The Tryptophan
Road.
The rarest amino acid in your food branches into your mood and sleep signals — and, far more of it, into a pathway that ends in the body's energy currency and passes through some powerful neuroactive molecules on the way. Which way it goes isn't fixed. It's a switch, and inflammation throws it.
One scarce molecule, pulled two ways.
This is the third of the four amino-acid roads. Tryptophan is the least abundant essential amino acid — the one your body guards most carefully — and it's asked to do jobs that compete for the same short supply. Most people know it as the precursor of serotonin and melatonin. That's true, but it's the small branch. The large majority travels a different road: the kynurenine pathway, ending in NAD, the currency every cell runs on.
Tryptophan comes only from dietary protein — meat, fish, eggs, dairy. After the body takes its share for building proteins, what's left faces a fork. A small fraction — only a few per cent — goes to make serotonin and melatonin. The large majority, roughly ninety-five per cent, goes down the kynurenine pathway toward NAD.[1]
Serotonin, melatonin — and an honest word.
The serotonin branch is short: tryptophan → 5-HTP (via an enzyme needing BH4, the same cofactor the phenylalanine road depends on, and which inflammation depletes) → serotonin → melatonin at night. A surprise for most people: about ninety per cent of the body's serotonin is made in the gut, not the brain, under the influence of the microbiome.[5] Serotonin doesn't cross into the brain, so the two pools are separate — one real thread tying gut health to mood.
Two arms, and the energy currency at the end.
Ninety-five per cent of the leftover tryptophan goes down the kynurenine pathway, which splits into two arms of opposite character — and ends in NAD, the same energy-and-repair molecule all of metabolism runs on.[2]
- The calming arm — kynurenic acid (made largely by astrocytes) blocks the NMDA receptor: neuroprotective.
- The excitatory arm — quinolinic acid (made largely by activated microglia) activates the NMDA receptor and is a pro-oxidant: neurotoxic in excess.
- The endpoint — NAD⁺, built from this pathway. Tryptophan is a genuine dietary NAD precursor, alongside niacin (vitamin B3).
In a healthy brain the protective arm dominates. The balance is not fixed, though — and in neuroinflammation it tips toward quinolinic acid. Which brings us to the switch.
The enzyme that opens the kynurenine pathway — IDO — is strongly switched on by inflammatory signals. So when the body is chronically inflamed, IDO pulls tryptophan away from serotonin and down the kynurenine road: the tryptophan steal.[3] Drag the fire up and watch what happens to the three products.
The effect is double: less tryptophan left for serotonin and melatonin, and more flowing into the arm that, in an inflamed brain, tilts toward the excitotoxic metabolite. Reduced calm-and-sleep substrate and raised neurotoxic metabolite, from one inflammatory switch.
A real thread — held honestly.
This is one of the better-evidenced biological accounts of how inflammation is linked to depression — not through a simple serotonin shortage, but through IDO activation, tryptophan diversion, and kynurenine metabolites acting on the brain.[3] And since insulin resistance and metabolic disease drive chronic low-grade inflammation, it's a plausible route by which the metabolic fire reaches mood and sleep.
- Adequate quality protein — tryptophan is scarce, so a real supply matters more than for common amino acids.
- Lowering the inflammatory load — the most on-mechanism lever, because it's inflammation that throws the IDO switch.
- Gut health — given how much serotonin is made in the gut, under microbial influence.
- Daylight and dark — the serotonin-to-melatonin conversion is timed by light; bright day and dark night support the sleep end.
A switch worth understanding — carefully.
The tryptophan road is the one this series has to handle most carefully, because it touches mood, and mood is where simple stories do the most harm. So the honest shape is this: tryptophan sits at a real switch; inflammation really does throw it; the kynurenine metabolites really do act on the brain; and metabolic health, by calming inflammation, really may protect this road.
Ask your body. Ask what the fire is doing to the road — and take a struggling mood to someone who can help.
One road among the many of the one fire. Back to the whole: The Invisible Fire.
Neither old slogan nor new one.
The old "low serotonin" story was simple, and it turned out not to be well-supported. The temptation is to swap in a new simple story — and that would be the same mistake in a new coat. What's true is more useful than either: a scarce amino acid, a switch thrown by the fire, kynurenine metabolites that reach the brain, and a metabolic route that may protect the road by keeping inflammation low.
One scarce amino acid, a switch thrown by fire, and an honest map of what we do and don't yet know.
[1] Platten M et al. (2019). Tryptophan metabolism as a common therapeutic target in cancer, neurodegeneration and beyond. Nat Rev Drug Discov 18(5):379-401. PMID 30760888. Read →
[2] Cervenka I, Agudelo LZ, Ruas JL (2017). Kynurenines: tryptophan's metabolites in exercise, inflammation, and mental health. Science 357(6349):eaaf9794. PMID 28751584. Read →
[3] Miller AH, Raison CL (2016). The role of inflammation in depression: from evolutionary imperative to modern treatment target. Nat Rev Immunol 16(1):22-34. PMID 26711676. Read →
[4] Moncrieff J et al. (2023). The serotonin theory of depression: a systematic umbrella review of the evidence. Mol Psychiatry 28(8):3243-3256. PMID 35854107. (No consistent evidence depression is caused by low serotonin — cited to keep the mood claims honest.) Read →
[5] Yano JM et al. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell 161(2):264-276. PMID 25860609. Read →