" 90 percent of the serotonin in our bodies—the neurotransmitter that when abundant makes us feel happy, and when depleted makes us feel depressed—is produced in our guts, and gut microbes play a major role in regulating its production (Yano et al. 2015). "

― Merlin Sheldrake , Entangled Life: How Fungi Make Our Worlds, Change Our Minds & Shape Our Futures