BACTERIA AND MENTAL HEALTH. “The vast majority GI tryptophan is not utilized for serotonin production, but rather metabolized via the kynurenine pathway. Alteration of kynurenine:tryptophan ratios has been linked with depression and anxiety. The gut microbiota may modulate this ratio directly.”
http://onlinelibrary.wiley.com/doi/10.1111/cmi.12585/full