV I was teaching children how to feign illness in order to avoid having to go to school. Following some tips on whipping up imitation blood, forming really ghastly sores, and pretending to chop off your fingers, came the highlight of the program: fake puke. I mixed cold tea with garlic, vinegar, food scraps, cinnamon, coffee and beetroot. It took a long time. Finally I called out, ‘There! Your puke is ready!’ But the director wasn’t going to let me get away with that. She wondered if I’d be willing to take a big swig. In a burst of obedience I complied with her request. Even before the fake puke could reach my stomach a wave of real vomit came up, so that fake and real together were sprayed all over the camera lens. The director was satisfied. I less so. The cameraman least of all.
The contents of the stomach are easy to examine, if necessary.
It’s simple. Once the vomit centre in the brainstem has decided it’s time to puke, an automatic response is triggered that cannot be stopped with all the will in the world. Diaphragm, belly and chest contract, and it’s as if you suddenly had to hiccup really badly. The difference in pressure between your inside and your outside increases, and then the uppermost sphincter of the oesophagus springs opens. Like a tsunami the contents of your stomach force their way out. Shutting your mouth tight won’t help; it only comes out your nose. All this violence is necessary because puke runs against the flow of traffic. Normally, the traffic in the alimentary canal is strictly a one-way street, from mouth to anus, from food to shit. You’d be dumbfounded if the turd you had just shat were to rise up out of the toilet, enter your lower intestinal tract and reappear again in your mouth bite by bite as steak with mushrooms. The reason the body has broken the rules is only because it hopes by so doing to prevent something worse. You puke in order to get rid of poisonous substances or to remove a blockage. To induce it deliberately all you have to do is ingest an emetic or stick a finger down your throat. The ancient Romans tickled their uvulae with feathers to keep their orgies going longer.
The most revolting thing about vomit is the sour smell. But that’s not the only thing about vomit that’s sour. If you forget to clean up your cat’s puke it will burn a hole in your carpet. With a pH of 1, gastric juice is among the world’s strongest acids. The second greatest mystery of the stomach is how it can tolerate all that acid; the greatest mystery is how glands can exist that produce an acid in which they themselves would dissolve. The secret lies in other miracle glands that produce a miracle mucus which keeps the burning acid away from the delicate stomach tissue. If these glands fail to deliver for one reason or another, you run the risk of developing an ulcer that may result in a perforation of the stomach if the ulcer festers. When that happens, the acid drains out of the stomach and ends up in organs that are absolutely defenceless against it because they have no miracle glands of their own. The acid is supposed to stay in your stomach, where it preserves your food until it’s ready to enter the rest of the digestion machine. In this ruthless chemical warfare, bacteria decompose and pathogens meet a miserable death in the gastric juices.
And then what? Does anything else happen in the stomach? Vomit is silent on the subject. In order to find out, you would again have to arm yourself with gun and camera, or a gnome’s cap. Or an endoscope, of course. But before the endoscope was invented, there was only one alternative outside of the gun, camera and cap: the break-in. If you can’t get in by the front door and there’s no visibility from the back, your only choice is to go straight through the wall. Operate. But all you get by operating is an arbitrary view. The only thing that really helps is a lucky break. Science needed continuous insight into human inner workings, and it was then that coincidence came to the rescue. With a marvellous accident.
On 6 June 1822, Alexis St. Martin became a man with insight. A musket ball had accidentally torn a hole in his stomach, and as a result everything that went into his mouth came back out through this hole. The doctor from Fort Mackinac, Michigan, kept him alive by means of ‘nutritious injections’ rectally administered. The patient recovered, albeit with an exit hole in his belly, which ‘was a genuine anus, except for the absence of a sphincter’. For Dr William Beaumont, an ambitious army doctor without a war to practise in, this was the chance of a lifetime. Science finally had a man with a car bonnet. Beaumont tied pieces of food to a silk thread and inserted them into Alexis’s stomach. In this way he could pull the thread out at regular intervals to see what had happened. But he went too far. At one point he shoved sixteen raw oysters into Alexis’s belly; another time it was a thermometer. Shamelessly leering in, he found that Alexis’s stomach turned red when he became angry. And he had every reason to be. Sick and tired of serving as a human guinea pig, Alexis put an end to the tests in 1832, the year before Beaumont became famous with his Experiments and Observations on the Gastric Juice and the Physiology of Digestion. But not everyone was happy with the discovery that you could reduce vis vitalis locally to an ordinary toilet bowl cleaner, hydrochloric acid.
