It really makes a lot of sense when you think about it. There are more bacterial cells in the human body than actual human cells. We need this bacteria to survive and we have formed a symbiotic relationship with them.
I've been reading a lot about Buddhism and it does seem like people understood the connection between gut and mind before, just not in the scientific manner we understand it now.
Also, being a teacher of little kids, if a kid is having a tough time emotionally, my reflex reaction is to ask if they ate breakfast. I whip out a little snack and 99 percente of the time they feel better.
I know emotions are connected to our bellies but I've not thought about mental illness being so connected.