Going with your Gut

The bidirectional pathway between the gut microbiome and the central nervous system (CNS) is becoming a major topic of conversation in health literature, given that disruption to this pathway leads to health concerns within both neurological and gastrointestinal (GI) diseases. In fact, healthcare professionals are now in the midst of finding an answer to the question: Does rebalancing of the gut microbiome work as a management option for related disease?

The gut, or inside of the intestines, hosts trillions of microorganisms, including thousands of species of bacteria, viruses, fungi, and parasites; ie, the microbiome. Initially inherited via breastfeeding in infants, this unique ecosystem is characterised by its environment within the body and its ‘inhabitants,’ developing and changing based on diet and lifestyle factors. A symbiotic, or mutually beneficial, relationship develops between our bodies (their physical host) and microbes: we provide a host for their ecosystem, and they provide services to function in a balanced manner. Digestion plays a central role in which the gut microbiome is involved, aiding the breakdown of complex carbohydrates and dietary fibres as well as providing the necessary enzymes to synthesise certain vitamins. Nonetheless, gut bacteria are also essential in metabolising bile in the intestines for what is called enterohepatic circulation. Once bile digests fats in the small intestine, bacteria and the correlated enzymes aid the breakdown of bile into bile acids for reabsorption by the liver. In the case of complications within this pathway, the body would be unable to recycle bile acids, and the liver would not have the necessary resources to produce new bile, interfering with the digestion and absorption of fats, effectively halting digestion. This ecosystem also plays a major role in immunity, as the gut constitutes the largest immune system organ in the body, with beneficial microbes in the gut training the immune system to differentiate these from pathogenic microbes. ‘Helpful’ gut bacteria directly compete with ‘unhelpful’ types for space and nutrients, preventing them from gaining an advantage over the entire microbiome. Short-chain fatty acids are an example of the byproducts of helpful gut bacteria and offer support to the body’s immune system, preventing bacterial toxins from reaching the bloodstream and suppressing inflammatory reactions in autoimmune diseases.

Another point of overlap with gut microbes is via the Gut-Brain axis — the network of nerves, neurons, and neurotransmitters that run through the GI tract. The expression ‘gut feeling’ highlights this communication exactly, between the gut and CNS, affecting feelings, mood, and decisions, as there are more nerve cells in our gut than anywhere else in the human body. The Vagus nerve is the main link between the enteric nervous system (neural network within the GI tract) and the brain, conveying sensory information from inside the gut to the brain. Experimentation in treating related neural conditions by treating the gut microbiome is now taking place among healthcare specialists, using probiotics (live bacteria and yeast), antibiotics, and faecal microbiota transplantation (transferring a medically processed faecal sample from one person’s gut to another).

Research suggests that individuals with Autism Spectrum Disorder (ASD) often show notable differences in their gut microbiome in comparison to neurotypical individuals. Multiple studies have found that people with ASD frequently have reduced microbial diversity, combined with an imbalance between beneficial and potentially harmful bacterial species. A 2024 study conducted by Peralta-Marzal et al. used machine-learning techniques to analyse microbiome data from multiple datasets in order to identify consistent bacterial patterns associated with autism. Researchers found that individuals with ASD showed distinct microbial signatures compared to neurotypical controls. By combining data from different studies, the analysis by Peralta-Marzal et al. demonstrated that certain microbial groups (Actinobacteria and Firmicutes) were repeatedly associated with ASD across populations. Furthermore, increased levels of Clostridium bacteria (belonging to the phylum Bacillota) have been observed — within this group, certain species produce neurotoxins and metabolic byproducts that possibly interfere with normal neurotransmitter signalling in the brain. Such microbial changes also contribute to the gastrointestinal symptoms that ASD patients have reported. However, work by Julia Yap and colleagues found no clear association between gut microbiota composition and ASD diagnosis, suggesting that observed microbial differences may instead result from ASD-related dietary and feeding behaviours.

Continued research into the Gut–Brain Axis will help determine a future strategy for managing neurological conditions, leading us closer to answering the question, ‘Does rebalancing of the gut microbiome work as a management option for related disease?’ 

Illustration by Mokshita Nagandla