The term “gut-brain axis” refers to the interrelationship between the gut microbiota and the brain. The gut microbiota is made up of the microorganisms that inhabit our intestines, and comprises billions of microbial cells including bacteria, viruses, fungi, and other microorganisms. These microorganisms are capable not only of influencing the overall health of the host, but also behavior and cognitive functions.
Dietary composition modulates the composition of the microbiota and the production of its metabolites in the gut, most likely due to the prebiotic effect of fiber found exclusively in plant foods. A diet rich in unprocessed plant foods has been shown to promote greater variety and abundance of beneficial species, thanks to its fiber and phytocompound content. Indeed, compared to omnivores, vegetarians show a higher Prevotella / Bacteroides ratio and a lower Firmicutes / Bacteroidetes ratio.
In turn, the microbiota produces indispensable molecules such as vitamin K, short-chain fatty acids (SCFAs), and other metabolites, including neuroactive molecules (such as serotonin, dopamine, and butyric acid), which act as neurotransmitters. Since some of these molecules are capable of influencing the functioning of our brain, regulating cognitive functions, behavior, and mood, the term gut-brain axis has been coined.
Alterations in the microbiota have been associated with the development of psychiatric disorders (major depression and bipolar disorder, schizophrenia, autism spectrum disorders, and attention deficit hyperactivity disorder), as well as neurodegenerative diseases: increased permeability of the intestinal barrier, with activation of the immune system and consequent chronic inflammation, is thought to be responsible for neuroinflammation and neurodegeneration.
Therefore, a healthy microbiota represents a factor that promotes optimal health, both physical and mental. The composition of the microbiota depends above all — though not exclusively — on diet: thus, deciding what to put on our plate can help prevent psychiatric and neurodegenerative diseases, which are increasingly common in Western societies.
