Did you know that there is a direct relationship between gut health and brain function? Growing research has established this link, which is known as the gut-brain axis. This article will look into the individual roles of the gut and brain, how they interact, and most importantly why the gut-brain axis is important.

Components of the gut-brain axis

The gut

Overview of the digestive system

The digestive system is responsible for breaking down food so that nutrients can be absorbed into the bloodstream to supply the cells of the body and waste/unwanted products can be excreted. The digestive system consists of the mouth, oesophagus, stomach, small intestine, and anus. Organs known as ‘accessory organs’ also play a role in the digestive process and include the liver, gallbladder and pancreas.  

Digestion

In digestion, the food first enters the mouth. In the mouth, salivary glands secrete saliva which sticks the masticated (chewed) food together to form a ball known as a ‘bolus’, which travels down the oesophagus. The smooth muscle of the oesophagus contracts in a process known as peristalsis, whereby the bolus is propelled into the stomach. 

In the stomach, the bolus is broken down by the churning actions of the muscular inner wall, and secreted enzymes then break down proteins, fats, and carbohydrates into a form that can be absorbed into the bloodstream. The secretion of digestive enzymes triggers the gallbladder, liver, and pancreas to release what are known as ‘digestive juices’. These digestive juices are aqueous solutions that help dissolve the bolus into a frothy liquid called chyme, which can easily pass through the intestines, have its nutrients absorbed and unwanted components excreted.

Figure Source: https://microbenotes.com/physiology-of-digestion/

Role of the microbiome in the digestive system

There is a misconception that microorganisms, particularly viruses, are all bad. However, there are good bacteria, viruses, and fungi that offer significant health benefits and are important for physiological functioning. The microbiome is a term that refers to all the microorganisms that live in or on the human body. The ‘good’ microorganisms in the microbiome, such as bacteria, viruses, and fungi, form a symbiotic relationship (both host and microorganism benefit) with our cells that plays an essential role in our health. Most of the microbiome is found in the gut, where it plays an important role in metabolism, host protection, and immune system development.¹

The microbiome can synthesise vitamins and essential amino acids, and cause biotransformation of bile. This, in addition to their ability to facilitate the breakdown of non-digestible carbohydrates, provides a means of recovering energy and useful substrates (a molecule on which enzymes act) for the body.¹

Distributed across the gut lining are attachment sites, at which pathogens (infectious microorganisms) can bind to and enter the cells lining the gut. Gut microbiota can provide benefits by competing with pathogens for these attachment sites, thereby inhibiting pathogen cell entry.¹

The brain

Overview of the central nervous system

The central nervous system consists of the brain and spinal cord. It’s similar in a sense to a central processing unit (CPU) in a computer – it receives and processes information and produces an appropriate response via an output.

General nervous system response

The nervous system consists of both the central nervous system and the peripheral nervous system. The peripheral nervous system consists of the nerves that are located at the extremities of the body – hands, arms, legs, etc. The peripheral nervous system detects changes in the outer environment and relays this information to the CNS. The information that is relayed via many neurons is known as a signal. Once the signal reaches the CNS, it is processed, and the CNS facilitates an appropriate response, which produces a favourable physiological reaction. In essence, the nervous system facilitates communication throughout the body.

Connection between the gut and brain

Research in recent decades has discovered that the gut and brain communicate in a bidirectional manner. This interaction between the gut and brain has been shown to play an important role in brain health. The gut achieves this communication with the brain through the release of several compounds which include vitamins, neurotransmitters, and neuroactive microbial metabolites. The exact mechanism by which these compounds exert their effects on the brain is currently unknown. However, studies have shown this connection between the gut and the brain.²

The gut microbiota is dynamic – it can be altered through diet and environmental factors. Therefore, the microbiota composition influences the interactions that take place with the brain and thus brain health. 

