Serotonin is vital to mental health but guess what? 95% of it is produced in the stomach! That means many of us could find good mental health through diet and probiotics! It also plays a major role in sexual desire/function for both men and women!
Links:
https://www.healthline.com/health/irritable-bowel-syndrome/serotonin-effects#outlook
http://m.caltech.edu/news/microbes-help-produce-serotonin-gut-46495