The Mind-Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health

by Emeran Mayer

Those in the control group ingest a placebo—a food that is indistinguishable from the treatment in appearance, taste, or flavor, but that has no known intrinsic action. To increase the reliability of such a study, neither the study participants nor the investigators are allowed to know until after the study is completed which treatment group a subject was assigned to.

By consuming certain types of probiotics—either contained in naturally fermented foods or enriched in dairy products or fruit juices—that can regulate levels of the vital neurotransmitter serotonin, we may be able to fine-tune a control system in our body that plays such a crucial role in many of our vital functions, ranging from mood to pain sensitivity and sleep.

Does the brain of a depressed patient send signals to the gut that changes the composition and function of the gut microbes, or do the microbes of depressed patients play a causative role in the patients’ symptoms. If the second hypothesis is correct, changing the signals the gut microbes send to the brain by the consumption of a probiotic or a particular diet could alleviate depression symptoms.

These experiments revealed that gut microbes can influence the development of the brain’s stress response at an early age.

He diagnosed traveler’s diarrhea, a common form of gastroenteritis that’s typically caused by bacteria in the local water.

endorphins when we’re happy, oxytocin when we feel close to our spouse or children, and dopamine when we’re longing for something.

Like many people with ASD, Jonathan had always suffered from a range of gastrointestinal problems, which in his case included abdominal bloating, pain, and constipation.

Like many patients with ASD, he had already tried several diets, including a gluten-free diet and a dairy-free diet, without any lasting benefit. His unusual day-to-day diet was not helping him, either, but that wasn’t surprising. He ate almost no fruits or vegetables, as he disliked both their texture and smell. Instead, his diet consisted largely of refined carbohydrates, including pancakes, waffles, potatoes, noodles, pizza, snacks, and protein bars, as well as some meat and chicken.

A series of studies in recent years has suggested that patients on the autism spectrum may have an altered mix of gut microbes relative to individuals without ASD symptoms, including proportionally more of a bacteria group known as Firmicutes and less of a group called Bacteroidetes. Patients with irritable bowel syndrome exhibit a similar pattern.

Even more astonishing, we humans differ very little from each other genetically, sharing more than 90 percent of our genes, but the assortment of microbial genes in our guts differs dramatically, and only 5 percent of them are shared between any two individuals.

Are the microbes influencing what goes on in our brains, our feelings, and emotions, or are the signals the brain sends to the gut based on a particular emotional state influence the gut microbes? Based on what we have learned so far, both mechanisms are at play, forming a circular, reverberating communication circuit which can be triggered or modulated both from the brain or from the gut.

You feel when something’s fishy. You sense when you have an instant personal bond with a stranger. You are positive that the charismatic politician on television is lying through his teeth. Gut feelings reflect an extensive and often deeply personal body of wisdom that we have access to, and that we trust more than the advice provided by family members, highly paid advisors, and self-declared experts or social media.

A hungry baby left in its crib to cry for an hour perceives the world very differently from the baby who is quickly picked up, cradled, and fed. Thus your earliest gut feelings serve as a model for “what the world is like and what I must do to survive in it.”

How? Some of it has to do with mother’s milk, which contains something akin to Valium. The gut microbes in all infants are adapted to optimally metabolize the complex carbohydrates in breast milk. One of the microbes best suited for this is a certain strain of lactobacillus that makes a metabolite of GABA—a substance that acts on the same brain receptors as the anxiety-reducing drug Valium. By producing endogenous Valium, a microbe may help to calm down babies’ emotion-generating system in the brain, and make them feel good by relieving them of hunger pangs. Human breast milk also contains complex sugars that are not only essential for the baby’s developing gut microbiome, but may also contribute to a baby’s sense of well-being when it’s fed.

In my experience with patients, many women seem to be better at listening to their gut feelings and making intuitive decisions than men are.

However, research by our group and others suggests that women tend to show greater sensitivity to the brain’s salience and emotional arousal systems attuned to physical feelings like abdominal pain and emotional feelings like sadness or fear, than men do.

In North America today, it’s hard to get away from an unnatural diet, one that’s full of sweeteners, emulsifiers, flavorings, and colorings, with extra fat, added sugar, and vital gluten, and loaded with calories.

