This knowledge has been around for thousands of years.
I knew this in my early 20s because I went to visit a naturopath regular. The ancient Greeks and ancient Egyptians knew this, which is well documented in modern times. All naturopaths know this as part of their basic learning. Now with the internet and more people becoming aware of alternate medical treatments and are going to alternate therapists. The medical industry has had their grubby hands forced to admit this truth about intestinal health and mental wellbeing.