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

by Giulia Enders

The first thing a sea squirt does after setting up home is to eat its own brain.

Daniel Wolpert is not only an engineer and medical doctor who has won many academic honors, he is also a scientist who believes the sea squirt’s attitude to having a brain is very significant. His theory is that the only reason for having a brain is to enable movement. On first hearing, that might sound like an annoyingly mundane statement. But perhaps we just consider the wrong things mundane. Movement is the most extraordinary thing ever developed by living creatures. There is no other reason for having muscles, no other reason for having nerves in those muscles, and probably no other reason for having a brain. Everything that has ever been done in the history of humankind was only possible because we are able to move. Movement is not just walking or throwing a ball. It is also pulling faces, uttering words, and putting plans into action. Our brain coordinates its senses and creates experience in order to produce movement: movement of the mouth or the hands, movement over many miles or over just a few inches. Sometimes, we can also influence the world around us by suppressing movement. But if you’re a tree and can’t choose whether you move or not, you don’t need a brain.

signals from the genitals are sent to the upper central part of the brain, just below the crown. Fear is found in the middle of the brain—right between the ears, so to speak. Word formation is located just above the

temple. Morality is located behind the forehead, and so on.

Signals from the gut can reach different parts of the brain, but they can’t reach everywhere. For example, they never end up in the visual cortex at the back of the brain. If they did, we would see visual effects or images of what is going on in our gut. Regions they can end up in, however, include the insula, the limbic system, the prefrontal cortex, the amygdala, the hippocampus, and the anterior cingulate cortex. Any neuroscientists reading this will be up in arms when I roughly define the responsibilities of these brain regions as, respectively, self-awareness, emotion, morality, fear, memory, and motivation.

This does not mean that our guts control our moral thinking, but it allows for the possibility that the gut might have a certain influence on it.

This boils down to one of the basic questions of our existence: how intensely are we prepared to strive for something that we believe exists? That might be something concrete, like dry land beneath our feet or high school graduation.

Or it might be something abstract, like satisfaction or happiness.

Mice with depressive tendencies do not swim for long. They simply freeze, apathetically awaiting their fate. It seems inhibitory signals are transmitted more efficiently in their brains than motivational or driving impulses. Such mice also show a stronger reaction to stress. New antidepressants can normally be tested on these mice. If they swim for longer after receiving the medication, it is an indication that the substance under scrutiny might be effective.

They fed half their mice with Lactobacillus rhamnosus (JB-1), a strain of bacteria known to be good for the gut. Back in 2011, the idea of altering the behavior of mice by changing the contents of their gut was very new. And, indeed, the mice with the enhanced gut flora not only kept swimming for longer and with more motivation, but their blood was also found to contain fewer stress hormones.

Furthermore, these mice performed better in memory and learning tests than their unenhanced peers.

When scientists severed their vagus nerve, however, no difference was recorded betwee...

The vagus nerve is the fastest and most important route from the gut to the brain. It runs through the diaphragm, between the lungs and the heart, up along t...

Experiments on humans have shown that people can be made to feel comfortable or anxious by stimulating their vagu...

Cooperation between the gut and the brain begins very early in life. Together, they are responsible for a large proportion of our emotional world when we are babies. We love the pleasant feeling of a full stomach, get terribly upset when we are hungry, or grizzle and moan with wind. Familiar people feed, change, and burp us. It’s palpably clear that our infant self consists of the gut and the brain.

A gut that does not feel good might now subtly affect our mood, and a healthy, well-nourished gut can discreetly improve our sense of well-being.

After four weeks of taking a cocktail of certain bacteria, some of the areas of the subjects’ brains were unmistakably altered, especially the areas responsible for processing emotions and pain.

A healthy gut does not transmit minor, unimportant digestive signals to the brain via the vagus nerve. Rather, it processes them with its own brain—that’s why it has one, after all. If it thinks something is important, however, it may consider calling in the brain. By the same token, the brain does not transfer every piece of information to the conscious mind.

The brain’s bouncer is the thalamus. When our eyes report to the thalamus for the twentieth time that the same curtains are still hanging at the living-room window, it refuses entry to that information—it is not important for the conscious mind.

a report of an unusually large intake of alcohol will make it from the belly to the head, where it informs the vomit control center; information about trapped gas will reach the pain center; and the presence of pathogenic substances will be reported to the officer in charge of nausea. These stimuli make it through because the gut’s threshold and the brain’s doorman consider them important.

Some signals can cause us to fall asleep on the couch, contented and full after a big Christmas dinner.

chronic inflammatory bowel disease like Crohn’s disease or ulcerative colitis

Like patients with irritable bowel syndrome, sufferers of these conditions also show increased rates of depression and anxiety.

When the brain senses a major problem (such as time pressure or anger), it naturally wants to solve it. To do so, it needs energy, which it borrows mainly from the gut. The gut is informed of the emergency situation via the sympathetic nerve fibers, and is instructed to obey the brain in this exceptional period. It is kind enough to save energy on digestion, producing less mucus and reducing the blood supply. However, this system is not designed for long-term use. If the brain permanently thinks it is in an emergency situation, it begins to take undue advantage of the gut’s compliance. When that happens, the gut is forced to send unpleasant signals to the brain to say it is no longer willing to be exploited. This negative stimulus can cause fatigue, loss of appetite, general malaise, or diarrhea. As with emotional vomiting in response to upsetting situations, the gut reacts by ridding itself of food to save energy so it is available to the brain. The difference is that real stress situations can continue for much longer than minor upsets. If the gut has to continue to forego energy in favor of the brain, its health will eventually suffer. A reduced blood supply and a thinner protective layer of mucus weaken the gut walls. The immune cells that dwell in the gut wall begin to secrete large amounts of signal substances, which make the gut brain increasingly sensitive and lower the first threshold. Periods of stress mean the brain borrows energy, and, as any housekeeper knows,...

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.

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.

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.

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.

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?

Lactobacillus reuteri is able to inhibit pain sensors in the gut.

Hypnotherapy has been shown to be effective in treating patients with irritable bowel syndrome, reducing their reliance on medication—in some cases to zero.

antidepressants

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.

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 this age, we find it more difficult to deal with sudden change, but the payback is a more stable, calmer disposition.

our gut brain possesses the same neural receptors as the brain in our head. So, antidepressants automatically treat both brains.

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 ...

95 percent of the serotonin we produce is manufactured in the cells of our gut,

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.

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.

GRUMPINESS, HAPPINESS, INSECURITY, well-being, and worry do not originate in isolation in the mind.

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.”

insula, or insular cortex.

his theory that human self-awareness originates in the insular cortex.

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.