The role of the gut and its microbiota. Gut-brain axis

The study of gut microbiota has been a growing field for some years now. In addition to supporting good digestive health (better digestion, better intestinal transit, lower risk of colon cancer, fewer gastrointestinal symptoms such as heartburn, bloating, etc.), the presence of a "healthy" gut microbiota has been associated with multiple health benefits. Nowadays, the microbiota-gut-brain axis is often referred to, and it is considered that having a good gut microbiota allows us to have better neurological, psychological, immunological or hormonal functions, among others.

     In the gut there is a large network of nerve endings originating from the sympathetic and parasympathetic autonomic nervous system, especially the vagus nerve and spinal cord nerves, as well as an intrinsic neural network, called the enteric nervous system. These networks are interconnected. Through the release of numerous chemicals (neurotransmitters, short-chain fatty acids, peptides, hormones, cytokines, etc.), the vagus nerve enables constant communication between the brain and the gut. The communication is bidirectional: the vagus nerve releases substances in the gut if it is an efferent information and the gut transmits other chemicals to the vagus nerve so that the information travels to the brain, if it is an afferent message. These chemicals can be produced both by nerve endings, intestinal or immune cells in the gut wall, and, to a large extent, by the microbiota. Thus, the microbiota plays a crucial role in gut-brain communication. I would like to clarify that the vagus nerve, while being the main communication pathway between the gut and the brain, is by no means the only one. Another important communication pathway is the blood pathway (hormones, cytokines or other chemicals produced in the brain travel to the gut via the blood, and vice versa). I will not go much further into the exciting world of the gut-brain axis, which could be the subject of several books, but, by way of example, I could say that it is more than proven that people suffering from psychiatric illnesses such as depression or anxiety, neurodegenerative diseases such as Alzheimer's or Parkinson's or neurodevelopmental disorders (autism spectrum disorders for example) often have a profound alteration of their gut microbiota, known as "dysbiosis". 

    The gut is home to around 80% of our body's immune cells, as it is the main "customs house" of our body, where the greatest exchange with the outside world takes place. The interaction between microbiota and gut immune cells from the first days of life is essential for a person's immunity to develop normally. The immune system thus learns to tolerate those micro-organisms that are part of our microbiota and that help us in multiple functions of our body, and to attack those micro-organisms that are dangerous to our health. 

     The intestinal microbiota is also our great ally when it comes to correctly digesting and assimilating many of the foods we eat. Part of the digestion of certain molecules in our food, especially carbohydrates and some proteins, is carried out by certain microorganisms. More importantly, these micro-organisms manufacture what are known as short chain fatty acids (SCFAs) (butyrate, propionate and acetate), which are thought to have numerous effects on our bodies. In fact, it is now thought that SCFAs are capable of regulating between 5% and 20% of our genes, thus acting on our metabolism as well as on cell differentiation and proliferation. They also regulate the gut's immune response, as mentioned above, hormone production by cells in the intestinal wall and intestinal motility, promoting transit. They are also an important source of energy, especially for our gut wall cells. The micro-organisms in our microbiota also manufacture neurotransmitters, as I have already mentioned, and vitamins (such as vitamin K or vitamins of the B group, mainly B12). Likewise, the "good" micro-organisms in the microbiota produce substances, such as hydrogen peroxide or bacteriocin, which kill or inhibit the growth of other "bad" or "less good" micro-organisms. So you can get an idea of how important it is to have a healthy microbiota, without dysbiosis.

Relationship between gut microbiota and urogenital system

The presence of certain intestinal micro-organisms can trigger different immunological or neuro-hormonal reactions that affect the neuro-immune-hormonal state of our body, and more particularly that of our urinary system. A state of chronic low-grade inflammation caused by the consumption of pro-inflammatory foods (cereals, cow's milk products, products containing toxic substances, etc.) or by the presence of certain pro-inflammatory micro-organisms can generate an altered systemic immune response, either by excess or defect, an alteration in the levels of certain hormones and neurotransmitters such as cortisol or serotonin, and a propensity to urinary tract infections or other urological problems. Similarly, an insufficient production by the microbiota of certain "relaxing" neurotransmitters such as GABA could theoretically lead to muscular hypertonia and a consequent deficit of sphincter relaxation, favouring the onset or worsening of uncoordinated urination or constipation. 

     The relationship between diet, gut microbiota and the urinary system is very complex, and we are still a long way from understanding it well. In recent years, it has been suspected that there is a constant exchange of information between the gut, urinary and vaginal microbiota (in the case of women) or prostate microbiota (in the case of men). This multidirectional connection is more complex than previously assumed, to the extent that the composition of one microbiota can influence the others, even without a "direct" transfer of micro-organisms. Much research is still needed to fully understand these mechanisms.

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