If your brain is the control room, then your gut is the engine room. It’s here where all the goodness (and badness) gets sucked out of the various things we munch throughout the day. This is where innumerable enzymes, transporter proteins and various other bits of the gastrointestinal machinery get the job done. It’s also home to a magnificently large number of tiny specks of life, collectively referred to as microbes. These bugs are mostly bacteria, but they also include fungi, viruses and even more obscure life-forms.
A common factoid is that there are more microbial cells in the body than human cells. I’m pretty sure no one has counted it out, but there is at least quite a few of them and they almost certainly play a major role in immunity, metabolism and digestion and likely also affect things like mood, and even behaviour.