The gut brain axis is the collective term for all the channels of direct and indirect communication now known to exist between the brain and the intestinal tract, providing a pathway for thoughts and feelings to influence the operations of the intestinal system and for the state of the viscera to affect all the ways the brain works. Over the past few decades, researchers have discovered that the brain and the gut communicate in many more ways than once thought and they talk about many things, from hunger to happiness to how much power your brain cells need to generate your thoughts.
In addition to direct nerve pathways through which the brain and the gut message each other, there are many biologically active substances produced in the gut through processes of digestion that enter the bloodstream or, through other means, affect the operations of the brain. What’s more, it has become clear that the actions in the gut are strongly influenced by the billions of bacteria that normally live in the intestines, collectively referred to as the microbiome .
For several decades, it has been known that the gut produces hormones such as ghrelin and leptin that send signals of hunger and satiety , respectively, to the brain. But only more recently have researchers discovered that there are many more things the gut and the brain talk to each other about and many more ways of doing so.
Key Takeaways
- Gut-Brain Axis affects mental, emotional, and physical wellbeing
- Understanding gut-brain axis is the first step toward managing it
- Evidence-based approaches can significantly improve outcomes
- Building daily habits is more effective than one-time interventions
Why Is the Gut-Brain Axis Important?
The gut-brain axis links the cognitive and emotional activity of the brain with the activity of the intestinal system, enabling talk between the two systems. Through the bidirectional communication network, the brain can influence the activities of the gut, and the activities of the gut, including the products of digestion, have an impact on all the ways the brain works.
The gut-brain axis is a channel by which the food we eat, after it is broken down by digestive enzymes and acted upon by the trillions of bacteria living in the gut, shapes our moods, our cognitive function, our reactivity to stress , our memory operations, how the brain ages, and much more. It’s also a pathway through which emotional experience can change the physiology of the body and affect how the intestinal system works.
Knowledge of the gut-brain axis opens a whole new channel not simply for understanding mental disorders but for treating them, too. Diet is becoming a necessary instrument of mental health and psychiatric treatment.
Psychiatrists have known for years that 60 to 70 percent of depressed and anxious people, for example, have gastrointestinal problems, such as irritable bowel syndrome (IBS). Approximately 84 percent of patients with IBS also have a depressive disorder and 44 percent have an anxiety disorder. Moreover, 45 percent of patients with anxiety and 30 percent of those with a depressive disorder develop IBS.
It was commonly assumed that the gut problems were peripheral to the mental disorders. But there is mounting evidence that they are related. Studies show that disruption in the gut bacteria—the result of a poor diet, a prolonged course of antibiotics, or chronic stress—affects the substances they produce, and many of those substances “speak” to the nervous system and brain. Many animals and human studies are now underway to better understand the connection.
In addition, the gut-brain axis engages in constant crosstalk with other major systems of the body, including the immune system and the neuroendocrine system, which plays a prominent role in the stress response. As a result, the microbiome is increasingly linked to immune function and immune disorders and a range of metabolic disorders as well as to mental health and psychiatric conditions.
How the Gut-Brain Axis Works
The gut talks to the brain constantly and rapidly. There are direct connections between the gut and the brain, the major one being the vagus nerve . The gut also produces neurotransmitters that relay messages to the brain. There are many indirect ties, such as short-chain fatty acids, substances produced by bacterial action in the gut that act on nerve pathways or circulate to the brain or stimulate processes that affect the brain. These activities are currently under intensive research, because they suggest new ways of approaching treatment of both psychiatric conditions and gastrointestinal problems.
Vagus means wandering, and the vagus nerve , the longest in the body—reaching from the brainstem to the abdomen, with branches to all the visceral organs—is a prominent component of the gut-brain axis. The vagus nerve oversees many body functions—heartbeat, for one—but as the main highway connecting brain and gastrointestinal tract, it sends information about the state of the gut to the brain, delivering it to important information about, say, hunger, or the need for specific nutrients.
In the course of digesting food, the bacteria of the microbiome produce many substances that act on the nervous system. Chief among them are neurotransmitters—including serotonin and GABA—known to be involved in many psychiatric disorders. There are many other neurally active substances produced in the gut as well, and they send signals to the brain via the vagus nerve or are directly transported to the brain by the vagus nerve. In addition, the vagus nerve brings to the brain news of the body’s inner sensations, an awareness known as interoception. Because it is a two-way road, the vagus nerve is also a pathway by which thoughts and feelings affect the operation of the intestinal tract.
The therapeutic implications of this understanding are huge. The vagus nerve can be used as a channel for improving disordered brain function; manipulation of the microbiome by diet is one significant way. It is also possible to directly target the vagus nerve as treatment for both psychiatric disorders and such somatic conditions as irritable bowel syndrome and inflammatory bowel disease.
