The Brain-Gut Axis, Part 1 – A Paradigm Shift In Neuroscience

Traditional Eastern medicine has long acknowledged the importance of the gut. But evidence-based medicine, colloquially called Western medicine despite being practiced all over the world, has long disregarded a possible link between the brain and the gut – until recently, because a paradigm shift in neuroscience is now taking place.

Both Ayurveda (the traditional Hindu medicine) and Traditional Chinese Medicine (TCM) attribute a tremendous impact to the digestive system in all health and disease, and underline its strong connection to the mind.

Ayurveda states that the health of the digestive system is the single most important determinant of health and well–being. Likewise, TCM recognizes the digestive system as the center of all bodily functions and strongly links it to the brain. In TCM, all the organs and functions of the body are associated with psychological components: the stomach is associated with temper and intelligence, the small intestine is associated with mental clarity, and the large intestine is associated with willpower.

Arguably, the connection between the gut and the brain is somewhat embedded in our collective unconscious. Expressions such as “gut feeling” are of common usage. Many common idioms using the word “gut” are somehow illustrative of a brain-gut connection: “have the guts” – have courage; “hate someone’s guts” – hate someone, which is basically hating their personality, temper or attitude; “spill one’s guts” – to confess, say what’s on one’s mind; “gut reaction” – a basic and instinctive reaction to something; “butterflies in the stomach” – you know, when you see someone you fancy…

The gut microbiota (the microbe ecosystem) has become our body’s superstar lately. There has been an explosion of research on the effect of the gut microbiota in all sorts of non-gastrointestinal diseases. In the scope of the diseases of the nervous system, and just to name a few, the gut microbiota has been linked to: migraine, chronic pain, autism, anxiety, depression, and cognitive decline. And it has even been linked to motivation, and higher cognitive functions.

The mind-gut connection is much more than unconsciously coined idioms. There are some obvious connections that everyone knows, even if not consciously aware of them. Stress is a great example.

Anyone who has experienced stress and anxiety knows that it can easily induce gastrointestinal symptoms. In fact, irritable bowel syndrome (IBS) is strongly associated with stress and anxiety. Psychiatric disorders, especially major depression and anxiety, occur in up to 94% of IBS patients. But although IBS has been classically mostly regarded as a consequence of these psychiatric disorders, it may actually also be the other way around.

It makes perfect sense, if you look at what the gut entails. More than 60 per cent of our immune cells originates in the digestive system – it therefore massively influences the immune system and, hence, the body’s reaction to pathogens; also, local and systemic immune activation can have marked neuronal and behavioral effects. In addition, the gut has its own nervous system – the enteric nervous system – deemed our seconded brain, and it is estimated to have around 100 million neurons, maybe more than the entire spinal cord.

And the gut microbiota is incredibly massive: it’s estimated to be comprised of at least a thousand different species of bacteria, adding up to trillions (!) of microbes that together can weigh up to three pounds (!) in normal conditions.

Research has been showing that the microbes in our gut can release compounds that can influence the brain’s chemistry. There are indications that they can actually produce neuromodulator molecules and even regulate the production and/or the metabolism of neurotransmitters.

And given the immense ecosystem that our gut actually is, doesn’t it seem logical that evolution would somehow allow us to effectively interact with our symbiotes? I think so.

Before we dwell any further into the information that has been arising on the brain-gut axis, I believe it is desirable to understand what is going on in our second brain – our gut – and how it communicates with the actual brain.

In Part 2 of this article series I’ll explain what the enteric nervous system is and how the brain and the gut talk to each other.


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Stilling RM, Dinan TG, & Cryan JF (2014). Microbial genes, brain & behaviour – epigenetic regulation of the gut-brain axis. Genes, brain, and behavior, 13 (1), 69-86 PMID: 24286462

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Image via wavebreakmedia / Shutterstock.

Sara Adaes, PhD

Sara Adaes, PhD, has been a researcher in neuroscience for over a decade. She studied biochemistry and did her first research studies in neuropharmacology. She has since been investigating the neurobiological mechanisms of pain at the Faculty of Medicine of the University of Porto, in Portugal. Follow her on Twitter @saradaes
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