How our microbiome affects our mental health

Our gut microbiome hosts a wealth of bacteria, viruses and fungi responsible for performing multiple essential functions. Steward of our digestive health, our microbiome maintains balance in our gut by regulating our pH, promoting effective intestinal muscle contractions while increasing nutrient uptake. But its role does not stop there! It secretes unique enzymes which help us digest nutrients, synthesize vitamins [3] and get rid of toxic or cancer-causing substances. It produces a crucial source of energy for gut cells. Above and beyond digestion, our microbiota also acts as a line of defence by strengthening the intestinal barrier and playing a key role in regulating the immune system [4].

Our gut and brain are in constant communication through a complex network involving the nervous system, endocrine system and immune system. Bacteria in our gut produce different substances such as short-chain fatty acids—important players only produced by the microbiome—, neurotransmitters like serotonin and GABA, and molecules which play a part in inflammation [5]. These substances can directly activate the vagus nerve or circulate in the blood as far as the brain. Microbiota uses these mechanisms to affect our brain functions as they relate to mood and stress.

As a result, an unbalanced microbiome can disrupt the connection and affect mental health, playing a role in anxiety and depression.

An imbalance in the microbiome, called “dysbiosis”, is caused when opportunistic bacteria overcome the good bacteria. That can disrupt balance in the gut, reduce production of crucial substances (such as short-chain fatty acids), increase inflammation, and lower the effectiveness of the intestinal barrier.

A study demonstrated that, when gut microbiota is transferred from depressive individuals to rats, the rats develop signs of depression. This reinforces the idea that the microbiota plays a role in mental health. Strangely, even though the diversity of bacteria remained relatively stable in persons with mental disorders, their microbiota makeup was unique [7]. Some bacteria which are crucial for producing short-chain fatty acids, themselves vital for the brain to function properly, seemed to be decreasing. However, other microorganisms, especially the ones that regulate mood and stress, were on the rise [8].

As fascinating as these findings are, they are only part of the story. The precise mechanisms connecting microbiota and mental health are not yet well understood; more research is needed to understand how to harness the connection for therapeutic purposes.

Some molecules produced by the microbiota are important in neurodegenerative diseases like Alzheimer’s and Parkinson’s. Microbiota affects the permeability of the barrier protecting the brain, allowing harmful substances to get through and cause inflammation of the brain, a key factor in the progression of these conditions [9].

Even though the direct connection between diet and mental health through the microbiome remains to be proven, we do know that a diversified microbiota is vital for overall health. A varied, balanced diet contributes to microbial diversity:

• Prebiotic fibres, found in fruits, vegetables and whole grains, feed the good bacteria in the gut. In particular, these fibres promote the growth of bacteria which produce elements needed for the gut to function properly [10]. Some studies have suggested that a fibre-rich diet may relieve symptoms of depression and anxiety. However, randomized clinical trials have not provided conclusive confirmation [11].
• Fermented foods, like yoghurt, kefir, kombucha, sauerkraut and miso can help expand enhance microbial diversity. However, there is not yet any demonstrated direct effect on mental health [12].
• Probiotics and postbiotics are being studied to learn about their potential effect on gut microbiome and, in turn, on some aspects of psychological well-being. More research is still needed to better understand their effects [13].
• Ultra-processed foods: limiting intake of ultra-processed foods, like foods supplemented with sugar, fat, dyes or preservatives is recommended, to maintain balance in the gut microbiome and mental health. Studies seem to indicate a connection between this kind of food and a higher risk of depressive disorders, possibly because they negatively affect microbial diversity and gut inflammation [14].

The relationship between stress and gut microbiome is both inseparable and complex. By releasing cortisol into the body, stress changes microbiome balance and modifies intestinal barrier function, which increases its permeability. That allows unwanted molecules to enter the blood stream, contributing to inflammation [15]. Stress is a major risk factor for irritable bowel syndrome. In turn, the microbiome impacts the brain through the substances it produces, thereby changing the response to stress. This interaction underlines the importance of a balanced diet and relaxation techniques, like meditation or sports, for maintaining mental health.

Physical activity is crucial for a balanced gut microbiome. Physical activity stimulates diversity of good bacteria, thereby strengthening gut flora. Doing sports increases production of those well-known short-chain fatty acids, which have positive impacts on the brain. Exercise has many positive effects on the microbiome and overall health. Keeping active is a way of maintaining our microbiota and our mental health [16]!

The gut-brain axis is a growing area of research which shows the microbiome’s central role in mental health. There are many studies suggesting that a balanced microbiota impacts mood and stress and plays a key role in some neurological conditions, but the precise mechanisms behind these processes still need to be clarified. A holistic approach which combines a varied diet, stress management and physical activity may improve psychological well-being. However, even though this hypothesis has been strengthened in animal models and some observational studies, clinical trials in humans remain limited and sometimes contradictory. Further research is needed to establish clear causal links and determine therapeutic strategies based on gut microbiota.