Short-chain fatty acids (SCFAs) are an important class of biologically active substances produced in the gut, specifically by the action of gut bacteria on plant-derived foods containing fiber that is otherwise resistant to digestion, such as artichokes and legumes. SCFAs are emerging as important contributor to metabolism, immunity, and mental health. But exactly what those roles are is very much a developing story. It is safe to say that the more scientists discover, the more important SCFA’s are becoming to general and mental health.
Fibrous foods pass through the stomach and small intestine intact but meet a special group of bacteria in the lower intestines, or colon, that work them over by fermenting them— releasing energy, gases, and SCFAs, the most common of which are acetate, propionate, and butyrate.
The Gut-Brain Axis in Health and Disease
Because the gut-brain axis is a two-way channel, it provides a pathway for the gut and its operations to influence the brain and its activities and for brain states to influence the gut. Just as disturbances of the gut microbiome can undermine mental health, and disturbances in thinking can throw gut operations into disarray, so can the influence be positive. Growing understanding of the gut-brain axis not only puts a new emphasis on the role of diet and the composition of the microbiome in health and disease but is expected to provide new ways of intervening in disorders of both mental function and gut function.
Stress and the gut are intimately connected, and the gut microbiome is a major influence on adaptation to stress. Stress negatively affects the diversity and complexity of the microbiome. But in a cruel twist of fate, the very reactivity to stress is affected by the composition of the microbiome. The relationship between stress and the gut takes on special importance as stress susceptibility plays an outsize role in common psychiatric conditions such as anxiety, depression, and PTSD . And it is thought to influence the waxing and waning of symptoms in many other conditions, including bipolar disorder and schizophrenia.
At the simplest level, stress can promote consumption of highly palatable comfort foods, loaded with simple carbohydrates, and directly influence which gut bacteria thrive. In addition, stress can reshape the composition of the gut bacteria through the actions of the hormone cortisol, released by the brain's hypothalamic-pituitary-adrenal axis in response to the perception of threat to prepare the body to meet a challenge or flee from it. An array of changes occurs quickly, including alteration in the diversity and function of the gut bacteria. Studies show, for example, that gut bacteria have the ability to sense cortisol in their environment and change their gene expression in response to it.
Researchers have found that specific strains of gut bacteria influence fear learning and extinction, processes at the heart of anxiety conditions such as PTSD. In studies of one strain of beneficial gut bug, Bifidobacterium longum 1714 , fed to healthy men for a month, the subjects displayed reduced levels of stress measured psychologically and physiologically after being subjected to a challenge; researchers recorded a blunted hormonal response to stress.
Studies suggest that there are specific times during development when the threshold of stress responsiveness is set by the composition of the gut microbiome—just before and after birth, and again at adolescence, all periods when there is rapid growth of connectivity between brain cells. Preclinical studies indicate that shifts in the normal composition of the microbiome early in life—through Caesarean birth, lack of breastfeeding, antibiotic exposure, infection, or significant stress exposure—can enduringly influence stress physiology and influence the risk for depression and many other psychiatric disorders.
But there’s more. An increase in levels of cortisol, the physiologic hallmark of stress, increases the permeability of the gut. That, in turn, allows the leakage of bacterial substances that set off inflammatory processes throughout the body, including the brain, activating yet another pathway of disorder.
Diet and the Gut-Brain Axis
What you eat is a primary determinant of which bacteria get to thrive in your gut and, as a result, of how the gut-brain axis functions. Eating a largely plant-based diet, for example, supplies an abundance of digestion-resistant fiber, which in fact supports the growth of the very bacteria that are capable of breaking it down in the colon and releasing substances important to mental health, such as short-chain fatty acids.
Manipulation of the microbiome by diet and by diet supplementation is emerging as a promising pathway for the treatment of many disorders, including mood and other psychiatric disorders. It’s not clear yet what a stress-proof or depression-proof or anxiety-proof microbiome looks like, but the day may not be far off when mental distress is cured in the kitchen.
The Standard American Diet
The so-called standard American diet is an industrial triumph but, it is becoming increasingly clear, a biological disaster. Highly processed, laced with stabilizers and emulsifiers and other additives to give foods appealing texture and a long shelf life, it is loaded with fats and with simple carbohydrates, particularly sugar, which can be highly palatable but bias the gut microbiome in favor of bugs that make a cheap living off glucose and, especially, fructose.
At the same time, the standard American diet supplies little plant-based fiber—fewer than 15 grams a day, far less than the human body requires for optimal functioning. Lacking digestion-resistant complex carbohydrates, such a diet cannot sustain the bacteria in the colon that produce short-chain fatty acids, important for mental and physical health. The bacterial composition supported by such a diet contributes to leaky gut, inflammation, and is associated with disorders ranging from diabetes to schizophrenia.
