THE GUT-BRAIN AXIS AND OUR VAGUS NERVE

There is an undeniable link between our gut and our brain. Have you ever had a ‘gut’ feeling or felt butterflies in your tummy when nervous? The bidirectional communication between our central nervous system (CNS; brain and spinal cord) and the enteric nervous system (ENS; the ‘little brain’ within our gut) means our emotions and cognition collide with intestinal function, and vice versa. Herein, let’s explore why this crosstalk is so important and how we can optimise it. First up, how exactly do the gut and brain talk to each other?

There are different mechanisms whereby gut microbiota are capable of influencing the CNS—immune (cytokines), endocrine (hypothalamic pituitary adrenal (HPA) axis), neural (vagus nerve), and metabolic (short chain fatty acids (SCFAs), tryptophan). The fastest and most direct is via the vagus nerve.

VAGUS NERVE

The vagus nerve is the longest nerve in the body and has been considered to be our sixth sense, given its role in interoception. Motivation, emotions, and homeostasis are all governed, at least in part, by our interoception. Recent research has shown the relevance of gut microbiota and their metabolites in interoceptive signalling via the vagus nerve—our sense of hunger/appetite, knowing and acknowledging emotions, and having self awareness. Put simply: The information relaying between our gut and brain is determining how we think, act, and feel.

Gut-to-brain (and vice versa) communication:

- Regulation of our neurotransmitters (like serotonin, GABA, BDNF)

- Protection of intestinal barrier and tight junction integrity

- Modulate enteric reflexes (like how we eat and poop)

- Mucosal immune activation and alterations in immune function

- Bacterial metabolites (secondary bile acids, short-chain fatty acids, and tryptophan)

- Entero-endocrine signalling (hunger/appetite, insulin requirements etc).

- Production of mucus and biofilm

- Regulation of motility and our Migrating Motor Reflex (MMR)

Gut microbes can hijack vagus nerve signalling to influence the brain. They transfer gut information to the CNS, where it is integrated and then a response is generated. In this way, dysbiotic microbes could have a deleterious effect on the brain, whereby beneficial species and their metabolites (i.e. short chain fatty acids) could have protective and adaptive effects. Then, from a brain-to-gut perspective, the vagus nerve plays a role in protecting the intestinal barrier function and tight junction integrity, it resolves inflammation, alters mucus and biofilm production, and regulates motility. It is part of our parasympathetic nervous system (rest and digest) and so all things considered will be impacted by excessive sympathetic nervous system activity. Therefore, understanding the role of stress for the gut-brain axis is important.

MITIGATING STRESS

Stress is the biggest factor when it comes to vagus nerve tone, and the way in which the gut and brain interact. Stress sends us into our sympathetic nervous system and if this becomes chronic, vagus nerve dysfunction can ensue. Generally focusing on stress reduction techniques and stress resiliency is great. Additionally, specific activities that have shown to improve vagus nerve tone are as follows:

  • deep, slow breathing (belly breathes) and longer exhalation breathing (inhale 3-4 counts, exhale 6-8 counts)

  • meditation

  • yoga

  • exercise

  • massage

  • listening to relaxing music (where music beat is slower than heart beat). Something like this.

  • singing and humming

  • gargling water

  • cold shower, or splashing face with cold water, or jumping in the ocean

  • Grounding practices (gardening, barefoot walking outside, being in water) 

GUT-BRAIN CONNECTION IN DISEASE

It is now widely accepted that Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Disease (IBD) are biopsychosocial diseases, a hallmark driver being an aberrant gut-brain axis. This leads to poor interoception and vagus nerve dysfunction. For example, visceral hypersensitivity seen in IBS is a sign of poor interoception. Additionally, IBS has been treated with antidepressants, exposing its psychological underpinnings. On the other end of the spectrum, mood disorders like anxiety and depression have been shown to be associated with dysbiosis and Alzheimer’s disease has been linked to poor barrier integrity and tight junction function. These have both been considered gut-brain issues. Therefore, treating only one system (either brain/nervous system or gut) is limited, and does not get to the root of the issue. There is much to understand in relationship to both systems and how alterations in either system can influence the other.

IN SUMMARY

This is an ever-evolving area that is in and of itself offering huge promise for those struggling with mental health (and also, those struggling with gut issues that could be offset by mood modulation, nervous system support and neurotransmitter regulation). We cannot consider remedies for mental health, without considering implications of gut health (and vice versa). Any plan to address either gut or mental health without acknowledgement of the other is incomplete. Improving vagus nerve tone, alongside understanding interoception, is a great place to begin to optimise both gut and brain function. If you’d like to take a deeper dive, this is an area I love to address in clinic and you are most welcome to book an appointment.

 
Claire Hargreaves