Later on, hydrochloric acid was also found in the stomachs of sparrows, panda bears, nine-banded armadillos, frogs and toads. Even greater was the astonishment in 1973 in Australia when a new species of frog was discovered, Rheobatrachus silus, that vomited living tadpoles. It seemed that the animal hatched her larvae in her stomach. Biologists couldn’t believe their eyes. It’s hard to imagine an environment less hospitable for the raising of youngsters than a stomach, where food is prepared for digestion. Tadpoles are definitely not resistant to toilet bowl cleaner. The only possible explanation was that when the frog hatched her young in her stomach she temporarily halted the production of hydrochloric acid. But how?
Tests showed that the trick is not carried out by the mother but by the young themselves, back when they were eggs. The mother is totally oblivious; she probably regards her eggs as food, which is how they get in her stomach in the first place. There they secrete a substance, prostaglandin E2, which suspends the production of hydrochloric acid. So the larvae fiddle with the body of their mother in order to transform a deadly witches’ kettle into a cosy little cot.
People don’t gestate in their stomachs. Our children have a room of their own, right in the parental body: the womb. That may seem convenient but it’s also rather a waste, having an organ that is used for only a short part of your life and otherwise just sits there, useless and unoccupied. While we fill our stomachs three times a day, that other organ is filled an average of only three times during an entire human lifespan. The reason is clear. An empty stomach convulses, which causes an unpleasant feeling called hunger, and the only way to make that feeling go away is to fill the stomach. Wombs are never hungry; a womb doesn’t growl. Satisfying the desires that go hand in hand with the filling of wombs requires a very different organ, a much smaller one, located a little further on.
The stomach also farms out the desires associated with filling it. That’s what it has the tongue for. As icon of gastronomic gratification, the stomach is highly overrated. You can actually do without it entirely. People whose stomachs have been removed, or who have had a gastric bypass, live quite peaceful lives, and they’re stylishly thin as well. Many species of animals have never had stomachs. We regard the stomach as the centre of digestion because of our need for hierarchy. If the heart rules over the circulatory system, then the stomach will rule over the intestines. But the heart is no more than a pump, and the stomach no more than a warehouse. Except for water, almost the only thing that is absorbed into your blood from the food and drink in your stomach is alcohol, so that wine or beer can soon be detected on your breath. Opinions are divided as to whether this is convenient or not.
The stomach is simply a broadening of the intestinal tract. At best you can see it as a single station on a long underground line that starts in the throat, dives through the oesophagus on the way to the stomach, climbs up a slight incline to the duodenum and, after a long and winding ride through the small intestine via the Caterpillar ride of the large intestine—straight up, along a horizontal bit, and then right and straight down—arrives at Anus Station, its final destination. One way only; no return tickets available.
The route’s first stretch goes via the oesophagus and carries the food and drink through the chest, behind the heart and lungs, towards the abdomen. Upon its arrival, the stomach opens its gates, which otherwise are tightly shut to prevent the stomach acid from percolating upwards and incinerating the oesophagus or the throat. As a station, the stomach is really no more than a waiting room, although there are snacks available there such as pepsin to initiate the digestion of proteins (carbohydrates have already undergone a preliminary treatment in the mouth with the enzyme amylase). Within six hours the transport is ready to continue. No matter what delectable ingredients you put in your food, no matter how sophisticated your cooking technique, your body feeds itself with nothing but the vomit dripping out of your stomach and into your intestines. No photos of food porn here. The only time we see what’s really keeping us afloat is when we have to throw up. Sometimes you recognise a tiny bit of chicken or caviar, and almost always chili con carne, even if you haven’t eaten it in quite some time.
