The Second Brain: an exploration of how our gut microbiota and our brain work together

For the last century or so, scientists have been intrigued by the link between our diets and our mental health.
In fact, during the early 20th century Dr George Porter Phillips had a hunch that by looking at a patient’s gut, we could begin to comprehend the origins of depression. He had observed that many of his patients diagnosed with melancholia (a subtype of major depressive disorder) suffered from severe constipation along with other signs such as brittle nails and an unhealthy complexion.

Phillips wondered if by targeting the gut it would have a knock-on effect on his patients’ wellbeing. So, in his curiosity to find out he fed his patients a carefully restricted diet and offered them a fermented milk drink known as kefir, which contains lactobacillus bacteria. A microbe that is now commonly used in the manufacturing of probiotic products, with certain strains commonly found in the well known fermented milk drink, Yakult. Astonishingly enough it worked, with many of his patients either reporting no symptoms of melancholia or showing signs of significant improvement. It became some of the first evidence in suggesting that our gut flora can have a profound impact on our mental health.
Since this shred of evidence, research into the microbes of our gut has continued to reaffirm the fundamental idea of a gut-brain axis.
Additionally, the effect probiotics have on our gut flora is still of academic interest as I remember not too long ago writing a lab report during my degree on how the presence of milk proteins in Yakult increased the survivability of L.casei (the strain of bacteria in Yakult) in the gastrointestinal tract. Implying somewhat that probiotics in fermented milk products were superior in terms of ensuring they survived in the gut than opposing probiotic capsules.
Which makes sense, given the volume of advertising aimed at informing us how consuming yoghurts and fermented milk drinks will help promote and provide “tummy loving care”.

However, not only have probiotics become a staple in what is seen as a healthy balanced diet but they have also led researchers to discover that the gut microbiota when thriving, can naturally synthesize valuable neurotransmitters such as serotonin. While some antidepressant drugs have been used to increase serotonin uptake in the central nervous system, scientists have come to find that studies in mouse models provide evidence that the gut microbiota influences such levels. They where also able to discover that acetylcholine, an important neurotransmitter in cognitive function, particularly important in our ability to memorise and learn is a component of some bacterial strains found within our gut. So not only does our gut influence our happiness but it can also influence our ability to remember important details and learn new things. It clearly goes without saying that the gut is pretty special and crucial in determining what are bodies can and cannot do.

Alright let us draw our attention back to the brain now, shall we? There are a vast number of factors that can impact our mental health, ability to process emotions and thoughts, retain memories, our ability to move our bodies and continue to learn as we grow older. The main question is what can we do about these things by understanding our gut-brain link? Well …all of these things are now being considered by experts who study the brain and the gut (along with all the thriving bacteria found within it). With the aim being to develop therapies that work in favour of our gut, to treat or alleviate symptoms from conditions such as irritable bowel syndrome, anxiety, depression, autism, multiple sclerosis, chronic fatigue syndrome and neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease.
These objectives provide us with hope that we can produce therapies that mirror the hidden world in our guts and that could help us improve people’s lives.

During my time at university, I came across the book ‘GUT: The Inside Story of Our Body’s Most Underrated Organ’ by Giulia Enders. Not only do I recommend it to those who have an interest and enthusiasm for gut health and science, but I also recommend it as a brilliant book for those interested in how our gut functions and impacts our central nervous system, the allergies and food intolerances we may have, how and why we poop (sorry not sorry) and how it really is our second brain.

“Were the gut solely responsible for transporting food and producing the occasional burp, such a sophisticated nervous system would be an odd waste of energy. Nobody would create such a neural network just to enable us to break wind. There must be more to it than that.”

Giulia Enders, Gut: The Inside Story of Our Body’s Most Underrated Organ

Sources for those that wish to crawl further into the world of our guts and what’s inside them:

  1. Phillips, J. (1910). The Treatment of Melancholia by the Lactic Acid Bacillus. Journal of Mental Science,56(234), 422-430. doi:10.1192/bjp.56.234.422
  2. Gustaw W, Kozioł J, Radzki W, et al. The effect of addition of selected milk protein preparations on the growth of Lactobacillus acidophilus and physicochemical properties of fermented milk. Acta Sci Pol Technol Aliment. 2016;15(1):29‐36. doi:10.17306/J.AFS.2016.1.3
  3. Wikoff WR, Anfora AT, Liu J et al. (2009) Metabolomics analysis reveals large effects of gut microflora on mammalian blood metabolites. Proc Natl Acad Sci U S A 106, 3698–3703.
  4. Girvin GT & Stevenson JW (1954) Cell free choline acetylase from Lactobacillus plantarum. Can J Biochem Physiol 32, 131–146.
  5. Rowatt E (1948). The relation of pantothenic acid to acetylcholine formation by a strain of Lactobacillus plantarum.J Gen Microbiol 2, 25–30.
  6. Horiuchi Y, Kimura R, Kato N et al. (2003) Evolutional study on acetylcholine expression. Life Sci 72, 1745–1756.
  7. Probiotics For Mental Health And Wellbeing. [online] Available at: <; [Accessed 1 May 2020].
  8. 2019. The Microbiota–Gut–Brain Axis. [online] Available at: <; [Accessed 1 May 2020].
  9. Image Source: [Accessed 1 May 2020]