The brain gut microbiome (BGM) system is a network of bidirectional connections between our brain, our gut and the trillions of microbes living inside the gut. It plays a crucial role in mediating the effects of diet, stress and emotions on our health, and alterations in the system and the resulting metabolic and immunological consequences are being increasingly identified as factors in the chronic non-infectious disease epidemic of today.
Besides being essential for the digestion and absorption of nutrients, the gut contains about 70% of our immune cells, a large portion of our endocrine (hormone-producing) cells and some 150 million nerve cells that make up the enteric nervous system. This unique arrangement makes the gut the most complex organ in our body and it plays a role in most physiological and pathological processes.
The impressive assembly of cells closely interacting with each other (also referred to as the gut connectome) continuously sends signals to the brain, mainly via hormones, sensory vagal nerve fibers, neuroactive molecules and immune signals, and in turn receives signals from the brain via the autonomic nervous system and stress hormones. The signals sent by the brain inform the gut about actual or anticipated threats to the homeostasis (stress) of the organism, and about the emotional states of the individual. This influence by the brain greatly shapes the habitat in which the microbes live. On the other hand, signals generated from the interactions between food and the gut connectome are translated into nerve and hormonal signals that can influence our emotions, cognitions, sense of wellbeing and pain sensitivity. “Gut health,” the popular term in this new understanding, cannot be assessed by endoscopic or radiological procedures, but refers to the balanced interactions within the gut connectome.
Based on a wealth of preclinical and epidemiological data, chronic low-grade immune system activation throughout the body has been implicated as a major risk factor in genetically predisposed individuals for the development of a wide range of chronic diseases, including metabolic syndrome, autism spectrum disorder, depression, Parkinson’s and Alzheimer’s disease, obesity and food addiction, non-alcoholic fatty liver disease and colon cancer.
A better understanding of the complex interactions within the BGM system in health and disease requires a combination of investigative strategies, from mechanistic studies in preclinical models to large-scale human studies. The latter include clinical interventions and so-called multiomics studies that look at the relationship between clinical, brain, gut, microbiome, genetic and dietary factors. The expected breakthrough of a better understanding of the BGM system will have dramatic implications in a new conceptualization of chronic disease mechanisms, and more cost-effective strategies to prevent and treat them.