Before the vomit in your belly can really be digested, the gastric juices have to be neutralised. This is done with an equally strong antacid: bicarbonate. In the shop you can buy it as antacid tablets; in your body it’s produced by the pancreas, which empties into the duodenum just below the stomach. Only when the last morsel of puke is sufficiently neutralised does the sphincter controlling the stomach’s exit door release a new batch. This sphincter is known as the pylorus—‘gatekeeper’ in Greek—but in fact is no more than a back door that is operated at a distance by the real gatekeeper, the pancreas, with the help of the bicarbonate. The pancreas is also responsible for producing enzymes such as trypsin, lipase and amylase, which break down proteins, fats, and carbohydrates respectively. Fats are the most troublesome. They need a lot more than lipase to be broken down properly. First they’re reduced to little droplets by the bile salts that the liver supplies to the intestine via the bile. But no matter how small, even the tiniest morsel of pork chop or Brussels sprout won’t fit through the openings in the intestinal wall and has to sit out the entire journey unutilised, only to get out at Anus Station, its final destination. Strictly speaking, everything in the intestines is located outside the body, the way all the air in the hole of a spool isn’t part of the spool but of the outside world. To break down the food particles so they can get through the intestinal wall, the intestine resorts to an old trick from the chemistry lab: it liquefies them. Solids are usually much more manageable once they’re liquefied. As tiny, water-soluble molecules, the nutritious substances can hazard the crossing from the intestine to the nearby blood vessels. After arriving in the blood they often combine immediately to form complex, insoluble substances, but that no longer matters now that they’re safely on the other side, in the actual body. Here, the ready-to-eat fragments travel via the blood and the lymph to the liver, which makes the building materials and fuel that we need in order to live. The actual building and burning of fuel then takes place in various cells all over the body. Each cell has its own power station, but it runs on raw materials that are supplied by the intestines.
In order to extract sufficient energy from your food you need long intestines. A human being has six to eight metres of them. The duodenum—which means ‘in twelves’ in Latin—makes up for twelve finger-widths of intestines, or twenty-five to thirty centimetres. After that come metres of small intestine before the large intestine spans the last one and a half to two metres.
With all their entrails, large animals soon run up against an intestinal shortage. The nutrients are absorbed through the intestinal surface. If an animal grows to twice its length, then its intestines attain a surface that’s 2 × 2 = 4 times as big. That sounds good, because it means the animal can process four times more food. But the animal itself becomes 2 × 2 × 2 = 8 times as heavy. If an animal eight times bigger gets only four times as much food, it falls short by a factor of two. There’s hardly any room to expand the surface by means of folds and bulges, since the surface has already been increased to the extreme. A human intestine, for example, has five hundred times as much surface area as the inside of a smooth tube, thanks to the intestinal villi; that’s a whole football field. To keep from starving, humans have no choice but to let their intestines grow faster than their body. That’s quite a squeeze. The intestines have to twist themselves in all kinds of convolutions to find enough space. In a human embryo the intestine starts out as a straight tube. Because it grows faster than the body in which it is housed, the intestine first makes a turn of 90 degrees anticlockwise, and then another turn of 180 degrees; it keeps on coiling in this fashion because the tissue to which it is attached at the back of the abdominal cavity doesn’t grow fast enough. By the time you reach adulthood, your intestines are still hanging from your backbone by means of this mesentery; in the front they’re held in by the stomach muscles. Except when they aren’t. Then you get a paunch.