And learning more about our true ancestral diet may even provide some answers to the never-ending debate over which diet is best for our bodies and minds: the high-fat/high-protein, low-carb variety, the high-fruit and -vegetable omnivore diet, the extremes of the vegan diet, or the tasty compromise of the Mediterranean diet.

a growing number of such studies is the fact that they consistently show that people living on the typical North American diet had lost up to one-third of their microbial diversity compared to individuals living a prehistoric lifestyle.

greater diversity and richness in a host’s microbial species and their metabolites is associated with greater resilience in the face of infections, antibiotics, variable nutrient supply, carcinogenic chemicals, and chronic stress.

In addition to the initial shaping of a baby’s gut microbiome during birth, the food the child receives from her mother plays a crucial role in this process.

But before he had consumed any formula or a bite of solid food, gut microbes such as Prevotella appeared that could metabolize complex carbohydrates from plants. This meant that the baby’s gut microbiota were prepared for solid food before the baby had ever eaten any.

From this and other studies, it’s now clear that those first two and a half to three years shape our gut microbiome for a lifetime.

We know today that it’s the infant’s food supply, in particular breast milk, which helps her gut fill with the initial healthy mix of microbes. Keep in mind that the composition of breast milk is crucially dependent on the diet the mother consumes.

breast milk contains prebiotics—compounds with the ability to feed particular groups of gut microbes. Specifically it contains oligosaccharides—complex carbohydrates made of three to ten linked sugar molecules—that are essential in shaping the baby’s gut microbiota by selectively promoting the growth of beneficial bacteria.

HMOs are the only food that has evolved strictly for the purpose of feeding the infant’s microbiota.

The results of the German study suggest that it does so in part through the action of oxytocin. A variety of sensory stimuli cause oxytocin release in the brain: gentle touch, nursing a child, or certain gut sensations caused by nutrients. The hormone is released in the brains of both the nursing mother (where it stimulates the flow of milk) and her infant. Oxytocin promotes affiliation and bonding, suggesting that oxytocin release during nursing enhances mother-child bonding.

All this means that when you are born into Western civilization, you acquire a Western microbiome as well. Even if you go vegan today, your gut microbiota will remain that of a typical omnivore, and even if you eat a paleo diet for the rest of your life, your gut microbiota won’t turn into that of a hunter-gatherer.

Most undigested plant-derived carbohydrates are metabolized by microbes in the colon into short-chain fatty acids such as butyrate—so named because it has a buttery odor—and acetate, as well as gases such as carbon dioxide, methane, and hydrogen sulfide (which gives stool a bad odor).

It’s well known that this excessive animal fat intake, together with excessive sugar intake, is a contributing factor to the American obesity epidemic.

ancestors evolved in an environment of not only a limited and hard-to-obtain food supply, but also the near absence of foods high in fat and refined sugars. In other words, evolution never anticipated the standard American diet of today.

But today’s animal products are fundamentally different from what our ancestors ate, and what their few remaining direct descendants, living in isolated prehistoric societies, continue to eat.

It’s clear that the increased availability of animal protein has had significant benefits. It has played a major role in enabling our brains to grow larger over the course of human evolution, and it has helped increase our average height over the past century. But in contrast to our ancestors’ protein supply, our livestock often live out their lives in small pens, eating feed (like corn) that their digestive systems are not built to handle, and which is designed to fatten them as efficiently as possible. They ingest antibiotics and other chemicals, which reduce the diversity of their gut microbes and make them more vulnerable to serious gut infections. For all these reasons, the meat, eggs, and milk that come from these animals—and derivatives of these products (often no longer recognizable as food) in today’s processed food—are dramatically different from only fifty years ago, and they have fundamentally altered our diet.

When you’re hungry, enteroendocrine cells interspersed within the cells lining your stomach release a hormone, called ghrelin, also known as the hunger hormone, which either travels through the bloodstream to the brain or stimulates the tips of the vagus nerve in the gut to signal the brain directly. On the other hand, when you’ve had enough to eat, a different group of appetite-suppressing hormones (including cholecystokinin and glucagon-like peptide) are released from enteroendocrine cells in your small intestine, and these hormones turn the system off and suppress appetite.

They had previously shown that the satiety hormone cholecystokinin, released by cells in the gut in the presence of fat, was able to switch these nerve endings from a “hunger mode” to a “satiety mode.”

high fat intake can have an emotionally comforting effect. We have already learned how the gut, its enteroendocrine cells, and the vagus nerve respond to the presence of fat in the small intestine. Based on these interactions, we can speculate that the fatty acids improved the subjects’ mood by stimulating the release of signaling molecules from the gut, which reached emotional brain regions via the circulation or via increased signaling of the vagus nerve.

