There is a lot of biochemical signalling going on between the gastrointestinal tract and the nervous system. So much that the idea of gut-feeling, or thinking with something that is not your brain is more real than it seems. There is indeed what researchers call the gut-brain axis, a deep connection between the gut microbiota and brain function.

This is why foods containing probiotics can reduce anxiety and stress, and can improve the mood.

On the other hand, certain types of stress have been shown to cause increased intestinal permeability, causing gut bacteria to move into the systemic circulation, as well as organs such as the spleen, liver and mesenteric lymph nodes.  This process, often referred to as ‘leaky gut’, has also been implicated in the inflammatory pathophysiology of patients with depression or chronic fatigue.