IIn the 19th century, a French Canadian named Alexis St. Martin was shot in the stomach while in a fur trading post when someone's musket accidentally fired at close range. He survived, but his injuries left a hole in his stomach wall. This provided an early insight - literally - into how our emotions and mental health affect the gut. Through careful experimentation, surgeon William Beaumont discovered that St. Martin's mental state had direct physiological consequences on the activity of his stomach: for example, when he felt irritable, his digestion slowed. Somehow his emotional states were expressed in the specific, local biology of his intestines.
Most people have experienced the gut consequences of their emotional feelings. Nerves before an exam can cause you to feel nauseous or even vomit. Deep sadness can cause you to lose your appetite, or perhaps create a hunger that is impossible to satisfy. Gut symptoms are common with mental illness, from appetite changes in depression to debilitating 'psychosomatic' stomach pain. Many of our feelings are gut feelings.
But the intestines don't just respond to emotional feelings: they also influence them. Take disgust. Disgust is deep-seated. Our stomach, like our heart, has a regular electrical rhythm; even just to see something disgusting is causing disruptions, called 'dysrhythmias', in this electrical signaling. Although disgust is critical to survival - to help us avoid disease and stay alive - in many mental health conditions, disgust becomes pathological. For example, in obsessive-compulsive disorder (OCD), dirt or germs can occupy a person's thoughts, causing symptoms such as compulsive handwashing. Self-disgust is common with depression and eating disorders. And even post-traumatic disorder can be caused by very upsetting traumas.
Pathological disgust is particularly difficult to treat: exposure therapy and other psychological approaches are much less effective than for fear-based mental health problems. A few years ago, when I was working as a neuroscientist at the University of Cambridge, I wondered whether abnormal signals from the stomach could trigger disgust avoidance. I conducted an experiment to test this hypothesis and found that altering someone's bowel activity with a common anti-nausea drug reduced disgust avoidance. This could be a new way to treat pathological disgust in mental disorders. For example, an anti-nausea medication could be administered immediately before exposure therapy, allowing patients to begin therapy in a more optimal intestinal state.
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Read more: How I learned to listen to what my feelings told meGut feelings are not 'all in your head', but also not 'all in your gut feeling'. Sensations from the intestines are transmitted to the brain via the vagus nerve, the main channel of information sent from the body to the brain. A second way to address 'gut feelings' is to electrically stimulate this nerve, which changes the electrical rhythm of the stomach. That said, the idea isn't new: vagus nerve stimulation for patients with major depression dates back to 2000.
A new theory, published in November 2023, states that vagus nerve stimulation amplifies signals from the internal body to the brain, helping us adapt our behavior to current challenges and needs. That could explain why the effects of vagus nerve stimulation are so far-reaching, altering learning, memory and motivation. This means that amplifying signals from the gut using vagus nerve stimulation may improve mental health in some cases, but may be ineffective or even harmful in others. Ultimately, we must consider the condition and needs of one's internal body before enhancing the body's influence on the brain.
But the importance of the vagus nerve extends to even more established treatments: Evidence from mice suggests that the most common type of antidepressants (SSRIs, or selective serotonin reuptake inhibitors) require the vagus nerve to work. This, too, could provide clues as to why antidepressants do or don't work for a particular person, and even help us understand why they can cause side effects in some people.
If the role of the vagus nerve helps us adapt to our physical needs, perhaps the most important internal need is energy. One function of the intestines, along with other organs, is metabolism, converting food into energy that the body can use. There are mysterious and extensive connections between our metabolic system and mental health. For example, the prevalence of depression in people with diabetes is two to three times higher than in the general population. It's not clear why: diabetes can increase the risk of depression, or vice versa. My lab is currently testing a third possibility: that common metabolic factors may increase the risk of both depression and diabetes because of interactions between the body and the brain. If we are right, this could provide opportunities for metabolic interventions that improve both physical and mental health.
Our brain and broader nervous system adapt to circumstances, including the body's internal metabolic needs, as well as our experience of the environment around us. As a result, your gut-brain connection is not static, but changes and adapts over time. A fascinating 2021 study found that brain cells can reactivate intestinal inflammation that an animal has previously experienced. The very 'memory' of intestinal inflammation, stored in cells in the brain, caused the physical condition in the body. Sometimes a 'gut feeling' actually comes from the brain. This role of the brain in 'gut feelings' means that our brains have the ability to produce dysfunctional gut symptoms solely through changes in the brain. This ability of the brain could also have benefits, which may explain why psychological therapy - which causes changes in the brain - can also treat certain intestinal disorders.
Gut feelings come from many sources: directly through the gut, through communication channels between the gut and the brain, or even through the brain itself. As we unravel the dynamic communications between the gut and the brain in neuroscience, we can begin to understand how these processes helped our ancestors survive - and how we can better harness them to improve emotional and mental well-being. A gut feeling can have many possible causes, but each of these represents a possible mental health solution.
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