Neurotransmitters and Hormones

GLP-1 and PYY are gut-produced peptides that affect mood, memory, and learning. Gut microbes can synthesize neurotransmitters or stimulate intestinal cells to produce them. Gut-derived neurotransmitters do not directly cross the blood-brain barrier, but they can influence the brain through vagus nerve stimulation. Gut inflammation can divert tryptophan away from serotonin production toward the kynurenine pathway. Most serotonin and more than half of dopamine production occur in the gut.