The “feel-good” brain chemical serotonin is made in the gut. Things down there have to be working to make serotonin. Here’s an interesting article:
https://www.amymyersmd.com/article/serotonin-gut-health
Yep. A healthy intestinal biome is also necessary for a healthy, responsive immune system. The brain, too, is affected by intestinal bacteria. Too many of the bad ones leads to psychological problems. In fact, diet has been used to treat everything from ADHD/autism to epilepsy and MS.
That’s why I eat fudgecicles