Gut: The Inside Story of Our Body’s Most Underrated Organ

by Giulia Enders

Every time we go to the toilet, it’s a masterly performance—two nervous systems working tirelessly in tandem to dispose of our waste as discreetly and hygienically as possible.

The first surprise is the sophistication of our sphincters. The vast majority of people are familiar only with the outer sphincter: the muscle we can consciously control, opening and closing it at will. There is another, very similar muscle close by—but this is one we can’t control consciously. Each of the two sphincters looks after the interests of a different nervous system. The outer muscle is a faithful servant of our consciousness. When our brain deems it an unsuitable time to go to the toilet, the external sphincter obeys and stays closed with all its might. The internal sphincter represents our unconscious inner world.

The main thing for the internal sphincter is to keep everything comfortable and in its place.

When what’s left of our food reaches the internal sphincter, that muscle’s reflex response is to open.

The space between the internal and external sphincter muscles is home to a large number of sensor cells. They analyze the product delivered to them, test it to find out whether it is solid or gaseous, and send the resulting information up to the brain. This is the moment when the brain realizes, “It’s time to go to the toilet!” Or maybe, “It’s just a bit of wind.”

when we are sitting or standing, our sphincters have to expend much less energy keeping everything in. If the lasso muscle relaxes, the kink straightens, the road ahead is straight, and the feces are free to step on the gas. Squatting has been the natural defecation position for humans since time immemorial. The modern sitting toilet has existed only since indoor sanitation became common in the late eighteenth century.

squatting does indeed lead to a nice, straight intestinal tract, allowing for a direct, easy exit.

Hemorrhoids, digestive diseases like diverticulitis, and even constipation are common only in countries where people generally sit on some kind of chair to pass their stool. This is due not to lack of tissue strength, especially in young people, but to the fact that there is too much pressure on the end of the gut. Some people tend to tense up all their abdominal muscles when they are stressed. Often, they don’t even realize they are doing it. Hemorrhoids prefer to avoid internal pressure like that by dangling loosely out of the anus. Diverticula are small light-bulb-shaped pouches in the bowel wall, resulting from the tissue in the gut bulging outward under pressure.

it remains a fact that the 1.2 billion people in this world who squat have almost no incidence of diverticulosis and far fewer problems with hemorrhoids.

Doctors believe that straining too much or too often on the toilet can also seriously increase the risk of varicose veins, a stroke, or defecation syncope—fainting on the toilet.

You can seek out these secrets with your tongue. These are four small points in your mouth. Two of them are located on the inside of your cheeks, opposite your upper molars, more or less in the middle. If you explore the area with your tongue, you will feel two tiny bumps. If they notice them at all, most people assume they must have bitten themselves in the cheek at some point, but they haven’t. These little nubs, which doctors call the parotid papillae, are found in the same position in everybody’s mouth. The other two points are lurking beneath your tongue, just to the right and left of the lingual frenulum, the fold of skin connecting your tongue to the floor of your mouth. These four little nubs supply your mouth with saliva.

The papillae in your cheeks secrete saliva whenever it’s needed right away—for example, when we eat. The two tiny openings under the tongue secrete saliva continuously. If you could somehow enter these channels and swim against the tide of saliva, you would eventually reach the main salivary glands. They produce the most saliva—about 1½ to 2 US pints (0.7 to 1 liter) a day. If you feel upward from your neck to your cheek, you will notice two soft, round raised areas. May I introduce you? They are the bosses.

THE SUBLINGUAL PAPILLAE, those two constant suppliers of saliva, are situated right behind our lower front teeth, which are particularly susceptible to the buildup of tartar. This is because there are substances in our saliva that contain calcium whose sole function it is to make our teeth harder. But if a tooth is constantly bombarded with calcium, it can be a case of too much of a good thing. Tiny molecules floating innocently by are caught up and “fossilized” without so much as a by-your-leave. The problem is not the tartar itself, but the fact that it has such a rough surface, affording a much better foothold for bacteria that cause tooth decay and gum disease than smooth, clean tooth enamel.

But what are fossilizing, calcium-containing substances doing in our saliva? Saliva is basically filtered blood. The salivary glands sieve the blood, keeping back the red blood cells, which are needed in our arteries, not in our mouth. But calcium, hormones, and some products of our immune system enter the saliva from the blood. That explains why each person’s saliva is slightly different. In fact, saliva analysis can be used to test for diseases of the immune system or for certain hormones. The salivary glands can also add extra substances, including those calcium-containing compounds, and even natural painkillers.

Our saliva contains one painkiller that is stronger than morphine. It is called opiorphin and ...

our mouth is such a sensitive thing. It contains more nerve endings than almost anywhere else in the human body. Even the tiniest strawberry seed can drive us crazy if it gets stuck somewhere. We feel every grain of sand in a badly washed salad. A teeny little sore, which we would not even notice if it were on our elbow, hurts like hell and feels monstrously big in our mouth—without our salivary painkiller, it would feel even worse!

