In my early 20's, I had pneumonia, and was treated with 3 rounds of anti-biotics. They may have saved my life so I'm grateful. And they also really messed up my digestion, and then my over all health. Within 2 years I went from being a fit, happy young man, to a fat, old and moody grump! I luckily met an environmental doctor who diagnosed my intestinal flora issues (all the good bacteria were also killed off) and I was on the road to recovery. For decades now I've had to manage this chronic issue, and the accompanying mental stresses... Eat the wrong thing, and I'd be noticeably depressed!
Now modern fMRI techniques and other studies as proving the brain/gut connection when it comes to our mental health and happiness. Fecal transplants have been shown to improve mental health and anxiety disorders.
https://www.sciencenews.org/article/microbes-can-play-games-mind
(In the vein of journalistic honesty, there are many who would say I was always moody!)
Yeah for science... for now I think I'll stick with my pro-biotics and careful diet. But I shall never say never.. just "Blech!"