Microbes, Mood, and Mental Health: How Do Our Gut Bacteria Affect How We Feel?

If you’ve ever gotten a stomachache before an important presentation, had a low appetite after receiving stressful news, or felt the fluttering of butterflies during a first date, you’ve experienced firsthand the relationship between your gut and your mind. 

While we know that the collection of bacteria, fungi, and other microbes in our gastrointestinal tracts make up the gut microbiome, you may be less familiar with the ‘psychobiome.’ This recently coined term describes how our gut microbes affect the ways we think, feel, and act.

The link between microbes and the mind has been acknowledged for centuries, with the ancient Greeks believing mental disorders came from the digestive tract producing too much “black bile.” Although we’ve come a long way since then, researchers are still uncovering the myriad ways that the health of our guts also affects the health of our mind, mood, and mental status.

The Gut-Brain Axis: Is the Gut Our “Second Brain”?

The intricate crosstalk between the gut and the brain plays a vital role in both gastrointestinal function and several aspects of mental and cognitive health. Referred to as thegut-brain axis, these two organs have a bi-directional relationship and are constantly in communication — so much so that the gut has been dubbed our “second brain.” 

Evidence of this can be seen in people with digestive disorders. For example, many patients with irritable bowel syndrome (IBS) also experience depression or anxiety, and stress often exacerbates symptomatic flares of inflammatory bowel disease (IBD). 

There are several routes by which intestinal microbes can act on the brain: 

1. Gut microbes cause newly discovered sensory cells in the gut called neuropod cells to stimulate the vagus nerve. (The “pod” in neuropod comes from their foot-like projection that extends outward to form a synapse-like connection with nerve cells, including the vagus nerve.) The vagus nerve leads a direct connection to the brain and is a central component of the gut-brain axis, as it runs from the base of the brain to the abdominal organs. 
2. A second route is through messenger molecules, which get secreted by gut microbes and can permeate blood vessels and travel directly to the brain. 
3. In an indirect route, gut bacteria trigger enteroendocrine cells to send signals throughout the body. Enteroendocrine cells live in the gut lining, releasing hormones and compounds that regulate digestion and insulin secretion. They also promote the release of serotonin — our “happy” neurotransmitter — from the gut into the rest of the body, influencing brain health and mood. 
4. Lastly, gut bacteria indirectly modulate immune cells and inflammatory pathways that affect the brain. Recent research has linked increased inflammation to brain-related disorders, including autism and depression. 

 Recent Research on the Microbiome, Mood, and Mental Health

Although scientists have known about the gut-brain relationship for decades, it has only been over the past few years that research on the role of gut microbes on mental health disorders has exploded. 

From mood and memory to behavior and brain cells, researchers are now uncovering much more about the psychobiome — and how specific bacteria may soon be used as treatments to help us feel better.  Let’s take a closer look at how the microbiome affects four common mental health concerns: depression, bipolar disorder, anxiety, and stress.

1. Major Depressive Disorder

Major depressive disorder (MDD), or clinical depression, is a highly prevalent mental health condition, affecting over10% of the global population. 

It’s thought that gut microbiome disturbances play a role in depression in several ways. These include impacting the release of the “feel-good” neurotransmitters serotonin and dopamine, causing an abnormal function of the hypothalamic-pituitary-adrenal (HPA) axis and triggering pro-inflammatory responses. 

In asystematic review of 24 observational and experimental studies, researchers found significant differences between the gut microbiomes of people with MDD and healthy controls. Those with MDD exhibited lower bacterial diversity and changes in the abundance of specific bacteria. In experimental studies, probiotic and synbiotic (prebiotic plus probiotic) treatment modestly improved patients’ depressive symptoms. 

A2016 randomized controlled trial looked at the effects of supplemental probiotics on people with MDD. Forty adults with MDD were randomized to take either a probiotic capsule containingLactobacillus acidophilus,Lactobacillus casei, and Bifidobacterium bifidum or a placebo. After eight weeks, those in the probiotic group exhibited significantly decreased (i.e., improved) scores on a depression scale and dampened markers of inflammation and metabolic dysfunction. 

