A Gut Feeling: How the Gut Affects Our Mental Health
A recent flurry of research suggests the gut microbiome has an impact on mood and mental health. So how does what happens in the gut affect what’s happening in the mind?
The human body is an ecosystem. As a tree might play host to birds, possums, termites, mosses, beetles, vines and fungi, the human body is host to much smaller, flourishing forms of life. Like a tree, the body depends upon these lifeforms to thrive, playing the (mostly) happy and willing host to trillions of microorganisms. Each of us carries along an invisible twin, hosting as many cells belonging to microscopic life as there are cells in our own body.
No part of the body is more jam-packed with microbes than the human gastrointestinal tract, home to fungi, protozoa, viruses and, yes, lots and lots of bacteria. This inner petri dish is called the gut microbiome. A flurry of research since the 90s has largely driven from many angles at the same question: how can we help the microbiome help us?
Gut microbes and the intestinal tract interact with the rest of the body via the gut-brain axis. Just like an axis of allied countries, the gut-brain axis refers to separate systems (with overlapping or parallel duties) that control divisions of a single space – though this space is the human body rather than a continent. Simply put, the gut-brain axis is a two-way highway between the brain-central nervous system (CNS) and the enteric nervous system (ENS), which governs the gastrointestinal tract.
This interlinking is sometimes more accurately called the microbiome-gut-brain axis. The extended hyphenation points to the critical role of the microbial community. A thriving gut microbiome works wonders for the body, converting fibre into useful short-chain fatty acids, synthesising vitamins, protecting against pathogens and boosting metabolism. The profile and proliferation of microbiota in our gastrointestinal system influences a host of health functions like nutrition, digestion and immunity. In the last ten years, research has turned to the microbiome’s impact on mental health, to emotions and behaviour.
Preliminary experiments in humans are finding some compelling connections between gut microbiota and the mind. It has long been observed that stomach disorders and stress go hand in hand: 61% of people with IBS also seek treatment for an anxiety disorder and are significantly more likely to have depression. In early 2019, the Flemish Gut Flora Project found that people with depression were lacking in two groups of bacteria, even those treated with antidepressants. Bacteria in the gut can synthesise the signals needed for the release of dopamine and serotonin. By analysing faeces, alongside participant and physician reporting of depression, Belgian researchers found that quality of life correlates with a healthy microbial life.
So how does what happens in the gut alter what’s happening in the mind? The short answer is that they talk to each other. Chatter occurs between the brain, CNS and ENS. The ENS is a mesh of nerve cells lining the gastrointestinal tract, often called the “second brain” as it operates somewhat in isolation to the brain and CNS, directing the gut, taking feedback from both in and outside the body. The ENS talks to the brain and CNS largely through the vagus nerve. (Vagus means “wandering” in Latin, which seems apt for a nerve that starts in the brainstem and travels down the throat, reaching down through the intestinal system to the colon.)
So, the gut-brain axis involves complicated pathways with crosstalk between the endocrine, immune and autonomic nervous systems. It puts the gastrointestinal system in touch with the cognitive and emotional centres in the brain; they have a conversation. This is where microbes speak up. So, if the gut is speaking to the brain, it’s doing so while playing host to fluctuating flora that have varied and vital functions.
These microbes are present from birth, perhaps before – new research suggests that microbiome acquisition might begin in utero. As a fetus emerges from the womb, it passes through a sort of car wash of microbes that is the vaginal canal. Despite how it sounds, this process seems to be healthy and protective for the newborn – research suggests that those born via caesarean section may have a greater tendency towards lifelong immune system impairment.
Though each person’s microbiome differs, increasing evidence tells us that mental health and stress levels benefit from a well-stocked menagerie of microbes in the gut. (A 2004 study in the field showed that germ-free mice experienced an exaggerated hormonal stress response compared to genetically identical but germy mice.) But general gut health is just a first step, and the focus has shifted to targeted methods of treating mental illness through the microbiome. Invisible to the eye, our microbial communities might seem too esoteric to influence, but treatments from faecal transplants through to targeted probiotics have demonstrated their curative potential.
Contemporary meta-analyses (reviews of multiple scientific studies in the same field) have found that probiotics can lead to a reduction in symptoms of mental illness, particularly affective and anxiety disorders, as well as a minimisation of stress responses. Projects including MetaHIT and the Human Microbiome Project look at the composition of gut microbiota, how they correlate with different diseases, and investigates the possibility of coaching our microbiome for better health. If the microbial balance is overturned – if too much of one colony grows at the expense of other colonies – it can promote or even spark serious illnesses.
How can we help the microbiome help us?
This new area of “psychobiotics” – the manipulation of the microbiome for better mental health – has opened up rapidly in the last five years, especially in terms of human experiments. But mice were the first true indicators of the bacteria-to-brain pathways. In a 2011 study, mice bred in the lab to be timid – you could call them genetically “mousey” mice – were colonised with microbiota from a braver breed of mouse. The treated mice experienced increased courage: they explored more and showed less fearful behaviour. Though there’s a lot to be said for an early start in life on structuring the microbiome, this research into psychobiotics points to a present ability to change our “gut feelings”, so to speak, and the expression of our very DNA.
The simplest initiative one can take to promote a happy gut-brain combo is building probiotics into a day-to-day diet. Probiotics have enjoyed a long reputation for promoting health: Nobel laureate and zoologist Élie Metchnikoff first theorised that the longevity of Bulgarian peasants could be attributed to their penchant for yoghurt. Probiotics refer only, as in Metchnikoff’s original conception, to comestibles that boost “useful” microbes; they encourage the friendliest microbiota to take up residence and hold their own in the intestinal arena. The generalised approach of taking in friendly foods may soon be the low-tech, sledgehammer approach of the past. Introducing the brain to specific strains of bacteria is the work of psychobiotics now and into the future, tuning our gut microbiota, to improve quality of life and build up the courage of mice.