Commentary: Why Is the Gut So ‘Emotional’?

by Ross Pomeroy

You don’t get a ‘sinking’ feeling in your feet, nor butterflies in your fingers, nor elation in your shoulders. You feel these sensations in your stomach. But why?

As RCS originally reported nine years ago, the gut is home to at least 100 million neurons, and perhaps as many as 500 million, by far the most outside of the brain. Concentrated in the lining of the gastrointestinal system, embedded in the esophagus and even the anus, these neurons constitute what scientists have dubbed the “enteric nervous system.” Through the vagus nerve, this ‘second brain’ has a direct line to the primary one in your skull, and as you’ve undoubtedly noticed, it likes to talk.

Sorting out all of these communications and their possible effects is now the focus of a burgeoning field: neurogastroenterology. It’s increasingly apparent that what goes on in the gut affects the brain and vice versa. Nowhere is this more obvious than with mood. More than 95% of the body’s serotonin, a hormone that stabilizes mood, feelings of well-being, and happiness, is found in the enteric nervous system. Is it any wonder then, that an empty stomach can make you irritable and impatient, clear signs of ‘hanger’? About half of the body’s dopamine is found in the gut, too. As the primary ‘pleasure hormone,’ it’s responsible for the bliss you feel while imbibing a milkshake or devouring a fried chicken sandwich.

The gut-brain relationship can go in the opposite direction as well – what’s going on in the brain can rouse the stomach. This explains ‘excited butterflies’ before a first date or ‘anxious aches’ as an important deadline approaches.

Sometimes the relationship can go awry. Habitually poor diet can result in depression. Chronic stress can contribute to irritable bowel syndrome. The gut-brain connection explains how mental health issues can trigger very real and debilitating physical symptoms.

Furthermore, recent research hints that diseases acquired in the gut can spread to the brain via nervous system connections, as well as vice versa. Autistic spectrum disorders, Parkinson’s disease, Alzheimer disease, and amyotrophic lateral sclerosis are a few of the maladies that may move between the two.

Even more enigmatic and arguably interesting are the effects that our 100 trillion gut bacteria might have on the enteric nervous system and even the brain, itself.

“Research (mostly in the laboratory, but some in humans) suggests that emotions can affect the gut microbiota, and that, conversely, certain gut microbes can be mind-altering,” Dan Gordon wrote in U Magazine.

“We have been cohabiting with these bacteria for hundreds of thousands of years, and we have developed a relationship we haven’t even started to understand,” Dr. Claudia Sanmiguel, program director of the Ingestive Behavior and Obesity Program in the UCLA Vatche and Tamar Manoukian Division of Digestive Diseases, told Gordon.

A study published in 2018 found that individuals with higher levels of a chemical metabolite called indole in their microbiome were more likely to eat for pleasure rather than hunger.

Research has also demonstrated that mice with and without microbiomes behave differently when placed in mazes. Gut bacteria could also contribute to neurodegenerative diseases by transferring misfolded proteins to the brain via the vagus nerve.

These findings may open doors to novel therapies, a potential not lost on supplement sellers, who market all sorts of probiotics that supposedly boost generic attributes like “digestion, health, and beauty.” Targeted probiotic infusions could very well prove efficacious for various disorders, including anxiety, depression, irritable bowel syndrome, and obsessive-compulsive disorder, but the data gathered thus far is not convincing. We’ll have to wait for new treatments and better data before we can conclusively say that treating the gut can also treat the brain and the whole body.

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Ross Pomeroy is a reporter at RealClearScience.