2. Bipolar Disorder

Bipolar disorder is another common mental condition, causing mood swings with manic or hypomanic “highs” and depressive “lows.” However, despite its prevalence, the available research on the microbiome and bipolar disorder is still in its infancy. 

Evans et al. (2017) first studied gut microbiome differences in people with bipolar disorder, finding that they had markedly reduced levels of the bacteriaFaecalibacteriumand a bacteria in theFirmicutes phylum.Other research has found bipolar disorder patients to harbor higher levels ofActinobacteria. 

One clinical trial has looked at the effects of supplemental probiotics on 38 hospitalized patients with bipolar disorder. Those taking a mixed probiotic (Bifidobacterium bifidum, Bifidobacterium lactis, Bifidobacterium langum,and Lactobacillus acidophilus) for eight weeks experienced significant improvements on tests assessing manic and depressive symptoms compared to their baseline levels. However, the results were not significantly different from the control group, suggesting a placebo effect. 

3. Generalized Anxiety Disorder

Anxiety disorders are the most common mental health condition, with18% of the American adult population being diagnosed with anxiety — and probably many more who have never sought treatment. Anxiety and depression are closely linked, with almost one-half of people with depression also suffering from anxiety. 

In a recent study, researchers monitored gut microbiota changes in people with anxiety. Compared to healthy controls, people with anxiety exhibited decreased bacterial diversity in their gut microbiomes with overgrowths of some unhealthy bacteria, likeStreptococcus. 

Further, after the anxious patients underwent Mindfulness-Based Cognitive Therapy, they had reduced anxiety symptoms, and their gut microbiota resembled those of the healthy controls. Future research should assess whether combining behavioral therapy and probiotics works synergistically to suppress anxiety.

4. Acute and Chronic Stress

Chronic stress has been shown to impact the intestinal barrier, leading to increased gut permeability and inflammation. Often referred to as “leaky gut,”increased barrier permeability is linked to mental health conditions, including major depressive disorder. 

Stress also impacts the HPA axis — the physiological system that mediates all stress responses by modulating the release of hormones like cortisol, our primary stress hormone. The HPA axis is highly involved in the gut-brain crosstalk, and dysregulation of the HPA axis is known to play a role in the development of mental health disorders. 

Some studies have looked at the effects of probiotics on acute stress.One trial subjected people to complicated mental math challenges in front of an audience combined with social threat components. (Imagine Gordon Ramsey watching and evaluating you while you publicly work on difficult math challenges — essentially, it’s designed to make you very stressed out.) 

In healthy people who underwent this stress test, those taking probiotic supplements for four weeks prior had subtly altered brain activity in regions known to regulate emotion and stress responses. 

Similar results were seen in ananimal study looking at chronic stress. Rats subjected to eight weeks of chronic mild stress were randomized to take either a probiotic supplement or a placebo. 

The probiotic-supplemented rats displayed significantly less anxiety and stress during behavioral and exploratory maze tests that were designed to assess stress. The probiotics also improved specific brain metabolites, suggesting that probiotics may stabilize stress-related neuro-metabolites to prevent stress and anxiety. 

Key Takeaway

The constant communication between our guts and brains plays a role in the development of mental health disorders, including depression, anxiety, stress, and bipolar disorder. Studies on the gut-brain axis — and how to improve the function of both — are ever-growing, hopefully leading to potential clinical implications in the future. This connection is not just for people with poor mental health — probiotics may also maintain or enhance mood and reduce stress in healthy people.  

As we learn more about which bacteria are reduced or elevated in people with specific mental health concerns, therapeutics may be able to be tailored to improve their symptoms. Although more research is needed, the available evidence suggests that modulating the gut microbiome via probiotics may help maintain or improve mental health and mood. 

For now, as we don’t know for sure which bacterial strains are best for promoting healthy and stable moods, supporting your gut microbiome in other ways is never a bad idea. For example, increasing your consumption of fermented foods, like sauerkraut, yogurt, kefir, kimchi, and pickles, and boosting your prebiotic fiber intake are good places to start to benefit your gut and its trillions of bacteria. 

Written By: 

Cambria Glosz, MS, RD

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