Introduction
The feeling of “butterflies” in the stomach or the phrase “gut feeling” has suggested a connection between the gastrointestinal tract and the brain for the longest time.
Scientifically, the theory of gut-brain connection was proposed as early as 1930. In this study, it was suggested that emotional states might interfere with the gut microbiome, resulting in changes in intestinal permeability and systemic inflammation.
This theory has since been validated by numerous clinical trials over the past few decades or so. Our gut, brain, emotion, and nervous system are now considered as far more interconnected instead of just being separate entities by themselves. This association between the gut and brain is known as the Gut-Brain Axis (GBA).
Advances in research throughout the years have described the importance of gut microbiota in these interactions.
Gut-brain Axis
The gut and brain are known to be physically and biochemically connected:
According to Carabotti et al., The GBA has bidirectional communication between the central nervous system (CNS) – the brain and spinal cord, autonomic nervous system (ANS) – including the sympathetic, parasympathetic and enteric nervous system, and the hypothalamic-pituitary-adrenal axis (HPA) – hypothalamus, pituitary gland, adrenal gland.
The ANS receives afferent signals from the lumen and transmitted through the enteric, spinal and vagal pathways to the CNS and efferent signals from CNS to the intestinal wall. This complex connection bridges the emotional and cognitive centres with peripheral intestinal function.
An example of the mechanism was introduced as such:
The body reacts to environmental factors such as emotion or stress. The HPA is triggered to secrete cortisol from the adrenal glands (the body’s primary stress hormone) driven by the complex interaction of the limbic system (amygdala, hippocampus and hypothalamus).
Cortisol can affect nearly every body organ system; its functions include mediating stress response, regulating metabolism, inflammatory response and immune function. In the case of GBA, cortisol affects the brain, impacting the activities of functional cells within the gut, including immune cells, enteric neurons, epithelial cells, smooth muscle cells, enterochromaffin cells etc. These intestinal functional effector cells were shown to be modulated by the gut microbiota as well.
Therefore, the interaction between the gut and brain appears to be bidirectional, where the brain receives signals from the gut microbiota, and the gut microbiota receives signals from the brain through neural, endocrine, immune and humoral links.
The Role of Gut Microbiota vs Brain
Evidence from both clinical and experimental studies has suggested gut microbiota interact with intestinal cells, the ENS and also directly with the CNS through neuroendocrine and metabolic pathways. Pathophysiological mechanisms underlying CNS diseases are often associated with GI symptoms.
Firmicutes and Bacteroidetes are the two main phyla within the human gut; these phyla constitute more than 90% of the human gut microflora. There is a delicate balance between the two phyla, where the imbalance between the phyla is regarded as dysbiosis and is often associated with diseases. For example, the composition of gut microbiota and disruption of gut bacteria is related to mental health such as autism, anxiety and depression.
Conclusion
The mechanism of bidirectional interactions between the gut and brain through the gut microbiota is sound, and the evidence is strong. Although not yet fully understood, several studies suggest microbiota treatment is a potential intervention for neurological and psychological symptom management.
Future studies with larger sample sizes, more extended periods and better-designed studies are needed to determine the efficacy of the microbiota approach, especially the use of probiotics and the dietary influence of gut microbiota.
References:
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