Gut: The Inside Story of Our Body's Most Underrated Organ (Revised Edition)

by Giulia Enders

  One theory proposed by research bacteriologists is that stress is unhygienic. The altered circumstances stress creates in the gut allow different bacteria to survive there than in periods of low stress. We could say stress changes the weather in the gut. Tough guys who have no problem with turbulence will reproduce successfully—and at the end of the day they are not likely to spread good cheer in the gut. If this theory is true, that would make us not just the victims of our own gut bacteria, but also the gardeners of our inner world. It would also mean that our gut is capable of making us feel the negative effects long after the period of stress is over.

  Feelings from down below, especially those that leave a nasty aftertaste, will cause the brain to think twice next time about whether it really wants to hold a speech in front of the entire office, or whether we really should eat that super-hot chili. So the process of making decisions based on gut feeling may involve the gut recalling how it felt in similar situations in the past. If positive lessons could also be reinforced in the same manner, then the way to a lover’s heart really would be through their stomach—and straight to the gut.

  The interesting theory that our gut is not only involved in our feelings and in making “gut decisions” but also may influence our behavior is the subject of various research projects. A team at McMaster University in Canada led by Stephen Collins designed an ingenious experiment using two different strains of mice with very well-researched behavioral characteristics. Members of the strain called BALB/c are more timid and docile than those belonging to the NIH Swiss strain, which exhibit more exploratory behavior and gregariousness. The researchers gave the mice a cocktail of antibiotics that affect only the gut, wiping out their entire gut flora. They then fed the animals with gut bacteria typical of the other strain. Behavior tests showed they had swapped roles—the BALB/c mice became more gregarious and the NIH Swiss mice were more timid. This shows that the gut can influence behavior—at least in mice. The result cannot yet be applied to humans. Scientists know far too little about the various bacteria involved, about the gut brain in general, and about the gut–brain axis.

  Until scientists have filled those gaps in their knowledge, we can make use of the facts we already know to improve gut health. It starts with the little things like mealtimes, for example, which should be enjoyed without pressure, at a leisurely pace. The dinner table should be a stress-free zone, with no place for scolding or pronouncements like “You will remain at the table until you’ve finished the food on your plate!” and without constant television channel hopping. This is important for adults, but it is vital for small children, whose gut brain develops in parallel with their head brain. The earlier in life mealtime calm is introduced, the better. Stress of any kind activates nerves that inhibit the digestive process, which means we not only extract less energy from our food, we also take longer to digest it, putting the gut under unnecessary extra strain.

  We can play around with this knowledge and test it experimentally. There are tablets and medicated chewing gum that prevent travel sickness by numbing the nerves of the gut. When the nausea abates, feelings of anxiety often disappear, too. If unaccountable grumpiness or anxiety can originate in the gut (even without nausea), is it possible that these drugs could be used to banish them? By temporarily numbing a troubled gut, so to speak? Alcohol reaches the nerves of the gut before it reaches those in the brain—so how much of the relaxing effect of that “just one glass of wine” in the evening actually comes from a sedated gut brain? What about the array of bacteria in the wide range of yogurt on our supermarket shelves? Is Lactobacillus reuteri better for me than Bifidobacterium animalis? A team of Chinese researchers managed to show in the laboratory that Lactobacillus reuteri is able to inhibit pain sensors in the gut.

  Lactobacillus plantarum and Bifidobacterium infantis could already be recommended as a pain treatment for patients with irritable bowel syndrome. Many patients with a low pain threshold in the gut currently take substances designed to treat diarrhea, constipation, or cramps. That might help with the symptoms, but it does not address the cause of the problem. If, after eliminating possible food intolerances and restocking the gut flora, there is still no improvement, then we must take the problem by the scruff of its neck—or in this case, by the nerve-cell threshold. So far, very few treatments have been scientifically proven to be effective. One of those is hypnotherapy.

  Really good psychotherapy is like physiotherapy for the nerves. It eases tensions and teaches us how to move in a more healthy way—at the neural level. Because the nerves of the brain are more complicated creatures than the muscles of the body, a trainer needs to have more creative exercises up his sleeve. Hypnotherapists often use thought journeys and guided imagery techniques. These aim to reduce the intensity of pain signals and alter the way the brain processes certain stimuli. Just like muscles, certain nerves can become stronger with increased use. The therapy does not involve hypnotism as seen in television shows. That would, in fact, be self-defeating, since this kind of therapy relies on the patient being in control at all times. Patients must make sure the hypnotherapist they choose is recognized by a reputable institution.

  Hypnotherapy has been shown to be effective in treating patients with irritable bowel syndrome, reducing their reliance on medication—in some cases to zero. This is particularly true of children with this condition. For them, hypnotherapy has been shown to produce a 90-percent reduction in pain, compared with a 40-percent reduction produced by drugs. Some clinics offer specific hypnotherapy for abdominal complaints.