Intestines have always had difficulty keeping up with the growth of their body, not just within the span of a single human life but within the evolution of a small to a large animal species as well. Stuffed into such an awkward space, they start twisting of their own accord, much like a rubber band in an undersized bag. But it can’t go on like that. At some point there’s no more room for twisting, and the animal has to adapt its lifestyle. This explains why cows can’t fly and birds can. It isn’t so much a question of wings as it is of intestines. Cows eat grass. There’s not much nutrition in a single blade of grass, so the cow has to eat a lot of it, which its massive amount of intestines also demands. They almost bulge out of its body. A cow is built around its digestive system. This gives it that ponderous, bony, old-fashioned hay-making machine look: in short, that poignant quality. But flying is out of the question. Birds fly, though, even if all they get to eat is grass. A wigeon duck, for example, eats nothing but grass, yet still it flies. Like a cow, it has unusually long intestines for this type of animal, but when they reached 130 centimetres the wigeon decided that was good enough for a bird. If its intestines had grown any longer it would have had problems. In aviation, there’s a heavy price to pay for excessive baggage in terms of fuel. To get the extra intestines airborne, a wigeon would have to eat more grass, for which it would need more intestines, etc. Somewhere there’s a golden mean between intestinal weight and flying capacity. That compromise is called ‘wigeon’.
At the end of the small intestine the food sludge reaches a fork in the road: go straight to the appendix or take a left and enter the large intestine. The only thing the human appendix can still do well is become infected and cause trouble. Like a murky alley it comes to a dead, worm-shaped end. So all the sludge moves on to the large intestine. There’s no turning back now; that option was ruled out for good at the T-junction by the ileocecal valve. At this point the food is as good as digested, and all that’s left of your meal is a watery sort of pea soup. Now it’s the job of the large intestine to make lovely turds out of it. This is mainly a question of thickening, which means the water has to be drained off. So it gets carried away through the intestinal wall along with all sorts of salts that are of use to the body. Every day, about a bucketful of water is removed. If you didn’t have a large intestine, you’d have to drink that bucketful every day to make up for it. It’s not always possible to find that much water, which accounts for the development of the large intestine during evolution: it makes it possible to live in dry climates. This requires life’s cooperation, however. The pumping station of the large intestine does heavy work due to the salt content. Once the salts are pumped to the other side the intestines absorb water automatically. But all that salt pumping takes a lot of energy, and therefore a lot of fuel.
Otherwise there’s not much more for us to do in the large intestine. We find ourselves in someone else’s territory. The large intestine is the domain of the microbes. For them, it’s a bustling metropolis full of nightlife, gangs and junk food. They eat what our enzymes in the small intestine aren’t keen on. While we need oxygen by definition in order for our metabolism to function, bacteria also thrive in the oxygen-free backstreets of the large intestine owing to the fact that they can ferment their food, a chemical process requiring no oxygen and better known as rotting. It can be relatively stinky. You only notice this when you fart. You might find farting embarrassing, but sensible people ought to be proud of this sign of life from all their little co-workers. Without them we would have been extinct ages ago.
With a hundred trillion bacteria, the large intestine accommodates ten times more cells than we have in our entire body. The only reason we aren’t weighed down by them is that bacteria cells are so much smaller than the cells in any random organ. All of them together weigh a kilo and a half, not much more than our brains. There are about a thousand different species of intestinal bacteria. Fortunately most of them have developed a friendly relationship with us over the course of evolution—until we start taking antibiotics. During a typical treatment half the species give up the ghost in an enormous massacre, which prevents the intestinal flora from doing their work. Not only do they process our leftovers but they also provide vitamins, protect us from pathogens and stimulate the immune system. What you ought to be doing is cherishing them. Instead of antibiotics, many doctors prescribe intestinal bacteria to fight allergies, obesity and vascular disease. If your own microbes are falling short, it’s time to call in the auxiliary troops. For a push in the right direction, many people take probiotics such as Yakult or Actimel. Sceptical doctors doubt whether the friendly bacteria in these probiotics can really pep up lethargic intestinal flora. They see more benefit in the transplanting of healthy faeces, which can be necessary when the virulent hospital bacterium Clostridium difficile seizes the opportunity to stage a takeover in a large intestine after a course of antibiotics. You can try hosing your intestine down, but C. difficile will just rear its head again—unless you inundate it with healthy faeces from someone else’s intestine, which can be done by inserting a tube in the nose and running it down into the small intestine. Fortunately there’s little reason to take such drastic measures with every rumble in your tummy. Such gurgling is not meant to alarm you but to reassure you; it’s a sign that your intestine is being properly looked after.