When we chew, we produce more saliva and with it more of such analgesic substances, which explains why a sore throat often feels better after a meal and even minor sores in the oral cavity hurt less. It doesn’t have to be a meal—even chewing gum provides us with a dose of our oral anodyne.

Saliva protects the oral cavity not only from too much pain, but also from too many bad bacteria. That’s the job of mucins, for example. Mucins are proteins that form the main constituent of mucus.

A more useful function is their ability to envelop our teeth and gums in a protective mucin net. We shoot them out of our salivary papillae like Spider-Man shoots webs from his wrists. These microscopic nets can catch bacteria before they have a chance to harm us. While the bad bacteria are caught in the net, antibacterial substances in our saliva can kill them off.

The fact that we produce so little saliva at night explains why many people have bad breath or a sore throat in the morning.

Building a healthy immune system is not only important for warding off colds, it also has an important part to play in keeping our heart healthy and in controlling our body weight.

more and more researchers are now becoming interested in the link between the immune system and body weight.

One example of those who benefit from having their tonsils removed is psoriasis sufferers. In psoriasis, an overreaction of the immune system causes itchy skin lesions—often starting at the head—and painful inflammation of the joints. Psoriasis patients also have an above-average vulnerability to sore throats. One possible factor in this is bacteria, which can hide in the tonsils for long periods of time and needle the immune system from there.

Despite the problems, our body loves sugary sweet treats because they save the body work, since sugar can be taken up more quickly. The same is true of warm proteins. In addition, sugar can be turned into energy extremely quickly, and our brain rewards us for a rush of rapid energy by making us feel good. However, there is one problem: never before, in the history of humankind, have we been faced with such a huge abundance of readily available sugar. Some 80 percent of the processed foods found on the shelves of modern-day American supermarkets contain added sugar. On an evolutionary scale, then, we could say our species has just discovered the secret stash of candies at the back of the cupboard, and it keeps returning to binge on the booty before collapsing on the couch with a stomachache and sugar shock.

When we eat too much sugar, our body simply stores it away for leaner times. Quite practical, really. One way the body does this is by relinking the molecules to form long, complex chains of a substance called glycogen, which is then stored in the liver. Another strategy is to convert the excess sugar into fat and store it in fatty tissue. Sugar is the only substance our body can turn into fat with little effort.

Glycogen reserves are soon used up—just about the time during your run when you notice the exercise is suddenly much harder work. That is why nutritional physiologists say we should do at least an hour’s exercise if we want to burn fat. It is not until we pass through that first energy dip that we start to tap into those fine reserves. We might find it annoying that our paunch isn’t the first thing to go, but our body is deaf to such complaints. The simple reason for this is that human cells adore fat.

Fat is the most valuable and efficient of all food particles. The atoms are so cleverly combined that they can concentrate twice as much energy per ounce as carbohydrates or protein. We use fat to coat our nerves—just like the plastic on an electric cable. It is this coating that makes us such fast thinkers. Some of the most important hormones in our body are made out of fat, and every single one of our cells is wrapped in a membrane made la...

Unlike other nutrients, it cannot be absorbed straight into the blood from the gut. Fat is not soluble in water—it would immediately clog the tiny blood capillaries in the villi of the gut and float on top of the blood in larger vessels,

So fat must be absorbed via a different route: the lymphatic system. Lymphatic vessels are to blood vessels as Robin is to Batman. Every blood vessel inside the body is accompanied by a lymphatic vessel, even each tiny capillary in the small intestine. While our blood vessels are thick and red and heroically pump nutrients to our tissues, the lymphatic vessels are thin and filmy white in color. They drain away fluid that is pumped out of our tissue and transport the immune cells, whose job it is to ensure that everything is as it should be throughout the body.

Lymphatic vessels are so slight because they do not have muscular walls like our blood vessels. Often, they work just by using gravity. That explains why we sometimes wake up in the morning with swollen eyes. Gravity is not very much help when you are lying down. The tiny lymphatic vessels in our face are nicely open, but it is only when we get up and gravity kicks in that the fluid transported there during the night by our blood vessels can flow back down. (The reason our lower legs do not fill up with fluid after a long day on our feet is that our leg muscles squeeze the lymphatic vessels every time we take a step and that squeezes the...

All the body’s lymph vessels converge in an impressively thick duct, where all the digested fat can gathe...

But merrily drizzling your olive oil into the pan for frying is not such a good idea as heat can cause a lot of damage.

Cooking oil or solid fats such as butter or hydrogenated coconut oil should be used for frying. They may be full of the much-frowned-upon saturated fats, but they are much more stable when exposed to heat.

Fine oils are not only sensitive to heat, they also tend to capture free radicals from the air. Free radicals do a lot of damage to our bodies because they don’t actually like being free, much preferring to bond with other substances. They can latch onto almost anything—blood vessels, facial skin, or nerve cells—causing inflammation of the blood vessels (vasculitis), aging of the skin, or nerve disease. That’s why you should always close the bottle or container of olive oil carefully after use and keep it in the fridge.

