What is the gut-brain connection?
The gut-brain axis is a continuous, bidirectional communication system between the central nervous system (the brain and spinal cord) and the enteric nervous system (the network of neurons living in the gut lining).
The gut microbiota is the most abundant and diverse microbiota in the body and plays a central role in producing metabolites and signaling molecules that influence the brain and immune function.2 The vagus nerve is the most widely distributed and complex nerve in the body, sending signals from the brain to the gut and also relaying information from the gut back to the brain regarding inflammation, satiety, and microbial activity.3
Around 200–600 million neurons live within the enteric nervous system (the lining of the gut).
This “second brain” operates semi-independently from the central nervous system and communicates with both microbes and immune cells in the gut. Together with the vagus nerve and microbiota, it forms the basis of the gut-brain axis.4
This system interacts with intestinal microbes, the intestinal immune system, and endocrine systems, creating a powerful network that helps to maintain the overall health and function of the gut.
The gut-brain axis includes:
- The nervous system: The vagus nerve acts as a direct line of communication between the brain and the gut.
- The endocrine system: Hormones released by the gut and brain influence appetite, satiety, and stress responses.
- The immune system: Immune signals between the gut and the brain regulate inflammation and overall health.
What is the purpose of the gut-brain connection?
The gut-brain axis allows the brain to influence digestion, immune function, and the stress response, while the gut can influence mood, behavior, and even cognitive functions.5
This communication system links emotional and cognitive centres in the brain with important gut functions, including immune responses, the gut barrier, reflexes and hormone signalling.6 These include areas such as the amygdala (emotion), hippocampus (memory), and prefrontal cortex (decision-making), all of which are influenced by signals originating in the gut.
What body functions are affected by the gut-brain connection?
The gut-brain axis influences many bodily systems, including digestion, hormone production and the activity of functional immune effector cells. The gut is able to influence mood, cognition and mental health. For example, changes in gut microbiome have been associated with depression and altered brain activity in emotional processing regions.7
Specifically, gut-brain communication may affect:
- Cognitive function and mood: The gut microbiome produces a range of neurotransmitters (serotonin, dopamine), short-chain fatty acids (SCFAs), and other bioactive molecules that influence neural processes.8
- Digestion: The brain can influence gut motility, secretion, and intestinal permeability through the CNS. Stress can trigger the ANS to alter gut motility, potentially causing diarrhea or constipation.
- Appetite, cravings, satiety: Hormones secreted in the gut can affect brain regions involved in regulating appetite and feelings of fullness.
- Immune function: Gut microbiome can activate intestinal or circulating immune cells that can migrate to the CNS.9
- Food sensitivities and intolerances: Visceral hypersensitivity can amplify pain and discomfort resulting from gut discomfort (e.g. bloating, cramping)
- Metabolism: The gut releases hormones and peptides including glucagon-like peptide-1 and ghrelin, which influence appetite, satiety, and energy expenditure.10
