Ever get that 'gut feeling'? Or feel those butterflies in your stomach? What is that? Is our gut trying to tell our brains something?
The Enteric Nervous System (ENS) resides in the digestive tract and can be considered to be our second brain, communicating bidirectionally. It is now understood that microbes in our gut communicate with the Central Nervous System (CNS) through neural, endocrine and immune pathways.
The vagus nerve is one of the largest nerves connecting the gut and brain sending messages in both directions. The vagus nerve is the principal component of the parasympathetic nervous system and has the ability the sense microbiota metabolites, transferring gut information to the CNS therefore influencing brain function and other behaviours. Stress in the body disturbs the vagus nerve, the composition of the gut microbiota and plays a role in the pathophysiology of many gastrointestinal diseases including Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Disease (IBD). The vagus nerve exerts anti-inflammatory properties in healthy individuals protecting against intestinal permeability and dysbiosis (imbalance in composition of gut bacteria) which are found in both IBS and IBD. Stress whether interoceptive or exteroceptive could disturb the overall protective effects of the vagus nerve on gut barrier function and favour disruptions in the composition of gut microbiota (dysbiosis). It is no coincidence that mood disorders such as anxiety are prevalent in patients with IBS. Therefore it is important when treating gut issues such as IBS that stress and lifestyle factors are considered as much if not more than dietary factors to regulate the vagus nerve and promote its protective effects.
The gut/brain axis also communicates through neurotransmitters which help to control emotions and feelings. Interestingly 90% of serotonin is made in the digestive tract and not our brains. The gut also manufactures other neurotransmitters including dopamine and GABA, all of which influence mood, motivation, anxiety, reward and concentration. A small randomised control trial considered a group of healthy women and provided them with either fermented milk with probiotics, a non-fermented milk or no intervention over a period of 4 weeks. The women were assigned to an emotional faces attention task and brain function was measured before and after. The women who consumed the probiotic fermented milk had a calmer brain response to the task compared to women in the intervention group who showed a more hyperactive brain response. Although this is a small study with some limitations, it does suggest the critical role that bacteria in the gut play in modulating emotional behaviour.
Mental health disorders including depression and anxiety can be associated with disturbances in the gut microbiota. Bacteria in the gut produce short chain fatty acids, such as butyrate, which promotes the production of serotonin. Serotonin is responsible for feelings of happiness and low levels are associated with depression. Anti-depressants work by increasing serotonin levels in the body. GABA is another important neurotransmitter involved in mental health and acts as a relaxant helping to reduce anxiety. Maintaining a balanced ecosystem in the gut is critical for modulating the production of neurotransmitters and influencing the communication between the gut and brain.