Psychobiotics: gut and mental health

Psychobiotic is a recently coined term and refers to the relationship between the mind, bacteria and the gut.

Only in the last 10-15 years has the direct connection between the intestine and the brain been discovered with certainty (so much so that the intestine is called “the second brain”). However, it had been several decades that researchers had begun to orient their studies on the relationships existing between the intestinal flora and our physical and mental health.

The human organism from birth is inhabited by a wide range of microorganisms (bacteria, fungi, archaea, protozoa and viruses) that live and colonize the exposed body surfaces, the mucous membranes communicating with the outside and especially the digestive tract, where they perform the main functions. This set of microorganisms, in the past called by the term “bacterial flora” or “intestinal flora” because in ancient times the bacteria were still little known and were considered as plants, has recently been called Microbiota .

Intestinal microorganisms

We currently know that this large community of intestinal microorganisms (the weight of microbes in our intestine is almost equal to the weight of the brain) can produce substances that are very beneficial for health. For example, vitamins or molecules such as neurotransmitters, which, reaching the brain, can generate different feelings and emotions. It is therefore scientifically proven that the state of our intestines affects our mood and mental health.

But it’s a two-way street, as the mind also affects our gut. The first assumptions were derived from experiments in rats which showed that a stressful condition , such as early separation from their mother, can alter their microbiota and consequently their behavior.

Scientific studies

The crucial role of the gut microbiota in brain development has been demonstrated in animals lacking the microbial heritage. One study found that mice raised in sterile environments and therefore devoid of indigenous bacteria (germ-free mice) showed abnormalities in the maturation process of the nervous system and exaggerated physiological reactions to stress, reversible through bacterial recolonization induced by specific drugs.

Gut bacteria have since been shown to participate in the regulation of various important physiological processes, including immunomodulation, adiposity and energy balance, as well as the activity of the intestinal nervous system.

Microbiota in the fetus and newborns

Confirmations in this sense have then come from more recent human studies which have ascertained that the characteristics of the microbiota are already determined at birth. In particular, babies born with caesarean section acquire a different microbiota than those born with vaginal birth due to the different exposure to microbes.

In fact, the intestine of the fetus, sterile until birth, can be colonized by the bacteria present in the vaginal canal in the case of natural birth. While, in the case of a caesarean section, this does not occur, with the consequence that the impact with environmental bacteria will be more dangerous.

The richness of the intestinal microbiota is then positively influenced by breastfeeding, which provides the most suitable nutrients not only for the growth of the infant’s organism, but also for the selection of the best microbiota for its intestine.

This affects the duration of weaning, which is useful in allowing the intestinal barrier to mature sufficiently before coming into contact with proteins or other potentially allergenic substances.

The microbiota throughout life

If, on the one hand, it is certain that the “core” of the microbiota will not change throughout life, it is also shown that a large part of its composition will instead be destined to change depending on various environmental factors such as lifestyle, use of drugs and disinfectants and of course nutrition. Exposure to antibiotics and the intake of low-fiber foods are considered negative factors for the maintenance of an adequate intestinal bacterial patrimony .

The gut behaves like a huge sensory organ, constantly feeding the brain with information, but how does the microbiota send signals to the brain? This is done through neurotransmitters – some microbes are capable of producing most of the important brain neurotransmitters. In particular, we know that 90% of our serotonin is synthesized in the intestine.

Diet is therefore considered one of the main factors that have an impact on the human intestinal microbiota, from infancy to old age, and the term Probiotics has therefore been coined to indicate live micro-organisms which, administered in adequate quantities, bring a benefit to the guest health.

Probiotics and mental health

Since probiotics help maintain intestinal well-being, several researchers have hypothesized that they could also positively affect our emotional well-being . According to this line of studies, therefore, an altered composition of the gut microbiota could play a role in the development of depression . At the same time, the reconstitution of a balanced bacterial patrimony can represent a goal for the prevention and treatment of this disorder.

Some research seems to confirm this. In fact, in 2016, an article was published which, based on 5 clinical studies in which volunteer participants had undergone treatment with probiotics, found that overall probiotics reduced the risk of developing depression in healthy people and that they relieved the symptoms of depression in those affected by it.

So the gut acts like a huge sensory organ, constantly feeding the brain with information.

But how does the microbiota send signals to the brain? This happens through neurotransmitters: some microbes are capable of producing most of the important brain neurotransmitters, in particular, we know that 90% of our serotonin is synthesized in the gut .

Innovative therapeutic strategies

Hence the rapid growth of new therapeutic strategies for the treatment of psychiatric disorders that target the gut microbiota. The International Society for Nutritional Psychiatry Research in 2015 published a study indicating which are the nutrients that help prevent and alleviate some mental alterations such as depression and anxiety .

In this list of substances we find omega 3 fatty acids, B vitamins, tryptophan, magnesium and zinc.

But if gut bacteria can communicate with the brain then why not try to control this communication to have a positive effect on the mind, for example by decreasing anxiety and depression?


It is on the basis of these considerations that the term psychobiotics was introduced , to indicate products containing live organisms that could make a contribution to the treatment of patients with psychiatric diseases if ingested in adequate quantities.

For example, Bifidobacterium longum was found to have profound anti-anxiety activity , appear to improve cognitive abilities in mice, reduce stress responses and improve memory.

In an article published in Trends in Neurosciences, Philip Burnet, a professor of psychiatry at the University of Oxford in the United Kingdom, argued that taking probiotics is just one of the possible approaches to psychobiotics. “In fact, we are aiming to broaden the definition of this term to include drugs such as antidepressants and antipsychotics that affect intestinal bacteria.”

In fact, it has recently been shown that some psychotropic drugs cause an increase in the variety and number of intestinal bacterial flora, thus opening the way to new knowledge about the mechanisms of action of these drugs.


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