The gut-brain axis

So far you have seen the important role that our microbiota plays. It is not only a collection of microorganisms that live in your gut, but they also act as a small factor of molecules that affect us at different levels and without which our life would be more complicated. As you have already learned, the microbiota is involved in the proper development and functioning of our immune system and in our metabolism. What if we tell you that this is not all they can do? What if we tell you that our microbiota also communicates and affects our brain and our social behaviour and vice versa. The science in this area of knowledge is just beginning to be uncovered.

New discoveries will help us to understand the fundamentals of how the gut-brain relationship works and it will continue expanding the frontiers of our thinking. Even if today this field is completely revolutionary, the first studies on the topic that lead us to our current knowledge started several decades ago, far before we were born.

For decades, researchers have observed that there seems to be a connection between our brain and our gut, through which our mental state could affect the gut microbiota composition and also the microbiota might affect host behaviour. Historically, the gut has been pointed to play a key role in the maintenance of well-being, but it was not until the last 18th century when doctors started to develop theories about how distal parts of the body, including the gut, were connected via the nervous system. In 1765, the Scottish physician Robert Whytt first described the concept of nervous sympathy, as the mechanism that connects the inner body organs.

He observed that the gut possessed an abundant supply of nerve endings, thus influencing his ideas and the thinking of 19th century doctors. It particularly impacted theories from an English surgeon called John Abernethy, who took this into a fanatical level and renamed it in 1811 as gastric sympathy. Throughout his career, Dr. Abernathy focused all his efforts on answering questions such as why a blow to the stomach can disorder the mind and why emotional conditions, such as excessive worrying, reduced appetite? One of his main conclusions was that humans need to eat natural foods instead of refined, although its relation with microbiota was unknown at this point.

Similarly, several doctors throughout the 19th century started to claim the importance of the gut and its connection to the brain. Some examples, in chronological order, are Dr. James Johnson, Whiting S., anon, and Murray W. For them, it was about the stomach’s nerves, nothing to do with the bacteria that colonised the gut. It was some years later when the first associations of the potential role of gut bacteria and mental health started to appear. For instance, in 1910, the British chemist, George Porter Phillips, proposed that major depression could be treated by the administration of a gelatin whey which contained acid bacteria. Quite a visionary, right?

The last two decades have been one of the most important advances in the unravelling of the mysteries of the gut-brain connection have been made. In 1998, Professor Michael Gershon, who has been called the father of neurogastroenterology, presented his popular book on the gut as second brain. There he shared 30 years of research and the rediscovery of nerve cells in the gut that act as a brain, which can control our gut all by itself, as well as the importance of the two brains working together. This relation and connection was still presumed to be a relatively new concept in medicine.

Although at that time a greater number of researchers also started to wonder if the gut microbiota might play a crucial role instead of being based uniquely on the nerve system, hence, new studies bloomed under this new hypothesis.

One of the pioneer studies regarding the evidence of this relation took place in 2004. It showed that germ-free mice, mice raised without intestinal microbiota, exhibit an upregulated hormonal response to stress. Then in 2011, seven years later, it was demonstrated that the lack of a conventional microbiota affected behaviour, gene expression in the brain, and development of the nervous system. Now studies look forward to unravelling the role of specific bacteria in our mood that some researchers have called psychobiotics and if their administration could be a valuable tool to benefit our health.

Publications about the mechanisms through which bacteria reach our brain and its importance regarding our mental health keep on growing. New studies keep being released every day, but still there is a lot to be discovered yet. Although this knowledge nowadays opens a huge window of new alternatives to prevent or treat certain mental disorders.