Of the thousand species of intestinal bacteria, every individual has about 150 in an entirely personal blend, each person with their own little garden. Tell me which bacteria you have and I’ll tell you who you are; an intestinal profile is as specific as a fingerprint. Yet as an embryo you didn’t even have one species of bacteria. An embryo enters the birth canal in as sterile a state as urine in the urethra. It isn’t until birth that a baby acquires its first portion of intestinal flora. Every vagina is a microbial paradise, and the composition of the flora is a reflection not of the newborn but of the mother. Unless the baby came into the world by way of Caesarean section, in which case it’s a reflection of the nurse who first handled it. But that would be skin flora, which differ from every kind of intestinal flora anyway.
In order to stay on good terms with your intestinal flora, you provide them with a steady diet of fresh intestinal mush. That’s not easy. How do you propel something along a conveyor belt many metres in length without wheels? Intestines have muscles, not wheels, and they can only do one trick: contract. With the circular muscles the intestine makes itself thinner, with the longitudinal muscles it makes itself shorter. When the circular muscles squeeze together the contents squirt from one side of the contraction to the other. There should be valves, as there are in the heart, which neatly press the blood to the other side. The small and the large intestines have only sphincters at the extreme ends. In the sections in between, movement is achieved by means of peristalsis. In peristalsis, the intestine doesn’t squeeze at once over its entire length but muscle by muscle, so the constriction runs through the intestine like a wave, rather like when you squeeze the last bit of toothpaste from the tube. While one circular muscle squeezes the ball of food forward from behind, the circular muscle in front of it relaxes to make it easier for the food to pass through. The longitudinal muscles pitch in at just the right moment to give the circular muscles a hand. In this way the intestinal contents move along at a good pace. After a stopover of a few hours in the stomach, the food takes about an hour and a half to get through the small intestine. The small intestine doesn’t care if the mush is thick or thin, but fatty food slows down the action because it pinches off the flow from the stomach. But even before digestion is complete, the muscles sweep the small intestine clean every other hour and a half to keep bacteria from settling there. Compared with the large intestine, the small intestine is practically sterile. It takes longer to get the contents through the large intestine than the small if only because its diameter is about twice as large. The fragments get tougher and tougher, and in order to keep them moving the large intestine has more powerful circular muscles than the small intestine and its inner wall is better lubricated with mucous. But it isn’t only up to the muscles to decide when to let the shit see the light of day. The large intestine also serves as a storage depot, waiting until the coast is clear and it’s safe to dump. The muscles can’t tell when that might be.
The tempo of one trajectory has to be coordinated with that of the other. The contractions cannot be allowed to collide. And not only do the contents of the intestine have to be pushed forward, but they also have to be kneaded and mixed. The circular muscles have to open and close on time, fluids have to be released on schedule and according to the right dosage, and bacteria have to be put to work. Compare it to brewing beer or baking bread; one mistake and your dough is ruined. So digestion makes heavy logistical demands, which call for scrupulous guidance and control. But from whom? You yourself wouldn’t know how to begin. Or do you know exactly when the gastric juices are sufficiently watered down, when to add more bile, when the duodenum has to be urged to speed up, when a turd is thick enough?
Are you in the driver’s seat, or are you just nothing but your own car? We drive our bodies the way we drive a car: we know where the steering wheel is and the gas pedal, but most of us have no idea how the engine works. Open the bonnet and you glaze over, look inside yourself and you’re nauseated by your own bowels. Recently something inside me broke down and the doctor invited me to look at the monitor with him so I could see inside. I saw my own bladder. Nothing about it looked familiar. Fortunately there wasn’t much damage. I gave my bladder a good talking to and I hope I never see it again. I’m perfectly happy to leave it to the competent authorities who know something about the autonomic nervous system.