In addition, the American diet contains a disproportionate amount of meat relative to fruits, vegetables, and other plant sources of food. A diet weighted toward animal proteins and fats disrupts the diversity of the microbiome in still other ways. For example, red meat is rich in the amino acid derivative L-carnitine. Eating red meat encourages the growth of the gut bacteria that are capable of metabolizing L-carnitine—but those very gut bacteria produce a substance, TMAO (trimethylamine N-oxide), known to contribute to atherosclerosis.
Then there are all the food additives in commercially produced foods, from breads and spreads to chocolate milk, ice cream, soups, and sauces. Most additives have no nutritional value, add no calories, and are eliminated in the feces unchanged. That turns out to be deceptive. Studies show that they nevertheless interact with bacteria in the gut in largely detrimental ways.
Even in healthy people food additives such as emulsifiers affect gut bacteria and create dysbiosis, altering both the composition of the microbiome and the way it functions. For example, common emulsifiers such as carboxymethylcellulose and polysorbate-80, used in baked goods, margarine, processed meats, chocolates, and other foods, have been found to wipe out healthy populations of gut microbes in ways that promote chronic gut inflammation, lead to inflammatory bowel conditions and metabolic disease, as well as to anxiety and disrupted mood states. Recent studies of many other food emulsifiers and stabilizers used in soups and sauces show that that the vast majority destabilize the gut, typically decreasing such beneficial bacteria as Akkermansia.
The standard American diet is saturated with simple refined sugar, that is, sugar stripped of its natural context of complex carbohydrates such as fiber. Americans consume an average of 77 grams of sugar daily—the recommended ceiling is 25 grams for women, 36 for men. While some of the sugar Americans consume comes from obvious sources such soft drinks, much of it enters the body surreptitiously; 68 percent of all packaged foods and beverages—from chicken nuggets to ketchup to salad dressings—contain added sugar.
Nutrition for a Healthy Microbiome
Establishing a balanced microbiome or resetting the balance of the microbiome is not only possible but likely to improve many aspects of health, including mental health, throughout the lifespan. Backing away from the overprocessed, fat- and sugar-rich standard American diet is likely to have the most far-reaching effects. Minimizing exposure to antibiotic drugs takes on a new importance for maintaining a healthy microbiome.
Then there are dietary actions to take on the positive side of the ledger to foster gut health and mental health. Eating foods that encourage the growth of specific groups of bacteria known to perform important functions, such as those that produce short-chain fatty acids, is well supported by data. In addition to their neural, metabolic, and immune importance, SCFAs help maintain the acidity of the gut, which in turn limits the growth of harmful bacteria. And there’s some evidence that consuming supplements with specific populations of beneficial bacteria is generally helpful and sometimes, such as following a course of antibiotic treatment, specifically warranted.
Nature supplies an abundance of foods that promote a healthy gut. Generally speaking, that equates to a diet tilted toward plants, and then in as close to their whole form as possible. Whole grains rather than refined flours. Whole fruit rather than juices.
Foods containing digestion-resistant fiber, composed of complex carbohydrates known as polysaccharides, are particularly beneficial to the gut. Because they nourish and prompt the proliferation and activity of beneficial bacteria that metabolize them (and release important bioactive substances from them), they are sometimes called prebiotics. All prebiotics are fiber, but not all fiber is prebiotic (some fiber resists even fermentation in the colon). Some prebiotics, such as pectin, abundant in citrus peels and apples, and inulin, found in oats, are soluble; often added to commercial foods as a thickener and stabilizer, pectin is one of the few food additives that has beneficial effects on gut bacteria.
A diet rich in fiber has long been associated with health. Consumption of dietary fiber is linked to reduced rates of all-cause mortality. Fiber is known not only to promote good bowel function but to prevent cardiovascular disease, for example. It also protects against diabetes, colon cancer, and obesity—diseases associated with the Western, or American-type, diet. The presence of fiber in foods slows down the release of sugar from the digestive tract, stabilizing blood sugar levels after meals. But it may be that the most far-reaching benefits of fiber are achieved through effects on the microbiome.
Probiotics is a term denoting microorganisms that confer health benefits to people. The term is also widely applied to foods that naturally contain live beneficial microbes—many kinds of yogurt and kefir, for example, and naturally fermented foods. But perhaps most often the term is used to refer to supplements containing various species of live bacteria that are beneficial to health.
Frequently Asked Questions
What exactly is gut-brain axis?
The gut brain axis is the collective term for all the channels of direct and indirect communication now known to exist between the brain and the intestinal tract, providing a pathway for thoughts and feelings to influence the operations of the intestinal system and for the state of the viscera to affect all the ways the brain works. Over the past few decades, researchers have discovered that the b
Is gut-brain axis a serious condition?
Gut-Brain Axis exists on a spectrum. While mild forms are a normal part of life, persistent or severe gut-brain axis can significantly impact daily functioning and quality of life. It's important to seek professional support if gut-brain axis is interfering with work, relationships, or wellbeing.
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