• When you think of the gastrointestinal tract, you might not immediately think of emotions and mood disorders, but the worlds of GIs and psychologists are starting to collide in important ways.
  • The all-important gut-brain axis refers to the communication pathways between the central nervous system and the enteric nervous system.
  • Gut microbial ecosystems can influence brain function, and the experience of stress can influence microbiota.
  • Experts are still uncovering the role of gut operations in health and disease, but meanwhile psychotherapy has been shown to help functional digestive issues like IBS.

Most people like to think of their brains and their bowels as completely separate units. Gray matter is where the higher thinking happens, where one contemplates art and one’s place in the universe. The gut is where our breakfast cereal goes. It’s a region of the body we only tend to think about when it’s malfunctioning. And at that point our digestion is pretty much all we can think about.

But some of the most exciting research in psychology today reveals the intimate connection between our gut and our brain, aka the gut-brain axis, or GBA. It turns out that gray matter is in constant communication with the gastrointestinal (GI) tract. Experts have even dubbed our gut the “second brain” because study after study demonstrates its importance to our conscious experience. Higher cognition is actually “lower” cognition, aka the alimentary canal, and vice versa. 

So how does this bidirectional relationship work, and what does it mean for our mental health? This article gives a broad, but detailed overview of your gut-brain axis. It might go too deep at times, but on the bright side, at least it’s only a figurative colonoscopy.  

The Link Between Gut Health and Mental Health

When experts talk about the gut-brain axis, they’re basically talking about the dense connections between our enteric nervous system (ENS) and our central nervous system (CNS). The ENS has its own nerve cells (enteric neurons)–up to 600 million of them–and it can actually behave independently of the CNS… but it doesn’t. Instead, it’s constantly chattering with the CNS through the vagus nerve, the immune system (70-80% of which resides in the gut), and our gut microbiota (more on that below). 

In this interactive environment, the gut and the brain speak the same language. For example, both the gut microbiota and the brain (which boasts 86 billion neurons) converse with the same neurochemicals, namely serotonin, norepinephrine, dopamine, acetylcholine, melatonin, and gamma-aminobutyric acid (GABA). They also have similar anatomical structure and signaling pathways

How Does Our Microbiome Affect Our Mental State?

Every human body hosts trillions of microorganisms, which together forms its unique microbiome. Biologists used to think that the cells of microorganisms in our bodies outnumbered human cells by about 10:1, but they’ve since revised that estimate to say that our cellular makeup is only about half inhuman. So that’s good news? Collectively, these microscopic colonizers (bacteria, archaea and eukarya) don’t weigh much–less than half a pound–but they seem to have an outsized impact on our physical and mental health.

A microbiota is a community of microorganisms within a specific location. In comparison with our other organs, our gut has the highest population of microbiota–one hundred trillion of them. But notoriously, the human gut microbiota has a tendency to get out of whack, or enter a state of dysbiosis, at least in the modern, Western world. And when we alter our gut microbiota, we also risk compromising our gut integrity–and our moods. 

Which brings us to inflammation. These days, inflammation seems to be implicated in a multitude of serious health problems, and unfortunately this article is just going to pile on. In the case of the gut-brain axis, gut microbiota can act as crucial regulators of epithelial (gut lining) health. Increased permeability of the intestinal epithelial barrier (i.e. “leaky gut”) can lead to inflammation. And because of the robust feedback loop between the brain and the gut, the brain is well aware when your gut is inflamed. And it tends to react defensively. 

Furthermore, research suggests that up to 95% of our serotonin (5-HT) is produced by gut microbiota. As you probably know, serotonin is associated with emotional states (moods)–but also with GI function and motility (the stretching and contracting of GI muscles). The gut also produces up to 46% of the human body’s dopamine, the so-called “pleasure” neurotransmitter. 

You can begin to see how all these points of gut-brain contact might be highly suggestive of a symbiotic relationship between mental well-being and the GI tract. And recent evidence seems to bear this out. 

The Gut-Brain Axis and Psychiatric Disorders

Roughly 40% of humanity suffers from functional gastrointestinal disorders (FGIDs)–and psychiatric issues are overrepresented in those populations. Let’s take irritable bowel syndrome (IBS) as an example. Is IBS a first brain problem or a second brain problem? People with IBS experience digestive issues like abdominal cramping and pain. They also tend to suffer from anxiety and depression more than people without IBS. 

Now, two things could be going on there. The first is that people with IBS are just responding emotionally to their digestive burden. They have to worry about finding a bathroom in time; they’re frustrated that their lives are being disrupted. The second is that intestinal disease and psychological disorders share an origin story.

Recent studies show that mental health conditions and neurodegenerative diseases as diverse as depression, autism spectrum disorder (ASD), schizophrenia, Alzheimer’s, and amyotrophic lateral sclerosis (ALS) are all associated with some form of dysbiosis in the gut microbiota. Further, researchers have actually been able to manipulate the behavior of mice by altering their microbiomes. For instance, they have been able to incite anxious states or calm states in mice through the transplant of certain strains of gut bacteria. Researchers have also demonstrated that mice with beneficial microbes in their guts actually experience less stress, which is important because chronic elevated levels of the stress hormone cortisol can lead to a systemic inflammatory response. 

When gut health is compromised and the immune system is responding with inflammatory cytokines, brain chemistry can be affected in ways that lay the groundwork for depression and anxiety. Or as very smart people say, “Psychiatric disorders and IBS appear to have bidirectional co-morbidities.” In fact, IBS patients also have other psychiatric disorders at rates of 54-94%

This is a huge, fascinating subject, and it’s impossible to cover all the emerging science in a brief article, but rest assured that the evidence is there to show that not only can gut microbiota influence how we think and behave, but our brain chemistry can also affect our gastrointestinal operations.

What Are Psychobiotics?

If the gut is continually signaling our brain and the brain is continually signaling our gut, then it would stand to reason that by helping one entity, you might help the other. And now we have landed at the mind-body connection. As Dr. Jay Pasricha, Director of the Johns Hopkins Center for Neurogastroenterology, states, “Our two brains ‘talk’ to each other, so therapies that help one may help the other. In a way, gastroenterologists… are like counselors looking for ways to soothe the second brain.” 

And studies have shown that not only can probiotic supplements (beneficial microbes as medicine, aka psychobiotics) and non-probiotic interventions (like adjusting one’s diet) help ease depression and anxiety, but antidepressants like selective serotonin reuptake inhibitors (SSRIs) and interventions like cognitive behavioral therapy (CBT) can help alleviate symptoms of IBS. Stress reduction techniques can also help people with functional bowel issues. The same interventions that act on nerve cells in the gut also seem to act on nerve cells in the brain.

But it’s not all about medications, many of which seek to mimic the natural metabolites of gut microbiota. Diet also plays a major role in establishing a healthy microbiome, whatever that may be. And since we’re all born with empty palettes in our intestines, childbirth itself, with all of its microbial excitement, can start shaping mental health outcomes straight from the get-go. 

Lastly, studies show that people in the developed world have less diversity in their microbiomes than non-western microbiomes. And when animals move from the wild to captivity, the microbe communities in their guts narrow and start conforming more to a human’s. The captive animals also tend to become anxious and depressed

Our knowledge of the gut-brain axis is still emerging, and there’s a big difference between animal models and studies on humans. But if you’ve ever felt sick to your stomach when anxious or scared, you may sense–deep in your gut–that these early GBA theories are on the right track. Maybe one day we’ll address our viscera as much as our conscious thoughts in our quest to achieve happiness.