Serotonin is central to mood, sleep and gut function — and roughly 90% of the body's supply is produced in the gut. Both serotonin and its competing pathways begin with the same dietary amino acid: tryptophan, which sits at a metabolic crossroads the microbiome helps control.
O'Mahony and colleagues (2015) reviewed how the brain-gut-microbiome axis governs tryptophan metabolism. Tryptophan can be routed toward serotonin synthesis or diverted down the kynurenine pathway, which produces both neuroprotective and neurotoxic metabolites. Because inflammation and microbial activity shift the balance, the microbiome can influence both serotonin availability and neuroactive kynurenine compounds.[1]
Yano and colleagues (2015) showed specific spore-forming bacteria promote serotonin biosynthesis by gut enterochromaffin cells; germ-free mice had markedly reduced gut serotonin, restored by the bacteria.[2]
Gut-derived serotonin does not simply cross into the brain to lift mood. Its relevance is indirect — via the vagus nerve, the gut environment, and the shared upstream supply of tryptophan.
Because tryptophan comes from food and the microbes that process it, this is one mechanism behind foods that support a steadier mood — and why feeding a diverse gut with fibre and fermented foods matters for the raw materials of neurotransmitters.