Hormones and their impact on the gut-brain axis

In addition to neuronal signalling in the nervous system, hormones also play an important role in the gut-brain axis. It has long been established that gut hormones play an important role in appetite and food intake. More recent research also suggests that gut hormones may be responsible for brain disorders such as anxiety and depression.^3,4,5

Importance of the gut-brain axis

Mental health

Impact on mood and emotions

Although in its infancy, research has found a link between the gut-brain axis and mood. Vagotomy studies in pre-clinical testing have found that the microbiota-vagus nerve interaction may be responsible for mood. Through blocking the vagus nerve, effects on mood were seen. Research looking into the impact of irritable bowel syndrome (IBS) on mood has also supported the notion that the gut-brain axis is important in mood. A one-year population-based study that assigned participants into three groups (Anxiety +IBS, Depression +IBS, and placebo) found that those with a higher baseline level of anxiety were more likely to develop IBS.⁶

Immune system

Role in regulating immune responses

The gut microbiota plays an important role in regulating immune function, whilst also aiding in the maturation of the immune system. The interaction between the gut microbiota and the host’s (own) cells influences antibody production and the population of immune cells in the body. Interestingly, these host-microbe interactions can influence immunity in the brain, affecting the development and activation of immune cells in the brain known as microglia and astrocytes.⁷

Digestive health

Impact on digestion and nutrient absorption

The gut is innervated by the enteric nervous system (ENS). The ENS is part of the peripheral nervous system and detects changes in the gut and sends signals back to the brain for processing. The ENS plays an important role in digestion by sending signals to the brain to stimulate the activation of processes such as swallowing and breaking down food. Thus, the ENS and brain rely on a healthy gut microbiota to carry out these functions appropriately.⁸

Factors that influence the function of the gut-brain axis

Diet and nutrition

The role of a balanced diet

The gut microbiota is dynamic and can change. One factor that can cause changes to the gut microbiota is your diet. Your diet greatly influences the type of microorganisms that inhabit your gut. To encourage a healthy gut microbiota, it is important to have a balanced diet, which introduces food that will benefit your microbiome. A poor diet can result in colonisation by microorganisms that can disrupt the normal functioning of the gut-brain axis. Western-style diets, which include processed, fried, and sugar-rich foods and low plant food consumption, have been shown to decrease gut microbial diversity and encourage the growth of opportunistic pathogens.⁹  

However, some diets support a healthy microbiome – for example, the Mediterranean diet (MD) offers a nutritionally balanced diet that provides many nutrients that benefit the gut microbiome. Higher adherence to an MD diet has been associated with reduced incidence of several disorders, which include obesity, type-2 diabetes, and metabolic syndrome.

Gut dysbiosis is common in these disorders and it has become evident that the MD diet influences the gut microbiome. Additionally, the MD diet can also suppress the growth of gram-negative bacteria that release endotoxins (toxic byproducts) and disrupt the gut wall barrier integrity.¹⁰

Lifestyle factors

Exercise and its impact

We all know how important exercise is and the multitude of benefits that it offers. In addition to its myriad of other benefits, exercise has also been shown to positively affect the gut-brain axis, by promoting gut microbiota diversity.^11,12,13,14

Stress management

Managing stress is another factor that influences the gut-brain axis. Stress has been shown to reduce microbial diversity, and numbers of ‘good’ bacteria such as Bacteroides and Lactobacillus spp. decreased. This consequently disrupts the communication between the gut and brain.¹⁵ Exercise can be an effective way to help reduce/manage stress.

Maintaining a healthy gut-brain axis

Probiotics and prebiotics

Beneficial bacteria

Through much research, the composition of the gut microbiota and species that constitute good bacteria have been identified. Bacteria belonging to the Lactobacillus and Bifidobacterium genera have long been established as safe and beneficial bacterial strains. Although microbiota composition varies from person to person, these bacteria are the most commonly identified in population studies.

Probiotics and prebiotics

Probiotics and prebiotics are commonly used terms in the health industry and have gained significant attention as a way of maintaining and improving gut health. Probiotics are supplements that contain many beneficial microorganisms in one dosage. These supplements tend to be in capsule form and are thought to introduce beneficial bacteria to the gut to help diversify and improve the gut microbiota.

Probiotics also include naturally fermented foods such as kimchi, kombucha, sauerkraut, kefir and natural fermented soya products. 

Prebiotics are plant fibres that provide food to good bacteria in the gut. By doing so, they promote the growth of good bacteria in the gut.

Summary

The gut-brain axis plays a significant role in our health. Through aiding in digestion processes to playing an important role in the immune system and mental health, maintenance of the gut-brain axis can improve various aspects of your life. 

Your gut microbiome is incredibly dynamic - so through lifestyle choices such as diet and exercise, you can effectively alter your gut microbiome, which will then generally improve the functioning of the gut-brain axis. To maintain a healthy gut-brain axis, you can also incorporate prebiotics and probiotics into your routine.