Your brain is in your guts – in many ways

You might already know that our nervous system extends to our gut – it’s called the enteric nervous system and it’s been popularized by many articles and books, such as The Second Brain. But there is something else there worth noticing there: bacteria.

We have between 10¹³ and 10¹⁴ microorganisms in our gastrointestinal system – yes, you read correctly: this is 10 times more than the total number of cells in our body and 100 times the number of genes… This huge and still relatively unknown population of bacteria has been called the “forgotten organ” and its composition, which varies a lot between individuals, is thought to be at least partially determined by our genes. Now that the human genome has been “discovered”, a significant worldwide effort is being invested in the characterization of the human microbiome and will probably lead among other things to a better understanding of the intimate relationships between our brain and our intestines.What emerges more and more clearly from research is the interdependence of that “forgotten organ” and the brain.

We all know that our brain can have a top-down impact on our gut flora: we can literally feel acute stress in our gut, through the intervention of what’s called the HPA (hypothalamus–pituitary– adrenal) axis on our gut flora, via the release of cortisol. Some experiments in rhesus monkeys have shown for instance that temporary maternal separation reduced the quantity of fecal lactobacilli, some rod-shaped bacteria. Other experiments in rats showed that such a variation in the flora could be permanent if maternal separation happened over a long period of time. These kinds of modifications in gut flora generally make the gut more vulnerable to inflammations. Chronic stress also creates leaks in the intestinal barrier, which increases the levels of the cytokines circulating in the gut that favor inflammations.

This adds itself to what is already commonly known: diet, infections, diseases and antibiotics all alter gut flora composition, and thus our moods. For instance, patients with symptoms of depression have been shown to suffer from what’s called fructose malabsorption and eliminating fructose from their diet resulted in a reduction of the level of their depression. Administering the bacteria Campylobacter jejuni — commonly found in animals feces (and sometimes unfortunately in food, thus being the main cause of gastroenteritis in human beings) — in very small doses in rats (to avoid direct immune reaction) resulted in anxiety-like behavior in mice. Gut microbiota can also modulate the function of the nervous system responsible for somatic pain perception. For instance, oral feeding of a Lactobacillus species to rats was capable of completely suppressing the perception of visceral pain.

What we eat and our relationships can thus have a direct and potentially long-term effect on our cognitive abilities, as we have always felt it to be the case – “in our guts”.