The kynurenine pathway — how chronic inflammation affects cognition and mood

Tryptophan is an essential amino acid — you cannot synthesize it, you must eat it. Most people know it vaguely as something in turkey that allegedly makes you sleepy, which is not actually how tryptophan works. What tryptophan actually does is occupy one of the most consequential metabolic forks in human biochemistry. When tryptophan is absorbed, roughly five percent of it enters the serotonin synthesis pathway: tryptophan becomes 5-hydroxytryptophan becomes serotonin, and some of that serotonin eventually becomes melatonin. That five percent is responsible for essentially all of the brain's associations with tryptophan in popular culture. The other ninety-five percent goes somewhere else entirely. It enters the kynurenine pathway — a metabolic route that most people have never heard of, and that turns out to be one of the most important bridges between the immune system and the brain.

The kynurenine pathway begins with an enzyme called IDO — indoleamine 2,3-dioxygenase — which converts tryptophan to kynurenine. IDO is upregulated by inflammatory signals, particularly interferon-gamma, the cytokine the immune system produces in response to viral infection, bacterial infection, and chronic inflammatory states. When IDO is activated, more tryptophan flows into the kynurenine pathway and less is available for serotonin synthesis. This diversion is not accidental. It appears to be an evolved mechanism: the immune system is deliberately consuming tryptophan during infection to deprive certain pathogens of an amino acid they need, while simultaneously generating metabolites — downstream products of kynurenine — with direct immunological functions.

Kynurenine itself doesn't stay as kynurenine for long. It is further metabolized along branching routes that produce metabolites with dramatically different biological effects. Kynurenic acid is one branch: neuroprotective, capable of antagonizing glutamate receptors — specifically the NMDA and AMPA receptors — and blocking the excitotoxicity that excess glutamate can produce. Quinolinic acid is the other major branch: neurotoxic, a potent NMDA receptor agonist that at high concentrations drives neuroinflammation, oxidative stress, and cell death. The balance between these two downstream products is not fixed. It shifts depending on the cellular environment, the inflammatory context, and the activity of the enzymes that govern each branch.

The consequences of that shift are significant. When chronic inflammation — from whatever source — persistently upregulates IDO, the result is sustained kynurenine production. Whether that kynurenine flows toward kynurenic acid or toward quinolinic acid determines what the brain experiences downstream. A shift toward quinolinic acid produces NMDA receptor activation, neuroinflammation, and is associated with the clinical pictures of depression, fatigue, and cognitive impairment. A shift toward kynurenic acid provides glutamate receptor antagonism and is generally neuroprotective — though at sufficiently high levels, even kynurenic acid has cognitive effects by reducing the glutamate-mediated neurotransmission that working memory depends on. The healthy state is a calibrated balance. Chronic inflammation tips that balance.

This is the mechanism that links depression to inflammation. The connection has been observed epidemiologically for decades — inflammatory conditions associate with depression at rates far above chance; depression patients show elevated inflammatory markers including IL-6, TNF-alpha, and CRP; treating inflammatory disease sometimes resolves depression; anti-inflammatory treatments show signal in depression clinical trials in some patient subgroups. The kynurenine pathway is one of the primary mechanistic explanations for these observations. Inflammation activates IDO, IDO diverts tryptophan away from serotonin and toward kynurenine, kynurenine shifts toward quinolinic acid in a pro-inflammatory environment, and quinolinic acid drives NMDA activation and neuroinflammation that manifests as depressed mood, anhedonia, fatigue, and cognitive slowing. The depressive symptoms of chronic illness are not merely a psychological response to being sick. They are, in significant part, a direct neurological consequence of the same cytokine signals driving the peripheral inflammation.