How much food you eat is controlled by three closely interacting systems in your brain: in addition to the appetite control system regulated by the hypothalamus, there are two other brain systems that play a prominent role: the dopamine reward system, and the executive control system, located in your brain’s prefrontal cortex, which can voluntarily override all other control systems if needed.

astonishing neurobiological similarities between the brain mechanisms that underlie substance abuse and chronic overeating. Based on questionnaire data, it’s estimated that at least 20 percent of obese individuals suffer from food addiction. Certain foods, especially high-calorie foods rich in fat and sugar, have been shown to trigger addictive eating behavior in both animals and humans.

In environments where such foods are plentiful and ubiquitous, however—as it is in many parts of the world today—this property has become a dangerous liability. In modern society, palatable foods, like drugs of abuse, represent a powerful environmental trigger, which can facilitate or exacerbate uncontrolled eating behavior in vulnerable individuals.

Indeed, today’s farm animals are kept completely separate from their natural environments and food supplies (grass) for most of their lives. Fattening of the animals with corn, a food source unsuitable for the cows’ digestive system, leads to diseases of their digestive system, resulting in a chronic, low-grade inflammatory state and often superimposed acute gastrointestinal infections that require continual administration of antibiotics.

we can’t escape the suspicions that the products that come from such chronically diseased animals are not good for our gut microbiota and not beneficial for our health. So the next time you buy milk, eggs, steak, or pork chops in the supermarket, be aware that they probably came from animals whose brain-gut-microbiome axis has been severely modified by the deplorable conditions in which they’re raised, the chronic stress that is associated with these living conditions, the unnatural diet they’ve been fed (not suitable for their digestive system), and the medications they’ve received—all of which pose unknown risks for the optimal function of our gut-microbiota-brain interactions and for our own health.

Industrial farming of corn, soybeans, and wheat is heavily dependent on fertilizers and pesticides, used to artificially maintain the growth and dominance of these plants over competitive plant species such as weeds and to defend them against pests and harmful insects.

One of the key reasons why ever-increasing amounts of chemicals are needed to maintain the “health” and dominance of these plants is the fact that these monocultures of often genetically modified single-crop fields, stretching across the landscape for miles, have completely lost their natural diversity in terms of both the genetic variety of the crops themselves and the variety of other species that coexist with them.

the collateral damage on our gut microbiome of the increasing deployment of weed killers (such as the notorious glyphosate, or “Roundup”)—necessary to overcome the weeds’ resistance to such chemicals—remains largely unknown, at least to the consumer.

three commercially available sweeteners—saccharin, sucralose, and aspartame—can induce glucose intolerance and signs of metabolic syndrome in mice.

sweeteners changed metabolic pathways in gut microbes so they produce more short-chain fatty acids, which can be absorbed by the colon, providing additional calories. This means that when you consume artificial sweeteners, your body enlists your gut microbiota to harvest more calories in the colon from the microbial metabolic products to compensate for the missing sugar available in the small intestine. It suggests that trying to cut calories with artificial sweeteners won’t work because your gut, with the help of its microbes, will just extract proportionally more calories from the food you eat.

these detergent-like molecules come with a downside. They can disrupt the protective mucus layer that covers the inner surface of the gastrointestinal tract, giving gut microbes easier access to the gut lining. Food emulsifiers also can disrupt the tight seal formed by the intact intestinal lining, enabling gut bacteria to cross and gain access to nearby immune cells, promoting metabolic toxemia.

Celiac disease has been on the rise for sixty years, and now it affects 1 percent of people worldwide. No one knows exactly why. One proposed hypothesis is the increased consumption of gluten-containing foods; another is a change in the immune system, possibly related to the alterations in the way the gut-based immune system is trained early on in life by interacting with foreign microorganisms. A third hypothesis is related to alterations in how wheat has been modified and is grown.

High fat content, artificial sweeteners, food emulsifiers, and other factors in our diet may have altered the set point of the myriad of sensors within our gut, including many of the receptors on nerve endings, enteroendocrine cells, and immune cells. Remember, the gut is our most complex sensory organ. Such changes may have altered the signals our gut sends to the enteric nervous system and to our brain. Is it possible that people with the most sensitive guts—people like Linda Schmidt—are now showing signs of food sensitivities and food allergies that they might not previously have developed?

As the earliest symptoms of constipation develop in the gut, is it possible that Parkinson’s disease begins in the gut and gradually makes its way to the brain? Could Parkinson’s disease be a gut-brain disorder? And could the gut’s microbiome be one of the culprits? Based on exciting new scientific evidence, the answer to all these questions may be yes. It