Tryptophan balance
Kynurenine pathway of tryptophan metabolism
Also searched as Kynurenine pathway of tryptophan metabolism.
The Mechanism Connecting Inflammation to low mood. Inflammatory signaling induces IDO, which pulls tryptophan down this branch and away from serotonin synthesis — so an inflamed person makes less serotonin from the same dietary tryptophan. The branch also terminates in NAD+ synthesis, which is why the same pathway appears in both mood and longevity discussions. Downstream metabolites divide into neuroprotective and neurotoxic arms; the balance, not the flux, is what matters.
In depth
The alternative fate of tryptophan — away from serotonin and toward NAD+ and neuroactive metabolites. Also carries the IDO enzyme step, which is what inflammation induces.
What drives it
Calms inflammation increases it.
Inflammation opens a second door for tryptophan. Pro-inflammatory cytokines — IL-1, IL-6, IFN-γ — upregulate indoleamine 2,3-dioxygenase (IDO1), the rate-limiting enzyme of the kynurenine pathway, in the periphery and the brain; the same happens after experimental LPS (PMC4631688, PMC5632654). In health the hepatic enzyme TDO handles about 90% of this traffic, but after immune activation the extra-hepatic IDO route takes on far greater weight. PMC4631688
Calms body-wide inflammation increases it.
Inflammatory signaling induces IDO, which pulls tryptophan down the kynurenine branch and away from serotonin synthesis. An inflamed person makes less serotonin from the same dietary tryptophan. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
What it drives
It reduces Mood (serotonin).
The arithmetic is the argument: the serotonin route consumes only about 1% of tryptophan, while the kynurenine pathway takes roughly 95% (PMC9142829). So a modest shift toward kynurenine starves a very small pathway of its only precursor. Shown by blocking it — after experimental brain injury, synaptosomal 5-HT fell and the fall was neutralized by the IDO inhibitor 1-methyl tryptophan (PMC9907831); IDO-inhibitor treatment likewise prevents inflammation-induced depressive-like behavior in rodents (PMC5753541). The limit, and it is a human null: a clinical study measuring tryptophan, kynurenine and the KYN/TRP ratio in serum and CSF found NO significant difference between depressed patients and controls (PMC5632654). The mechanism is solid in animals and in principle; the human biomarker case is not settled. PMC9142829
It reduces Mood.
The mechanistic route from inflammation to low mood. Downstream metabolites divide into neuroprotective and neurotoxic arms; the balance, not the flux, is what matters. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
It increases Cellular fuel.
The branch terminates in de novo NAD+ synthesis - which is why one pathway appears in both mood and longevity discussions. Carried by the intermediary-metabolism and clinical reviews (PMC9967593; doi 10.1016/j.molmet.2023.101755; PMC12293478; PMC7005133). Verbatim: “The kynurenine metabolic pathway is the sole de novo NAD(+) biosynthetic pathway, generating NAD(+) from ingested tryptophan”. doi 10.1016/j.molmet.2023.101755 · PMID 31954874 · PMC9967593
It increases Brain inflammation.
Quinolinic acid, a downstream metabolite, is an NMDA agonist. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
It reduces Calm pain signaling.
Tryptophan-metabolite imbalance is named in the central-sensitization literature. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
Blends that modulate it
Each blend page names this pathway and carries the study behind its claim.
See it on the map
Open the causal map to follow this pathway upstream and downstream, arrow by arrow, each with its study.