The Gut-Brain Connection
How gut microbes can influence mental health and what you can do about it.
Posted January 26, 2021 | Reviewed by Ekua Hagan
We are delighted to have Tanya Nguyen, Ph.D., Assistant Professor of Psychiatry at the University of California San Diego (UCSD), as a guest blogger.
Over the last decade, scientists have been learning about how the communities of microorganisms that live in our gut, called the gut microbiome, can influence human health and well-being.
What is the microbiome?
The human body is one big ecosystem, home to trillions of microbes. In fact, there are more microbial cells living on and inside the human body than there are actual human cells. Just as different parts of the world have different landscapes of plants and animals, different parts of our bodies have distinct and characteristic communities of bacteria, viruses, and fungi. The gut is the biggest and most famous microbiome, but the skin, mouth, and lungs harbor diverse populations as well.
These microbes aren’t just along for the ride. They are there for a reason. We have co-evolved with and participate in an intimate symbiotic relationship with our microbiome — we give them a place to live, and they help keep us alive. Our different microbes do different things, including many functions that cannot be performed by our own human cells. They digest and extract nutrients from our food, metabolize drugs, regulate our immune system, and provide protective defenses that support our health.
Many of the chronic diseases of modern life are significantly associated with systemic inflammation and gut “dysbiosis” — a loose term that implies an unbalanced, unhealthy gut microbiome. Many age-related and chronic diseases, including diabetes, hypertension, and heart disease, are associated with the gut microbiome. Read more about The Enormous Impact of Tiny Microbes.
How is the microbiome is linked to mental health?
One of the most intriguing aspects of this exciting new research is how the microorganisms in our gut influence our brain and mental health.
The gut microbiome is part of a network, termed the “gut-brain axis,” that links the emotional centers of the brain with peripheral intestinal functions. For example, when we feel stressed or anxious, we may end up with an upset stomach due to the signals our brain has sent to our gut. On the other hand, disruption to gut pathways may affect our body’s stress response, emotional arousal, mood, motivation, and even higher-order cognitive functions such as decision-making.
Studies in humans have shown that the gut microbiome is associated with personality traits and psychological states, including self-compassion, empathy, emotional well-being, and wisdom. Furthermore, the microbiome may play an important role in social behavior. People with larger social networks and lower levels of loneliness tend to have more diverse gut microbes.
Additionally, people with psychiatric illnesses, including depression, bipolar disorder, PTSD, and schizophrenia, have significantly different microbiome communities compared to people without mental health conditions. Importantly, this is true even when taking into account other factors that are known to impact the microbiome, such as age, body mass index, and medical diseases.
The connection is complex
The nature of this interaction between the gut and the brain in mental health and illness is not fully understood and is still an active area of research.
Some studies have suggested that people with schizophrenia have a “leaky gut,” which allows for gut microbes to pass through the gut barrier into the bloodstream. White blood cells in circulation can sense these microbes and cause an inflammatory reaction by releasing small proteins, called cytokines, that are able to pass through the blood-brain barrier and act directly on the brain. This unhealthy inflammatory cascade has been also been linked to depression and anxiety. Another study in schizophrenia suggested that gut microbes may change immune signals or regulate glucose and fats, which have implications for diabetes and heart disease, conditions commonly associated with psychiatric disorders.
The microbes in our gut also produce short-chain fatty acids (SCFAs), which are formed from breaking down fibrous foods like vegetables and grains. SCFAs are believed to mediate the gut-brain axis crosstalk. They support the integrity of the blood-brain barrier and can have neuroactive properties. They may also be involved in critical phases of neurodevelopmental and neurodegenerative disorders.
How do we nourish our guts and minds for better mental health?
Taking care of our mental health may lie in our microbiome. The fast-developing science of the microbiome is only beginning to identify potential diagnostics tools and interventions to promote mental health and well-being. Though rapidly developing, it is still a relatively young science where much is still unknown. We caution that many businesses and health influencers may try to capitalize on this exciting area of research by making claims filled with overpromises and even fraudulent advertising for tests (e.g., that identify specific bacteria that cause specific symptoms or diagnose diseases) and products (e.g., that cure all illnesses or make you smarter).