  Patients with intestinal disease who also suffer from extreme anxiety or depressive disorders are often recommended antidepressants by their doctor. However, they are rarely told why. And there is a simple reason for that: no doctor or scientists knows. It was not until they noticed the mood-enhancing effects of these drugs that scientists began to explore the mechanisms behind this phenomenon. They still have not come up with a clear answer. For decades, it was thought to be due to an enhancing effect of the so-called happiness hormone serotonin. More recently, depression researchers have also begun investigating another possibility—that such drugs may increase the plasticity of the nerves.

  Neuroplasticity is the nerves’ ability to change. It is nerve plasticity that makes puberty such a confusing time for an adolescent brain—so much is still being molded into shape. The possibilities are endless and nerves are constantly firing off messages in all directions in a pubescent brain. This process is not complete until we reach the age of about twenty-five. After that, nerves react according to well-rehearsed patterns. Patterns that have proved useful in the past are retained; others are rejected as failures. This explains the disappearance not only of the inexplicable fits of laughter and temper tantrums of the teenage years, but also of the posters plastering the bedroom walls. After this age, we find it more difficult to deal with sudden change, but the payback is a more stable, calmer disposition. This can also result in negative thought patterns taking root, such as “I am worthless” or “Everything I do goes wrong.” The nervous messages from a worried gut can also become embedded in a person’s mind. If it is the case that antidepressants increase neuroplasticity, they may work by loosening up such negative thought patterns. This is most beneficial when accompanied by effective psychotherapy to help patients resist slipping back into old habits.

  The side effects of commercially available antidepressants, such as Prozac, also provide us with important clues about the so-called happiness hormone serotonin. A quarter of patients report typical side effects such as nausea, an initial phase of diarrhea, and constipation when the drug is taken over a long period of time. This is explained by the fact that our gut brain possesses the same neural receptors as the brain in our head. So, antidepressants automatically treat both brains. The American researcher Dr. Michael Gershon takes this line of thought one stage further. He is interested in the possibility of developing effective antidepressants that only influence the gut and do not have an e
ffect on the brain.

  That is not as outlandish as it might first seem. After all, 95 percent of the serotonin we produce is manufactured in the cells of our gut, where it has an enormous effect on enabling the nerves to stimulate muscle movement and acts as an important signaling molecule. If its effects on the gut can be changed, the messages sent from there to the brain would also be changed enormously. This would be particularly useful in treating the sudden onset of severe depression in people whose lives are otherwise fine. Perhaps it is their gut that needs a session on the therapist’s couch and their head is not to blame at all.

  Anyone who suffers from anxiety or depression should remember that an unhappy gut can be the cause of an unhappy mind. Sometimes, the gut has a perfect right to be unhappy—if it is dealing with an undetected food intolerance, for example. We should not always blame depression on the brain or on our life circumstances—there is much more to us than that.

  Where the Self Originates

  GRUMPINESS, HAPPINESS, INSECURITY, well-being, and worry do not originate in isolation in the mind. We are human beings, with arms and legs, genitals, a heart, lungs, and a gut. Science’s concentration on the brain has long blinded us to the fact that our self is made up of more than just our gray matter. Recent gut research has contributed significantly to a new, cautious questioning of the philosophical proposition “I think, therefore I am.”

  One of the most fascinating parts of the brain that can receive information from the gut is the insula, or insular cortex. This part of the brain is studied by one of the most brilliant brains working in research today: Bud Craig. With superhuman patience, he has spent the last twenty years staining nerve fibers and tracking their paths through the brain. Eventually, he emerged from the lab and gave a one-hour talk revealing his theory that human self-awareness originates in the insular cortex.

  The first part of his hypothesis goes like this. The insula receives information about feelings from the entire body. Each piece of information is like a pixel. The insula then organizes these pixels to form an overall image. This image is important because it represents a map of our feelings. So, when we are sitting on a chair, we feel the cheeks of our behind pressed against the seat, and we might also feel cold or hungry, for example. Taken together, this gives the overall picture of a cold, hungry person sitting on a hard chair. We might not find this image particularly great, but it is also not awful—it’s just okay.

  Hypothesis, part two. Daniel Wolpert tells us the purpose of the brain is to create movement—irrespective of whether you are a sea squirt searching for a comfy rock beneath the sea or a human being striving for the best life possible. The aim of movement is to bring about an effect. The brain can use the insula’s map to plan meaningful movement. If I am sitting around feeling cold and hungry, other areas of the brain will be motivated to do something to change my situation. I could start shivering, or I could get up and head for the fridge in search of food. One of the main purposes of movement is to shift us constantly toward a healthy equilibrium—from cold to warm, from sad to happy, or from tired to alert, for example.

  Hypothesis, part three. The brain is an organ of the body. So, if the insula creates an image of the body, that image must also include the on-board computer in our head. It has some interesting areas, such as those responsible for social empathy, morality, and logic. The social areas of the brain might give rise to negative feelings when we argue with our partner; logic regions might induce despair when we try to solve a difficult puzzle. In order for the insula to create a reasonable image of our self, it probably also takes in perceptions of our environment and experiences from the past. So, when we are cold, we don’t just feel the low temperature, we are able to contextualize the feeling and think such thoughts as “This is weird. I’m cold, but I’m in a well-heated room indoors. Maybe I’m coming down with something?” Or alternatively, “Okay, maybe I shouldn’t be parading around naked in the conservatory in winter.” In this way, humans are able to react to the stimulus of feeling cold in a much more complex way than other animals.

