The link between brain and gut is now being considered as a peek into the mystery of human behaviour.
Writer & Artist: Jamie Hau
Editor: Dan Jacobson
The possibility that your stomach influences your brain might sound like something out of a sci-fi movie, or an amusing birthday card, but there is increasing evidence that your gut microbes may be influencing your day-to-day thoughts and behaviour.
For decades, scientists have been deciphering the complex link between the bacteria in our gut and a myriad of metabolic, neurodegenerative and autoimmune disorders. The classic example is the causative role of Helicobacter. Pylori in patients with gastritis and peptic ulcers, which earned its discoverers the Nobel Prize in 1982. Until recently, the idea that our gut and central nervous system are associated was widely reviled. Now, there is an emerging hypothesis that the microbiome plays an important role in the gut-brain axis.
The gut-brain axis is a bidirectional link between the central nervous system and our intestines. Chronic stress has been shown to alter intestinal permeability, which is associated with low-grade inflammation that can be linked to psychiatric disorders, such as depression. Other studies have suggested that the gut microbiota can produce neuroactive substances influencing the key symptoms of neurodegenerative disorders, such as Parkinson’s or Alzheimer’s disease.
So how does this “gut-brain axis” work?
The bacteria in our gut produce metabolites that are important for our general well-being. In healthy humans, commensal microbiota and their host display a pleasant symbiotic relationship. In states of dysbiosis, however, shortage of advantageous metabolic end-products may contribute to several neurological issues. For example, many species of Lactobacillus and Bifidobacterium produce GABA, the main inhibitory neurotransmitter in the brain. Likewise, Candida, E. coli, and Enterococcus produce serotonin, whereas some Bacillus species have been shown to produce dopamine, neurotransmitters strongly implicated in pleasure and depressive disorders. Our gut bacteria also break down dietary fibres into short-chain fatty acids that stimulate serotonin release.
To address the mechanisms underlying microbiota-gut-brain communication, a group of researchers, in 2019, developed a DNA sequencing method for characterising the neuroactive potential of gut bacteria, and their impact on quality of life (QoL) and depression. They analysed microbiota in the faeces of participants, and then correlated different microbial taxa with the participants’ QoL and incidence of depression. With these data, the researchers described the microbiota’s capacity to produce or degrade molecules that can interact with the human nervous system. They found that the relative abundance of two groups of bacteria populations, Coprococcus and Dialister, were reduced in people with depression. There was also a positive correlation between QoL and the ability of the gut microbiome to synthesise a breakdown product of dopamine. These are the strongest results yet, to show that a person’s microbiota can influence their mental health.
Despite the increasing evidence of the association between microbiome dysfunction and central nervous system-related comorbidities, much of what we know so far is based on studies showing correlations between specific gut bacteria, their metabolites and neurological symptoms, without proving a direct causal link. The challenge at present is to find out how these microbe-derived molecules interact with our central nervous system, and whether that alters a person’s behaviour or disease risk. At least for now, answering these questions is a sensible pursuit, and not a ludicrous one.