The animal fats found in meat, milk, and eggs contain far more arachidonic acid than vegetable fats. Arachidonic acid is converted in our body into neurotransmitters involved in the sensation of pain. Oils such as rapeseed (canola), linseed, or hempseed oil, on the other hand, contain more of the anti-inflammatory substance alpha-linolenic acid, while olive oil contains a substance with a similar effec...

ONE THEORY ABOUT the origin of allergies begins with the digestive processes in the small intestine. If we fail to break down a protein into its constituent amino acids, tiny bits of it will remain. Under normal circumstances, those tiny particles simply don’t make it into our bloodstream and there is no problem. However, hidden power often lies in the most inconspicuous places—in this case, in the lymphatic system. Those tiny particles can enter the lymphatic system, embedded in fat droplets, and once there, they attract the attention of ever-vigilant immune cells. When the immune cells discover a tiny particle of peanut in the lymphatic fluid, for example, they naturally attack it as a foreign body.

The next time the immune cells encounter a peanut particle, they are better prepared to deal with it and can attack it more aggressively. And so it goes on, until we reach the stage where just putting a peanut in our mouth causes our immune cells to whip out the big guns straightaway. The result is increasingly severe allergic reactions, such as extreme swelling of the face and tongue. This explanation applies to allergies caused by foods that are both fatty and rich in protein, such as milk, eggs, and, most commonly, peanuts. There is a simple reason almost no one is allergic to greasy bacon. We are made of meat ourselves, and so we generally have few problems digesting it.

Another theory about how allergies develop is this: the wall of our gut can become temporarily more porous, allowing food remnants to enter the tissue of the gut and the bloodstream. This is the theory under most scrutiny from researchers who are interested in gluten—a protein found in wheat and related grains. Grains do not like us to eat them. What plants really want is to reproduce—and then along we come and eat their children. Instead of creating an emotional scene, plants respond by making their seeds slightly poisonous. That sounds much more drastic than it is—neither side is going to lose much sleep over a few guzzled wheat grains.

the more danger a plant senses, the more poisonous it will make its seeds. Wheat, in particular, is such a worrier because it has only a very short window of opportunity for its seeds to grow and carry on the family line.

In insects, gluten has the effect of inhibiting an important digestive enzyme. A greedy grasshopper might be put off by a little stomachache after eating too much wheat, and that is to the benefit of both plant and animal. In humans, gluten can pass into the cells of the gut in a partially undigested state. There, it can slacken the connections between individual cells. This allows wheat proteins to enter areas they have no business being in. That, in turn, raises the alarm in our immune system. One person in a hundred has a genetic intolerance to gluten (celiac disease), but a considerably higher proportion suffer from gluten sensitivity. In patients with celiac disease, eating wheat can cause serious infections or damage to the villi of the gut wall; it can also damage the nervous system. Celiac disease can cause diarrhea and failure to thrive in children, who may show reduced growth or winter pallor. The tricky thing about celiac disease is that it can appear in more or less pronounced forms. Those with more...

Gluten sensitivity, by contrast, is not a sentence to a life of gluten avoidance. Those with this condition can eat wheat without risking serious damage to their small intestine, bu...

The gut can also become porous for a short time after a course of antibiotics, after a heavy bout of drinking alcohol, or as a result of stress. Sensitivity to gluten resulting from these temporary causes can sometimes look the same as the symptoms of true gluten intolerance. In

The body requires a digestive enzyme to break that bond, but, unlike other enzymes, this one does not come from the papilla. The cells of the small intestine secrete it themselves on the tips of their tiny little villi. Lactose breaks down when it comes into contact with the enzyme on the gut wall, and the resulting single sugars can then be absorbed.

Unlike in celiac disease, however, no undigested lactose particles pass through the gut wall. They simply move on down the line, into the large intestine, where they become food for the gas-producing bacteria there. Consider the resulting flatulence and other unpleasant symptoms as votes of thanks from extremely satisfied, overfed microbes.

Every human being has the genes needed to digest lactose.

Recent estimates say about 25 percent of people in the United States lose their ability to break down lactose after weaning.

Fructose intolerance can also affect our mood. Sugar helps the body absorb many other nutrients into the bloodstream. The amino acid tryptophan likes to latch on to fructose during digestion, for example. When there is so much fructose in our gut that most of it cannot be absorbed into the blood and we lose that sugar, we also lose the tryptophan attached to

long-unrecognized fructose intolerance can lead to depressive disorders.

The fructose intake of the average American is currently close to 3 ounces (80 grams) a day. Our parents’ generation, consuming just honey on their toast, far fewer processed foods, and a normal amount of fruit, took in no more than ½ to 1 ounce (only around 16 to 24 grams) a day.

Serotonin not only puts us in a good mood, it is also responsible for making us feel pleasantly full after a meal. Snack attacks or constant grazing on snacks may be a side effect of fructose intolerance if they are accompanied by other symptoms, such as stomachaches.