Our brain has bidirectional communication with our digestive system via the vagus nerve which extends from our brainstem through the neck and the thorax down to the abdomen, forming a communication network called the gut-brain axis (GBA).

The vagus nerve is responsible for regulating digestion, respiratory and heart rate as well as certain reflex actions, such as coughing, sneezing, swallowing, and vomiting. (Vagus nerve is shown in Figure 1 in yellow).

When we are in the ‘rest and digest’ mode, the vagus nerve regulates our bodily functions via the parasympathetic nervous system for digestion and relaxation to conserve energy, slow the heart rate, increase digestion, decrease inflammation and relax the sphincter muscles in the digestive tract. 

But when we are in a stressful situation, our body switches to the ‘fight or flight’ mode where our body requires a spurt of energy and fast reaction. This response is driven by the sympathetic nervous system which stimulates blood flow to major muscles, dilates the pupils, increases sweating, accelerates the heart rate and widens the bronchial passages. This is also the time when digestion is not a priority, hence indigestion and gut motility decrease.

Another nervous system that influences the gut and how we feel is the enteric nervous system, or sometimes called ‘the second brain’. This second brain makes us feel like there are butterflies in our belly when we are excited or scared. It is an extensive nervous system consisting of sheaths of neurons embedded in the walls of our gut (shown in Figure 1 in blue dots). The enteric nervous system controls our bodily functions such as the contraction of intestinal muscles, secretory glands of digestive organs, absorption in digestive tract and gastrointestinal blood flow.

When our body is experiencing stress (refer to Figure 2), stress hormones such as cortisol will be released in the body, alerting our body to switch from parasympathetic nervous system to sympathetic nervous system. When the body is in ‘fight or flight’ mode, gut function is reduced, and it is especially bad when this happens during or just after meal time. The food will be indigested or partially digested, resulting in lesser fuel for our body and our gut microbiome, making the environment in the gut unhealthy. Because of the lack of healthy microbiome to further digest the food particles, irritable bowel syndrome (IBS) symptoms such as diarrhoea, bloated stomach, stomach pain or indigestion will occur. Stress relieving practices such as deep breathing, meditation, yoga, walking or swimming are simple but are very effective towards alleviating IBS related to stress. These practices are able to do so by flipping the body’s sympathetic nervous system back to parasympathetic nervous system, where the ‘rest and digest’ mode is switched back on.

Importance of postbiotics produced by microbiome in GBA

It is a game changer when we realise how great an effect postbiotics or short-chain fatty acid (SCFA) have on our mental health via GBA.

In recent years, there has been growing recognition that gut microbiota exerts a profound influence on key brain processes through the highly interconnected GBA. Studies have showed that disruption in the neuronal and gut microbiome in prenatal and postnatal periods of mammalian development may lead to the onset of other brain disorders later in life. In addition, several brain disorders such as depression, Alzheimer’s and Parkinson’s diseases and autism spectrum disorder, have been linked to imbalances in the microbial composition of the gut.

There is growing evidence pointing towards postbiotics or SCFA and their pivotal role not only in gut health but also in brain health through the GBA.

SCFAs improve our gut health through a number of local effects, ranging from maintenance of intestinal barrier integrity, mucus production and protection against inflammation to reduction of the risk of colorectal cancer. In addition to exerting local effects in the colon and in the peripheral tissues, increased evidence supports a potential key role of SCFAs in GBA signaling and regulating central nervous system (CNS) processes through direct and indirect means and ultimately shape behavior and cognitive function.