Overview

Did you know that your gut can have a direct influence on every chemical process that happens in your body? This includes your mood and overall well-being. This is because of the enteric nervous system, which controls your gastrointestinal (GI) tract. Operating independently from your brain, the enteric nervous system is often referred to as “the second brain”.

However, this nervous system is not the only key player in establishing the gut-brain connection. Another essential component is a fascinating collection of tiny tenants called the gut microbiome. The gut microbiome is important in producing vital chemicals in your body, such as GABA, serotonin, hormones, etc.

What is the gut microbiome?

The gut microbiome is a powerful group of trillions of microorganisms that reside in your gut alongside their genetic material. These microorganisms include bacteria, fungi, and viruses, but mostly comprise bacteria. Microbial populations typically inhabit 5 major areas in the body:

• Airway passages
• Skin
• Mouth
• GI tract
• Vagina

The largest microbiota population is in the GI tract, that is your gut.¹ 

Your gut microbiome extends other benefits beyond your digestive tract. These microorganisms are involved in:

• Immune regulation
• Metabolism
• Mood and overall well-being

As each person has a specific DNA profile, each person also has a unique bacterial fingerprint. Some conditions can also affect the type of bacteria that lives in your gut. These can include illness, stress and lifestyle habits (i.e. diet and exercise).

A healthy gut microbiome simply means having a well-balanced and diverse range of microorganisms in your digestive tract. The gut microbiome is dominated by the Bacteroides and Firmicutes groups of bacteria.² However, over 50 bacterial phyla (the classification of different groups of bacteria) and 1000 microbial species-level phylotypes have been detected in the gut.³

Why do you need a healthy gut microbiome?

In a healthy state, your gut microbiome performs a myriad of functions. For instance, carbohydrates and simple sugars, the main components of the food you eat, are metabolised by your gut microbiome. Bacteroidetes and Firmicutes can ferment fibres (indigestible carbohydrates) to produce branched-chain and short-chain fatty acids (SCFAs) which are microbial metabolites, such as butyrate, propionate, and acetate. 

These SCFAs perform several essential functions in the human body.⁴ Butyrate is regarded as the most essential SCFA for human health. Butyrate has the potential to act as an anti-carcinogen as it promotes programmed cell death of colon cancer cells and regulates gene expression.⁵ Propionate serves as an energy source for the epithelial cells in the liver and plays a vital role in gluconeogenesis.⁶ 

Additionally, a healthy gut microbiome is crucial for mental health. The gut-brain axis is a strong line of communication between your gut and brain (although your gut does most of the talking). A healthy gut can enhance mood and cognitive function as well as reduce anxiety and depression. Contrarily, an unhealthy gut or dysbiosis has been linked to mental health issues, such as anxiety and depression.⁷ 

How does your gut microbiome affects your mood?

Your gut microbiome affects your mood and well-being through these pathways:

The vagus nerve

This is the longest nerve of the autonomic nervous system and serves as a direct link between your brain and gut microbiome. However, the vagus nerve doesn’t cross the epithelial barrier of your gut, instead, it senses microbial signals in the form of microbial metabolites (SCFAs).

These microbial metabolites (SCFAs) stimulate the enzyme responsible for serotonin synthesis from dietary tryptophan (i.e. when you eat protein foods rich in tryptophan). Serotonin is often dubbed the “happy” or “feel-good” hormone, as it is associated with positive emotions and calmness.

Therefore, the secretion and synthesis of serotonin from your gut will help to boost your mood. In fact, 95% of the body’s serotonin is said to be produced by your gut – this shows how vital our gut is in influencing mood.

The vagus nerve also signals information from the gut to the nucleus of the solitary tract (NTS) and the dorsal raphe nucleus (which houses the brain’s serotonin neurones). These two areas then interact with the emotion-regulating networks of the brain that influence mood.⁸

Immune signalling

The epithelial cells are not in direct contact with the gut microbiome. This implies that the immune cells can only be stimulated when the barrier (i.e. mucus layer) separating them is broken or compromised. For instance, in the state of chronic stress, cortisol is released by the HPA axis leading to a decrease in mucus production.

Once this permeability is affected, it allows bacterial antigens and lipopolysaccharides (molecules consisting of fats and sugars found on bacteria cell membranes) to enter into circulation and activate immune cells. This intestinal permeability is associated with gut-related disorders, such as irritable bowel syndrome (IBS) and necrotising enterocolitis.⁹ Additionally, increases in intestinal permeability are now recognised as features of:¹⁰ 

• Rheumatoid arthritis
• Alzheimer’s disease
• Asthma
• Inflammatory bowel disease

Simply put;

The gut bacteria stimulates the release of chemicals (e.g., cytokines, neurotransmitters, neuropeptides, and microbial metabolites).

These chemicals infiltrate the bloodstream and lymphatic system (part of the immune system), where they activate immune cells. 

The immune cells send signals to the brain through nerves.

These signals regulate the brain’s activity and behaviour.

In response to these signals, the brain can send signals back to the gut to alter the gut environment.

Factors that influence your gut microbiome

Things you do in your daily life can affect the composition of your gut microbiome. It is clear that our relationship with the gut microbiome is complex and almost always mutually beneficial until things go wrong. The following are factors that influence the microorganisms residing in your gut:

• Diet
• Antibiotics
• Chronic stress
• Your living condition
• Age 
• Exercise and sleep

The most prevalent of all these factors is your diet. This is because when you eat, you are not just feeding yourself, but also the trillions of microorganisms that live in your gut. This, in turn, modifies our microbial diversity in either a positive or negative way depending on your diet. This gut microbiome can further affect our health/disease status and overall well-being. 

FAQs

How do you know if your gut microbiome is unhealthy?

The signs of gut microbiota dysbiosis include:

• Frequent digestive issues
• Sleep problems
• Constant sugar cravings
• Mood problems
• Sleep disorders

However, it should be noted that if you happen to experience these symptoms, it doesn't necessarily mean your gut is unhealthy. If you are concerned about your symptoms, seek advice from a healthcare professional. 

What kills the good bacteria in the gut?

Taking antibiotics or consuming processed foods and sugars frequently can eliminate certain bacteria in your gut, resulting in a less diverse gut microbiome.

How can the gut microbiome affect your mental health?

The gut microbiome produces and regulates certain neurotransmitters, such as serotonin, dopamine, and glutamate. These play vital roles in neurological and immunological activities in the brain and reduce the risk of brain-related disorders.

Summary

• Your gut microbiome plays a significant role in your physical, mental, and psychosocial health
• The diversity of gut microbiome is positively associated with health; a well-balanced gut microbiome keeps us from getting sick and prevents worsening of health conditions. It helps our bodies work more smoothly and reduces the development of mental conditions
• The gut microbiome produces as much as 95% of the body’s serotonin which transmits messages between nerve cells and helps to regulate sleep, mood, and digestion. Gut microbiota also aids in the synthesis of other neurotransmitters which play a role in anxiety and depression
• The gut even has its own nervous system called the enteric system which can function independently of the brain. “You are what you eat” isn’t just a popular phrase, it is also our reality when it comes to the composition of our gut microbiome and its effect on our health