  The more information we connect, the cleverer the movements we can make. In this respect, there is probably also a hierarchy among our organs. Information that is particularly important for the maintenance of a healthy equilibrium has more sway in the insula. The brain and the gut are well qualified to take a central role—if not the central role.

  So, the insula creates a picture of our entire feeling body. We can then use our complex brain to embellish this image. Bud Craig believes the picture is refreshed approximately every forty seconds. Through time, those images merge into a kind of movie—the film of the self, of our life.

  A great deal of what makes up this movie is certainly contributed by the brain—but not everything. It may be time to expand René Descartes’ proposition: “I feel, then I think, therefore I am.”

  (PART THREE)

  THE WORLD OF

  MICROBES

  LOOKING DOWN ON Earth from space, it is impossible to make out any human beings. The Earth itself is easily identified—a bright, round spot among the other bright spots in the darkness of space. Closing in a little, it becomes apparent that we humans inhabit all sorts of different places on the planet. Our cities shine at night as patches of light. Some groups are concentrated in big urban centers, others live scattered over wide areas. We inhabit the cool northern climes of Europe and North America, but we also occupy tropical rainforests and the margins of arid deserts. We are everywhere, even though we are invisible from space.

  Looking closer at human beings, it becomes clear that each of us is a world of our own. Our forehead is a breezy meadow, our elbows are arid wastelands, our eyes are salty lakes, and our gut is the most amazing giant forest ever, populated by the weirdest of creatures. Just as we humans occupy the planet, our bodies are occupied by a population that reveals itself only under the microscope—bacteria. Viewed at great magnification, they resemble bright, little spots against a dark background.

  For many centuries, humans concerned themselves with the human-sized world. We measured it, examined its flora and fauna, and philosophized about life. We constructed huge machines and flew to the Moon. Explorers keen to discover new continents today must turn to the microscopic world within us. Our gut is perhaps the most fascinating continent of that world. It provides the habitat for more species and families of creatures than any other landscape. Exploration of this region is only now beginning in earnest. A new sense of excitement is brewing among scientists, not unlike that associated with the decoding of the human genome and all the promise that holds for the future. Of course, this excitement about gut research could still fizzle and come to nothing.

  Work on an atlas of human bacteria did not begin until 2007. This project involves taking cotton swab samples from all over the bodies of lots and lots of people. Samples are collected from three different areas of their mouths, under their armpits, and on their foreheads. Stool samples are analyzed and genital smears are evaluated. Places once thought to be bacteria-free turn out to be populated after all—the lungs, for example. When it comes to drawing up a bacteria atlas, the gut is the supreme challenge. Of our entire microbiome—that is, all the microorganisms that teem on the inside and outside of our bodies—99 percent are found in the gut. Not because there are so few elsewhere, but because there are simply so inconceivably many in the gut.

  I Am an Ecosystem

  WE ARE FAMILIAR with bacteria—those little creatures consisting of a single cell. Some live in boiling hot springs in Iceland, while others luxuriate in the moisture on a dog’s wet nose. Some require oxygen to produce energy and “breathe” almost like us and others die when exposed to the fresh air. Those who don’t draw their energy from oxygen rely on metal atoms or acids—and that can result in some rather interesting smells. Almost every body smell is, in fact, produced by bacteria. From the pleasant scent of a loved one’s skin to the dog-breath of next-door’s frisky hound—it is all the product of the mic
roscopic world in us, on us, and around us.

  We like to watch athletic surfers as they ride the waves, but we are unaware of the spectacular surfing scenario that takes place in our nasal flora every time we sneeze. We pant and sweat when we exercise, but no one notices how delighted the bacteria who live in our running shoes are over the sudden, summery change in climate. When we take a sneaky bite of cake, we never hear the roar of excited bacteria in our gut, as they gleefully shout, “HERE COMES CA-A-A-KE!” It would take an entire international news agency to report on all the events constantly unfolding in just one person’s microbiome. While we lounge about feeling bored, any number of exciting things are happening inside us.

  People are slowly beginning to realize that the vast majority of bacteria are harmless, or even helpful. Some facts are now known to science. Our gut’s microbiome can weigh up to 4½ pounds (2 kilos) and contains about 100 trillion bacteria. One-thirty-second of an ounce (1 gram) of feces contains more bacteria than there are people on Earth. We also know that this community of microbes cracks open indigestible foodstuffs for us, supplies the gut with energy, manufactures vitamins, breaks down toxins and medications, and trains our immune system. Different bacteria manufacture different substances: acids, gases, fats. You could say bacteria are tiny factories. We know that gut bacteria are responsible for blood groups and that harmful bacteria cause diarrhea.