The clinical evidence supporting this framework is most developed in a few specific contexts. Interferon-alpha therapy — used historically in the treatment of hepatitis C and some cancers — produces depression in a large fraction of treated patients, often within weeks. The mechanism has been directly traced: interferon-alpha robustly induces IDO, kynurenine levels rise, and the ratio of kynurenic acid to quinolinic acid shifts toward the neurotoxic end. Patients who develop depression on interferon-alpha treatment have demonstrably altered kynurenine pathway metabolites compared to those who don't. This is not a correlation — it is a mechanism demonstrated in a controlled pharmacological context. The implication for understanding how any chronic inflammatory state produces cognitive and mood effects is direct.

Long COVID has brought this pathway into renewed attention. Persistent fatigue, cognitive impairment (the "brain fog" that has become a defining feature of the condition), and mood disturbance in long COVID patients have been associated in multiple studies with elevated kynurenine pathway activation. A 2022 study in Cell found that persistent tryptophan metabolism dysregulation — specifically reduced serotonin availability and elevated kynurenine — was associated with vagal nerve dysfunction and cognitive symptoms in long COVID. The researchers proposed a mechanistic chain: low gut serotonin (produced by enterochromaffin cells in response to microbial signals that may be disrupted post-COVID) reduces vagal afferent signaling, which impairs hippocampal memory function — with the concurrent kynurenine pathway shift compounding the neurological effects. This is not a simple story, but it illustrates how the same metabolic pathway sits at the intersection of gut health, immune activation, vagal function, and cognition.

Autoimmune disease provides another angle. Rheumatoid arthritis, lupus, inflammatory bowel disease, and multiple sclerosis are all associated with elevated IDO activity and kynurenine pathway dysregulation. The cognitive and mood symptoms that accompany these conditions — which patients often describe as among the most disabling aspects of living with the disease — have historically been attributed to the psychological burden of chronic illness or to the medications used to treat it. The kynurenine pathway literature suggests a more direct biological mechanism: the same inflammatory cytokines driving the peripheral joint or tissue inflammation are also driving central nervous system changes through tryptophan diversion.

The NAD+ connection adds another dimension to this picture that is worth understanding in its own right. One of the terminal products of the kynurenine pathway — produced through the quinolinic acid branch — is nicotinic acid, which is a precursor to NAD+. This means the kynurenine pathway is the body's primary de novo synthesis route for NAD+: when the exogenous supply of NAD+ precursors from food is insufficient, the kynurenine pathway is where new NAD+ gets made. Chronic IDO activation, then, does not merely divert tryptophan from serotonin toward quinolinic acid. It also represents an attempt by an inflamed system to produce more NAD+ — a metabolite that immune cells consume at extremely high rates during an active inflammatory response. The competition is real: immune activation increases NAD+ demand, IDO activation provides one route to meet that demand, but in doing so it produces quinolinic acid as an intermediate with neurotoxic consequences.

This connection between the kynurenine pathway and NAD+ biology is part of why NAD+ supplementation has been explored in the context of chronic inflammatory conditions, fatigue, and cognitive impairment. The rationale is not simply that "more NAD+ is better" — it is that chronic IDO activation may be depleting available NAD+ precursors in ways that compound cellular energy deficits, and that supplementing NAD+ precursors (NMN, NR, niacin) from outside the pathway may relieve some of the demand pressure that drives kynurenine pathway overactivation. The clinical evidence for this specific mechanism is limited — most NAD+ precursor research has measured metabolic and cellular endpoints rather than the kynurenine-specific or neurological outcomes that would directly test the idea, so the rationale remains mechanistically plausible but under-confirmed.

What the kynurenine pathway ultimately offers is a concrete biochemical bridge between inflammation and the brain — an explanation for why the fatigue, low mood, and brain fog of chronic illness are not simply psychological but trackable to how a single amino acid gets routed. The practical implication is that addressing inflammation upstream, through the foundational levers of omega-3 status, exercise, and sleep, may do more to rebalance tryptophan metabolism than targeting any single enzyme — and that the cognitive and mood symptoms of inflammatory states deserve to be taken as seriously as the physical ones.