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, fr
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.
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.
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.
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.
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