What would you think if I were to tell you that your intestines—and more specifically, the bacteria inhabiting your intestines—communicate information to your brain that affects your mood? That is exactly what recent medical research and even human clinical trials are telling us: optimizing the bacteria in the human gut (the gut microbiome) can reduce symptoms of anxiety, depression, and other mood disorders. In addition, we are beginning to understand why and how this works. If you or somebody you love is challenged by psychological distress of any kind, read this article to find out how we might be able to help.

Human Studies:

Probiotics are the beneficial microbes inhabiting the human intestines, known to improve digestive health and mediate immunity. Probiotics can be taken as dietary supplements or consumed in foods—particularly fermented foods, such as yogurt, kefir, and sauerkraut. Earlier this year, a study in 710 young adults found that consumption of fermented foods decreased symptoms of social anxiety.[i] A second study found that supplementing Lactobacillus and Bifidobacterium (2 common types of probiotics) alleviated psychological stress in healthy adults.[ii] Probiotic supplementation has also been shown to affect brain activity in the areas that process sensation and emotion in healthy women.[iii]

How this Works:

The gastrointestinal tract is sometimes called the second brain because of the extensive system of nerves and neurotransmitters it contains (neurotransmitters are commonly known as “brain chemicals,” but they actually act throughout the body). The medical term for this intestinal system of nerves is the enteric nervous system. For years, medical textbooks have taught that the brain communicates information to the enteric nervous system, subconsciously sending messages to speed up or slow down digestion or even to create physical sensations related to emotion, such as “butterflies” in the stomach. Recent medical research has now revealed that communication also goes in the opposite direction: your gut can talk to your brain.

To our current knowledge, there are 3 main ways that communication is sent from the gut to the brain: by direct activation of the vagal nerve; by production of inflammatory compounds; and by production of neurotransmitters, such as serotonin. These mechanisms make up what we call the gut-brain axis:

1. The Vagal Nerve. The most important nerve extending from the brain to the intestinal tract is called the vagal nerve, and we now know that communication in the vagal nerve can go both ways: brain to gut and gut to brain. The bacteria inhabiting the intestines can directly stimulate enteric nerves, which in turn send messages to the vagal nerve, which communicates directly with the brain to produce changes in mood.[iv]

2. Inflammatory Compounds. Whenever digestive health is compromised, the composition of the gut microbiome can change, and intestinal permeability can change. This is commonly known as leaky gut. A leaky gut triggers a local inflammatory response, accompanied by inflammatory compounds, called cytokines. Inflammatory cytokines can influence brain activity and mood via the vagal nerve or by crossing the blood-brain barrier. Inflammatory cytokines also activate stress hormones. There is good medical research showing that depression is actually an inflammatory disease, so it is no surprise that gut-derived inflammation might play a role in mental health and disease.[v]

3. Neurotransmitter Production. Studies have shown that some strains of beneficial bacteria that inhabit the human intestines, including Lactobacillus and Bifidobacterium, are able to produce neurotransmitters: specifically, serotonin and gamma-aminobutyric acid (GABA).[vi] Serotonin is the target of most antidepressant medications, and GABA is the target of most anti-anxiety medications. Gut-derived serotonin and GABA can be delivered to the central nervous system in similar ways that inflammatory cytokines can be delivered.

Even when an intense psychological stress triggers anxiety or depression, the gut-brain axis might play a role in perpetuating the symptoms. Stress of any type activates production of cortisol, which can both alter the composition of the gut microbiome and create a leaky gut. This disruption of digestive health then feeds into the gut-brain axis described above, ultimately resulting in anxiety, depression, or other mood problems.

Why this Matters:

It is estimated that 1 in 10 Americans struggle with anxiety, depression, or a combination of both. Among these people, 1 in every 3 or 4 do not improve with conventional treatments—even when 2 or more antidepressant medications are prescribed. Anxiety and depression are the most common mood disorders, but a variety of other mental illnesses also plague Americans. We have seen a surge in rage and violent crimes on our streets, in our schools, at our businesses, and even in our medical clinics. These crimes are not being committed by people who are feeling content, happy, or at peace in our world.

Mood disorders and mental illnesses are complex conditions that are influenced by societal, psychological, as well as physical health. Effectively treating these conditions requires a team approach involving community support, professional therapy, and medical treatments to balance the body chemicals and physiology. One way to support a physiology that favors mental health is by correcting intestinal health. Hippocrates, who is known as the “Father of Modern Medicine,” taught that all disease begins in the gut. Modern research suggests that this may even be true for mental or psychological disease.

We use an impressively successful protocol in many of our patients to balance the intestinal microbiome and correct leaky gut. I hope that after reading this article, you will trust that this approach makes sense as part of a comprehensive plan for your mental health.

Learn more about leaky gut here


[i] Hilimire MR, DeVylder JE, Forestell CA. Fermented foods, neuroticism, and social anxiety: An interaction model. Psychiatry Res. 2015;228(2):203-208.
[ii] Messaoudi M, Lalonde R, Violle N, et al. Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects. Br J Nutr. 2011;105(5):755-764.
[iii] Tillisch K, Labus J, Kilpatrick L, et al. Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology. 2013;144(7):1394-401, 1401.e1.
[iv] Fichna J, Storr MA. Brain-Gut Interactions in IBS. Front Pharmacol. 2012;3127.
[v] Wium-Andersen MK, Ørsted DD, Nielsen SF, Nordestgaard BG. Elevated C-reactive protein levels, psychological distress, and depression in 73, 131 individuals. JAMA Psychiatry. 2013;70(2):176-184.
[vi] Dinan TG, Stanton C, Cryan JF. Psychobiotics: a novel class of psychotropic. Biol Psychiatry. 2013;74(10):720-726.

About the Author: Dr. Gerard Guillory, MD is Board Certified in Internal Medicine and has published two books on Irritable Bowel Syndrome (IBS). In 1985, he opened The Care Group, PC. Today, his clinic is a Primary Care facility that is a hybrid of functional and traditional medicine treating patients with digestive disorders, autoimmune disease, and other conditions. You can learn more about Dr. Guillory here.