The Gut-Brain Axis: How Probiotics Influence Mental Health

The Gut-Brain Axis: How Probiotics Influence Mental Health

Have you ever felt a "gut feeling" before an important decision? Or lost your appetite due to anxiety? These everyday experiences point to something that science is only beginning to fully understand — the intimate communication between your gut and your brain.

What Is the Gut-Brain Axis?

The gut-brain axis is a bidirectional communication network connecting the gastrointestinal tract to the central nervous system. It involves:

• The vagus nerve — the main physical connection between gut and brain
• Neurotransmitters — signalling molecules produced by both the brain and the gut
• The immune system — gut microflora influences immune responses that in turn affect the brain
• Metabolic products — short-chain fatty acids produced by bacteria

This communication happens in real time and affects everything from mood to sleep quality.

Psychobiotics: A New Scientific Concept

Cryan and Dinan (2012) introduced the term "psychobiotics" — live organisms that, when ingested in adequate amounts, produce a health benefit in patients with psychiatric illness. This was a turning point because it formally recognised that certain probiotic strains could influence brain function.

The psychobiotics concept rests on the following logic:

1. Gut bacteria produce neuroactive substances
2. These substances influence the brain via the vagus nerve
3. Specific probiotic strains can positively direct this process

Serotonin: 95% Is Produced in the Gut

One of the most striking facts about the gut-brain axis is that approximately 95% of the body's serotonin is produced in the gut, not the brain (Yano et al., 2015). Serotonin is a neurotransmitter involved in mood regulation, sleep, and appetite control.

Although gut-produced serotonin does not directly cross the blood-brain barrier, it influences the brain indirectly — through vagus nerve signalling, immune responses, and metabolic products. The composition of gut microflora directly affects serotonin production.

Scientific Evidence: Liu et al. 2019

Liu et al. (2019) conducted an extensive meta-analysis covering multiple randomised controlled trials on probiotics and depression. Key findings:

• Probiotics had a small but statistically significant positive effect on depression scores
• The effect was stronger in participants under 60 years of age
• Multi-strain combinations showed better results than single strains

Important note: the authors emphasised that while results are promising, probiotics do not replace professional mental health treatment.

Studied Strains and Their Properties

Not all probiotics affect mental health equally. The most researched strains in this context:

Lactobacillus rhamnosus JB-1

This strain has shown remarkable effects on anxiety and stress behaviour in animal studies. Researchers found that the effect disappeared when the vagus nerve was severed — directly proving the vagus nerve's role in gut-brain communication.

Bifidobacterium longum 1714

In human studies, B. longum 1714 has demonstrated potential for:

• Reducing perceived stress
• Moderately attenuating cortisol response
• Supporting cognitive performance under stress conditions

The Vagus Nerve: A Biological Highway

The vagus nerve is the body's longest cranial nerve, stretching from the brainstem to the abdominal cavity. It serves as the gut-brain axis's main physical connection and functions as a bidirectional "highway":

• Bottom-up (80%): from gut to brain — informing the brain about gut status
• Top-down (20%): from brain to gut — influencing digestion and gut functions

Probiotics can modulate vagus nerve signals, thereby altering brain chemistry and behaviour.

A Practical Perspective

While gut-brain axis research is rapidly advancing and highly promising, it is important to maintain a realistic perspective:

What we know:

• There is clear bidirectional communication between gut microflora and the brain
• Certain probiotic strains can positively influence this communication
• Diet directly affects gut microflora composition

What is still being studied:

• Optimal strains and dosages for mental health support
• Long-term effects
• Individual differences in response to probiotics

Supportive Lifestyle Changes

Probiotics are one part of a holistic approach. Gut-brain axis health is also supported by:

• High-fibre diet — prebiotic foods (oats, garlic, onion) feed beneficial bacteria
• Fermented foods — sauerkraut, kefir, yoghurt contain natural probiotics
• Regular exercise — physical activity increases microbiome diversity
• Adequate sleep — sleep deprivation disrupts gut balance

Summary

The gut-brain axis is one of the fastest-growing fields in neuroscience. Cryan and Dinan (2012) laid the groundwork for the psychobiotics concept. The Liu et al. (2019) meta-analysis confirmed a small but significant effect of probiotics on depression scores. The vagus nerve is the primary physical connection between the gut and brain, and the gut produces approximately 95% of the body's serotonin.

Although more large-scale clinical trials are needed, the current evidence is strong enough to consider gut health an important component of overall well-being.

Dietary supplements are not a substitute for a varied, balanced diet and healthy lifestyle.

Browse our probiotic selection at MaxFit.ee →

References

1. Cryan JF, Dinan TG. (2012). Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nature Reviews Neuroscience, 13(10), 701-712.
2. Liu RT, Walsh RFL, Sheehan AE. (2019). Prebiotics and probiotics for depression and anxiety: a systematic review and meta-analysis of controlled clinical trials. Neuroscience & Biobehavioral Reviews, 102, 13-23.
3. Yano JM, Yu K, Donaldson GP, et al. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161(2), 264-276.
4. Bravo JA, Forsythe P, Chew MV, et al. (2011). Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proceedings of the National Academy of Sciences, 108(38), 16050-16055.
5. Allen AP, Hutch W, Borre YE, et al. (2016). Bifidobacterium longum 1714 as a translational psychobiotic: modulation of stress, electrophysiology and neurocognition in healthy volunteers. Translational Psychiatry, 6(11), e939.