This system has traditionally been divided into two parts: the parasympathetic nervous system and the sympathetic nervous system. Eating and digestion are the work of the parasympathetic division. This is the nervous system of repose. It slows down the heartbeat, the breathing and lots of other stuff. The only things that are stimulated are the appetite, the pumping of the intestines, and the sex drive. It’s the fun nervous system. Unfortunately the sympathetic nervous system gets in its way. It wants to turn all of us into scrawny managers with ulcers. Sensible managers know how to switch off this nervous system, however: with a business lunch or dinner. Like a coin in a vending machine, these kinds of meals switch on the fun nervous system so you can calmly get down to business. At least that’s how I learned about it as a student.
In the late twentieth century this picture was drastically overhauled. The running of the stomach and intestines was declared independent. In and around the intestinal tract there are hundreds of millions of nerve cells that do everything on their own initiative. Within the autonomic nervous system they form a kingdom all their own, in addition to the sympathetic and parasympathetic nervous systems. This is where the sense of rhythm is to be found that keeps the intestinal muscles dancing; this is where extra bile is delivered; this is where the decision is made that you’re hungry. This enteric nervous system is what neurologist Michael Gershon called ‘the second brain’ in his book of the same name. Thanks to this second set of brains, the supreme brain high in our heads doesn’t have to interfere with our intestines way down south, thus freeing up its cells for more elevated things. But what do we mean by elevated, Gershon wonders:
The enteric nervous system may never compose syllogisms, write poetry, or engage in Socratic dialogue, but it is a brain nevertheless. It runs its organ, the gut…When the enteric nervous system fails and the gut acts badly, syllogisms, poetry, and Socratic dialogue all seem to fade into nothingness.
Even though the second brain is connected to the first brain by means of a main cable, the vagus nerve, so the first brain can oversee what’s going on, if you were to sever this nerve connection the intestines would still go on working. It’s more likely that any problems would occur in the opposite direction. Far more signals are sent from the belly to the head than the other way around, as if the intestines were telling you what you ought to do. It’s just a matter of how you look at it. Seen from the vantage point of the first brain the intestines serve us as a power station, a typical public utility. But if the second brain were to have any capacity for self-consciousness, it would know that life is not based on philosophy but on physiology. We feel ourselves with our first brain, but we are our intestines with their second brain. The fact that our brain is only a walnut-shaped appendage of our intestines takes some getting used to, but when all is said and done it doesn’t have a lot to contribute. The second brain has a powerful weapon to get our first brain to do its bidding: hunger. If it came down to a direct clash between intestines and intelligence, then the intestines would win, as we see from the report by stomach-intestine specialist Akkermans on attempts to tame a stubborn bit of bowel:
When a large section of the small intestine is removed during an operation, the rest of the intestine may be too short to absorb sufficient nutrients. To deal with this problem, surgeons cut away a piece of small intestine, turned it around, and then re-attached it in its original place, only backwards. The direction of the peristalsis in this piece of intestine would then be the reverse of the peristalsis in the rest of the intestine. As a result, the speed at which the food components were transported through the intestinal lumen would be inhibited, leaving more time for the absorption of nutrients. This trick worked for about a month. The neurons in the enteric nervous system grew out through the connecting seam and the peristalsis in the implanted piece of intestine reversed itself.
Akkermans’s conclusion: ‘Apparently it is difficult to fool the enteric nervous system.’ But it may be possible to drive it crazy, according to Michael Gershon. ‘Since the enteric nervous system can work independently, we should consider the possibility that the brain in the belly also has its own psychoneuroses.’ This raises hopes for the treatment of nervous disorders such as irritable bowel syndrome. Maybe the brains on the psychiatrist’s couch should be those in the belly and not those in the head.
