Why do we say that the intestinal microbiota is the "second brain"?

Here's a quick look at how Prof. Sokol answers this question from his patients

The gut microbiota is often called the "second brain" for several reasons. First, the gut is the organ that contains the most neurons after the brain. It contains about 500 million neurons forming what is called the enteric nervous system.

Secondly, the gut interacts with the brain through different mechanisms: nerve connections,  chemical molecules produced in the gut by human cells or by the microbiota, which can act either directly on the brain or indirectly via effects on the immune system, for example.

And finally, the brain has effects on our gut. For example, it is common to have digestive symptoms (abdominal pain, diarrhea) during a stressful event like an exam or a job interview.

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