Still, there are some practices that people can do to support a healthy brain and body.
Bottom-up (gut ⇒ brain) strategies
1. Embrace plant-based foods. Diets rich in fiber are associated with a highly diverse and balanced microbiome. Fiber is composed of long chains of sugars that are too complex for our own digestive tract to break down. The bacteria in our gut ferment fiber and, in the process, produce SCFAs that are anti-inflammatory and nourish the cells lining the gut.
2. Follow principles based on the Mediterranean diet. Studies consistently point to a Mediterranean diet as anti-inflammatory. The foundation of the Mediterranean diet is vegetables, fruits, herbs, nuts, beans, and whole grains. Meals are built around these plant-based foods, with moderate amounts of seafood, poultry, dairy, and eggs and sparing amounts of red meat.
3. Supplement with probiotics and prebiotics. Studies suggest that probiotics and prebiotics may improve mood and behavior, including symptoms of depression and anxiety. Don’t expect that these will remedy all of your problems. But like vitamins, they may be a useful supplement to your health routine.
Probiotics are foods or supplements that contain live microorganisms that alter the microbiome and confer health. Live Lactobacillus and Bifidobacterium cultures are found in fermented foods, such as pickled vegetables, kimchi, tempeh, and yogurt. Certain strains of these bacteria have the potential to produce neurotransmitters that are important in mood and anxiety disorders. Some bacteria may also exert activities that empower the immune system.
Prebiotics are foods or supplements that are not digestible by our own human cells but selectively stimulate the growth and activity of our gut microbes. Prebiotics lead to increased production of SCFAs. The main dietary sources of prebiotics are indigestible carbohydrate fibers and oligosaccharides, which are found in plant-based foods and human breast milk.
Top-down (brain ⇒ gut) strategies
While no published study has yet to investigate the effects of psychological interventions on microbiome composition, psychosocial interventions such as cognitive-behavioral and mindfulness-based therapies can improve the functioning of the immune system, suggesting that they may affect the gut microbiome and/or its related pathways.
1. Be mindful. In the face of a challenging year overwhelming with stress, isolation, and loss, it is challenging not to regret or ruminate about the past or worry about the future. Practicing mindfulness — purposeful awareness of the present moment — can help quiet the mind and relax the body. There are many ways to practice mindfulness, including deep breathing, meditation, yoga, guided imagery, gratitude, etc. Shut down social media apps and reach out to family and friends.
Dr. Nguyen is a clinical neuropsychologist and expert in schizophrenia, microbiome, and healthy aging. She earned her Ph.D. in the Joint Doctoral Program at UCSD and San Diego State University and completed a postdoctoral fellowship in psychosis research at the Desert Pacific Mental Illness Research, Education, and Clinical Center (MIRECC) before becoming faculty at UCSD and the VA San Diego. Dr. Nguyen was awarded a K23 to study gut-brain relationships and the immune system in schizophrenia and is also a clinical psychologist at the VA San Diego Healthcare System.
Cryan J. F., Dinan T. G. (2012). Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nature Reviews Neuroscience, 13, 701-12.
Nguyen, T. T., Hathaway, H., Kosciolek, T., Knight, R., & Jeste, D. V. (2019). Gut microbiome in serious mental illnesses: A systematic review and critical evaluation. Schizophrenia Research. doi.org/10.1016/j.schres.2019.08.026
Nguyen, T. T., Kosciolek, T., Daly, R. E., Vázquez-Baeza, Y., Swafford, A., Knight, R., & Jeste, D. V. (2021). Gut Microbiome in Schizophrenia: Altered Functional Pathways Related to Immune Modulation and Atherosclerotic Risk. Brain, Behavior, and Immunity, 91, 245-256.
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