Just as the underground railways in all the world’s cities look alike, so the intestines of all animals are made the same way. The food enters in the front, releases its energy along the way, and at the last station everyone is kindly asked to leave. The senses keep a close watch on everyone when they enter, but there’s hardly any checking at all at the exit. Yet one city isn’t quite the same as the next, and there are also significant differences among the various animals. One is bigger than the other, or more active, faster, more modern. Large animals have long intestines, but those of a lion are shorter than those of a gnu, who’s just as big. The intestines of a lion have less to do because it’s a carnivore. Meat is easier to digest. It consists of animal cells, which have thin walls, so you can easily get to the contents. Plant cells are wrapped in thick capsules of cellulose, which is difficult to digest. First you have to crack them open like nuts. Babies can’t do that at all. That’s why every human being, vegetarian or not, must begin life as a carnivore. Not with a lamb chop or a hot dog but with mother’s milk, which is a pure animal product. This white, liquid meat is full of amino acids that a baby would never be able to extract from a vegetarian diet on its own. By nursing, every mammal has at least one carnivorous period. Birds don’t nurse, although doves feed their young a milk-like substance from their crops. But in many cases the species that eat sunflower seeds like proper vegetarians and dutifully cling to the peanut baskets hanging from trees are the same ones that grew up on nutritious caterpillar mush.
While a carnivore quickly jams its tender prey through its short intestines—a cat has to return to its dish within thirteen hours—a herbivore needs more time to get the job done. Big herbivores take from forty to sixty hours. Is that why vegetarians stay so thin? Or do they only appear to be thin? Thin, not to say gaunt, is the general impression that the butcher’s regular customers have of those who abstain from meat. But Bernard Shaw, Leonardo da Vinci and Greta Garbo aren’t the only well known vegetarians. The hippopotamus, the elephant and the bison are just as horrified by meat. Yet they’re all certified heavyweights. In order to extract enough energy from its paltry fodder, each hippopotamus has fifty-five metres of intestine continuously running full tilt. A lot of the energy produced is needed right away for the kneading and pumping of the next serving of plants. And it takes a great deal more energy just to lug all those guts around, which means having to eat even more. All the livelong day. In order to chew all those plants, the head of a herbivore is almost entirely taken up by jaws, molars and chewing muscles; there’s little room left for brains. Predators, on the other hand, need a lot of intelligence in order to overpower their prey by means of cunning and deception, strategy and deliberation. Herbivores use their wits too, to stay out of the clutches of the carnivores, but they’re less successful; otherwise the carnivores would soon die of hunger. Eating meat makes you smart. It’s fun being smart. That’s why we have warmer feelings for cats than for rabbits. Carnivores are more playful, more intelligent, and more human-like than herbivores. Nicer. The only thing vegetarians have going for them is being right. And unfortunately you can’t eat that.
For grass-eating animals, a long intestine isn’t enough. They need extra bacteria to ferment the tough cell walls. To keep these auxiliary troops from being defecated before they’re able to do their work properly, the intestinal underground has built additional stations for them. With a cow these are the four stomachs. Here the bacteria break into the cell walls before the grass can get to the small intestine, which soaks up the nutrients. To help the bacteria along, cows vomit up the contents of their stomach every now and then so they can chew it some more. In this way they put their leisure time to good use (when they aren’t grazing) by maintaining their energy supply with their jaws. For all the work the bacteria do, they’re fobbed off with 6 per cent of the total extracted energy. The efficiency of the system is reflected in the huge milk yield as well as in the squishy cow pats, which contain few nutrients. A horse does it differently. Instead of four extra stations it only has one, a three-part room en suite that comes after the small intestine rather than before it. This is where the large intestine is made ready for the bacteria. Hindgut fermentation like this is not as efficient as the foregut fermentation of a cow. That’s too bad for the horse, but it does result in lots of very fine horse droppings.
