What is Gut-brain axis (GBA)?

The gastrointestinal tract communicates bi-directionally with the central nervous system (CNS) by means of enteric nervous system or the ENS(also called the GI brain), the autonomic nervous system (ANS), the neuroendocrine (HPA axis), and neuroimmune systems. This communication is responsible for many important physiological processes such as gut motility, regulation of digestive processes, modulation of the gut-associated immune system, secretion, visceral sensations, and control of bowel movements. The third arm of the gut- brain communication is the gut microbial community. The role of gut bacteria in mediating changes in the brain is exemplified by a study where the introduction of a pathogen Citrobacter rodentium induced anxiety-like symptoms that were likely mediated by the vagus nerve. Gut microbes are known to release neurotransmitters such as serotonin, which is responsible for the feeling of well-being and happiness. Gut microbiota interacts with the CNS via the vagus nerve.  It is the interplay between the GI system, the CNS and the gut microbiota that forms the basis of gut-brain axis.

Gut-brain axis in Irritable Bowel Syndrome

Homeostasis in the gut-brain axis is critical to gastrointestinal tract physiology and also for a balanced emotional state. Disruptions in the gut-brain communication have been considered as one of the underlying mechanisms of IBS and manifestation of comorbid physiological conditions. Alteration of gut-brain axis in IBS may explain abnormal gut motility, visceral hypersensitivity, anxiety, and depression experienced by IBS patients. In this context, 20-60% patients with IBS show comorbid conditions such as anxiety and depression.  Also, the symptom of IBS such as abdominal pain is exacerbated by stress.

How can GBA be modulated in Irritable Bowel Syndrome?

The concept of IBS as a result of altered gut-brains axis is becoming increasingly evident. Current treatment approaches include treatment with antidepressants such as tricyclic antidepressants (TCAs).  Antibiotic rifaxmin also prevents visceral hypersensitivity in stress-induced mouse model. Since gut microbiome is a crucial part of this axis, some studies are aimed at treating IBS symptoms with probiotics. In one study, probiotics improved gut dysfunction in rat pups separated from their mother (which induces stress) partially by normalization of HPA axis. In another study, chronic treatment with Lactobacillus rhamnosus (JB-1) reduced stress-induced corticosterone and anxiety- and depression-related behavior and this was dependent on vagus nerve integrity.

Gut-brain axis provides a conceptual framework for advancing our knowledge of IBS. Further research, basic and clinical, is needed for investigating novel therapeutic approaches targeting the altered gut-brain axis in IBS.

Role of Gut Microbiome in Irritable Bowel Syndrome

The role of gut microbiome (bacteria) in the development of IBS is supported by several scientific studies. There is evidence that an increased amount of bacteria in the small intestine known as (SIBO) is closely linked to IBS.

The human gut is colonized by 100 trillion microbes that play role in the functions and maintenance of the gi tract. The number and composition of gut microbes vary along the length of GI tract. The number of microbes increases in number the further down the GI tract you go. The closer to the mouth the fewer microbes while the closer to the anus the more microbes are found.

What happens in SIBO: The gut will function normally under a healthy condition, but there are factors that can disrupt normal gut microbial environments. These factors include repeated antibiotic use, western diet, infections, and stress. When enough stress or environmental factors occur, the gut can enter a state of imbalance called dysbiosis. Dysbiosis can cause colonic bacteria to migrate into the small intestine, leading to increased colonic bacteria in the small intestine. This this is referred to as small intestine bacterial overgrowth (SIBO) .

Small intestine bacterial overgrowth can often be detected by hydrogen breath test (HBT). Hydrogen and methane gas appear in the breath as a result of abnormal fermentation of carbohydrates by the colonic bacteria in the small intestine in the setting of SIBO. HBT can also be used to test lactose intolerance.

SIBO has been often observed in many patients with IBS and can be tied to many of the IBS symptoms. SIBO leads to abnormal fermentation in the small intestine by gut bacteria which causes gas, bloating, characteristics of IBS. Other pathophysiological mechanisms of IBS such as altered motility, visceral hypersensitivity, and abnormal brain-gut interaction may also be explained by SIBO. Moreover, studies show that the treatment of SIBO relieves symptoms of IBS.