The small intestine isn’t the best place for fermentation because bacteria aren’t very good at dealing with enzymes, and the small intestine is full of them. So which is better: letting your plants ferment before or after the small intestine? That depends on the available food. In a juicy Dutch meadow it’s best for the plants to ferment first, in which case you become a ruminant, like the cow. But if grass is scarce or straggly you’d be better off as a horse, because then you don’t have to chew your cud and you can spend the whole day grazing. But what if you’re a rabbit? Even though a rabbit’s food is more nutritious than mere grass, it still needs an efficient digestive system because it’s so small. Small animals need more energy, relatively speaking, than large ones. Once it was thought that rabbits chewed their cud. The endearing way they have of wrinkling up their nose was taken for chewing. That almost cost the rabbit its head. According to the laws of ritual purity in the Authorised Version of the Bible, ruminants were fair game. Only the fact that the rabbit does not have a cloven hoof saved it from the Old Testament cooking pots. Only later was it discovered that in the Bible the word ‘rabbit’ was a translation error. While the Authorised Version speaks of rabbits, the Hebrew has ‘hyrax’, or dassie. Hyraxes may look a little like rabbits but they live like marmots, are related to the elephant, and move their jaws like ruminants, although they don’t chew their cud. Rabbits aren’t big on cud-chewing either. Their food is fermented like that of the horse, following the small intestine—not in the large intestine, however, but in the appendix. The appendix in a rabbit is a sizeable, vital organ, just as it is in many of the apes. We humans almost lost our appendix over the course of our evolution because we went from being herbivores to being omnivores and could easily live without it. For a rabbit, on the other hand, the appendix is essential. In addition to nutrients, a special kind of faeces is made there, softer than ordinary rabbit droppings, darker and more strongly scented. You never see these pulpy droppings out in the fields, and for a simple reason: rabbits eat them as soon as they’ve shat them, nice and juicy and covered in a membrane, straight from the anus. In this way the rabbit profits from the nutrients, vitamins and bacteria that would otherwise be lost. It really does resemble chewing the cud in that sense. Just as in cud-chewing, the extra energy is released mainly when the animal is at rest, giving it a 10 to 15 per cent energy boost.
As an omnivore, a human being can manage with a simple intestine. Human food is rich and digestible enough so there’s no need for cud-chewing, re-pooing, or foregut and hindgut fermentation. And in terms of length, a human intestine—which is longer than that of a carnivore but shorter than an herbivore’s intestine—is just what you’d expect of an omnivore. Unless you were Dr John Harvey Kellogg of cornflakes fame. He believed that with all that cooked food, human intestines weren’t as active as they had been in prehistoric times and it wouldn’t hurt to cut off a bit. Otherwise they just kind of hung there. Food got stuck in them and began to rot. The rottenness entered your bloodstream, and before you knew it you were pushing up the daisies. Fortunately, you could have your intestine shortened in the Mecca of American food fanatics and religious lunatics, Battle Creek, where Kellogg ran a sanatorium. Following in the footsteps of surgeon Arbuthnot Lane, who cut away substantial sections of the large intestine in order to give it a boost, Kellogg himself took scalpel in hand:
Of the 22,000 operations I have personally performed, I have never found a single normal colon.
Only later did it occur to Kellogg that not only can you adapt your intestines to the food you eat but you can also adapt your food to your intestines. That saves a lot of blood. His brother Will began running a factory for making special food for the large intestine, and cornflakes were born. In no time at all, cornflakes had driven ham and eggs from many an Anglo-Saxon breakfast table. To this day you still have to be careful about where you spend the night in America or England. Before you know it there’s a bowl of milk-soaked kibble staring up at you. The same thing is threatening to overtake the Dutch. Never have I understood how someone could trade a splendid breakfast food like coloured sugar sprinkles, or syrup that you can write your own name with, for chicken feed. Until I read Dr Kellogg’s loathsome little books. Finally I get it. Cornflakes aren’t for eating. They’re for shitting.