Kynurenic Acid: Its Role in Your Brain and Health

Kynurenic acid is a naturally occurring molecule your body makes from tryptophan, and it acts as one of the brain’s built-in brakes on excitatory signaling. By blocking certain receptors that drive neurons to fire, it helps protect brain cells from being overworked to the point of damage. But the relationship between kynurenic acid and health is not simply “more is better.” In conditions like schizophrenia, levels in the brain run too high and appear to contribute to cognitive problems, while in Huntington’s and Parkinson’s disease, levels drop and leave neurons more vulnerable. That push-and-pull makes kynurenic acid one of the more fascinating and frustrating molecules in neuroscience.

Where It Comes From

Kynurenic acid is produced through what researchers call the kynurenine pathway, a series of chemical steps that breaks down the amino acid tryptophan. Tryptophan is the same amino acid often associated with turkey and sleep, but the vast majority of it is not converted into serotonin or melatonin. Roughly 95% of the tryptophan you consume is funneled into the kynurenine pathway instead.¹ Along the way, tryptophan is first converted into a compound called kynurenine. From kynurenine, the pathway splits in two directions. One branch produces neurotoxic metabolites, most notably quinolinic acid. The other branch, driven by enzymes called kynurenine aminotransferases, produces kynurenic acid. The balance between these two branches has enormous consequences for brain health.

What It Does in the Brain

Kynurenic acid’s best-established role is as an antagonist of NMDA receptors, a type of glutamate receptor that plays a central part in how neurons communicate, learn, and form memories. By sitting on these receptors and dampening their activity, kynurenic acid essentially turns down the volume on excitatory signaling in the brain.² That might sound counterproductive for learning and thinking, and in excess it can be, but in normal amounts it prevents the kind of runaway excitation that kills neurons.

For years, some researchers proposed that kynurenic acid also blocks nicotinic acetylcholine receptors, which would give it a second way to influence cognition, attention, and dopamine release.³ However, a detailed review of the evidence concluded that at least eight earlier studies found kynurenic acid blocked NMDA receptors but not nicotinic receptors, and five independent attempts to replicate the nicotinic blocking effect failed. The reviewers’ conclusion was blunt: there is no confirmed, reliable evidence that kynurenic acid antagonizes nicotinic receptors.²

Beyond NMDA receptors, kynurenic acid has at least two other confirmed molecular targets. It activates GPR35, a receptor found on brain cells called astrocytes, where it reduces certain signaling molecules and appears to quiet synaptic activity.⁴ It also acts as a potent activator of the aryl hydrocarbon receptor, a protein better known for sensing environmental toxins. Kynurenic acid’s activation of this receptor can influence immune responses and the expression of detoxification enzymes in the liver.⁵ These multiple targets help explain why kynurenic acid shows up in research on everything from gut disease to cancer.

A Shield Against Excitotoxicity

The kynurenine pathway produces metabolites that act in clearly opposite ways. On one side sit neurotoxic compounds like quinolinic acid and 3-hydroxykynurenine, which can overstimulate neurons and generate damaging free radicals. On the other side sits kynurenic acid, which counterbalances the toxic compounds by blocking the very receptors they exploit.⁶ The ratio between kynurenic acid and quinolinic acid has become a key measure in brain research, since shifts in that ratio reflect whether the brain’s chemical environment is leaning toward protection or toward damage.⁷

Animal experiments have directly tested this protective role. When researchers boosted endogenous kynurenic acid levels in rat brains, they could prevent the loss of dopamine-producing neurons that normally results from quinolinic acid exposure or NMDA administration.⁸ That finding is especially relevant for diseases like Parkinson’s, where dopamine neurons in the same brain region progressively die.

Why the Blood-Brain Barrier Matters

One detail that shapes the entire clinical picture is that kynurenic acid does not easily cross the blood-brain barrier. Early transport studies found that kynurenine, its precursor, is readily carried into the brain by an amino acid transporter, but kynurenic acid itself crosses at a far lower rate, essentially relying on slow passive diffusion.⁹ This means the brain’s kynurenic acid supply depends almost entirely on local production inside the brain, not on what is circulating in the bloodstream. The practical consequence is that even large changes in peripheral kynurenic acid levels, whether from food, exercise, or supplements, do not directly raise brain levels. Instead, those changes affect what happens in the rest of the body, a distinction that matters for understanding both exercise research and dietary claims.

Too Much in Schizophrenia, Too Little in Neurodegeneration

The diseases linked to kynurenic acid fall into two broad categories: those where brain levels are abnormally high and those where they are abnormally low. The pattern is consistent enough to suggest that deviation in either direction creates problems.

In schizophrenia, a systematic review and meta-analysis found that kynurenic acid levels are elevated in patients, specifically within the central nervous system, based on cerebrospinal fluid and brain tissue measurements.¹⁰ Elevated cerebrospinal fluid kynurenic acid has also been consistently linked to bipolar disorder.¹¹ The leading idea is that excess kynurenic acid over-blocks NMDA receptors, impairing the glutamate signaling that underpins working memory, attention, and cognitive flexibility. This hypothesis has driven interest in developing drugs that lower brain kynurenic acid as a way to treat cognitive symptoms in psychotic disorders.

Depression presents a different angle. When the immune system is chronically activated, as happens in inflammatory illness, the enzyme that launches the kynurenine pathway gets ramped up. This diverts tryptophan away from serotonin production and toward neurotoxic metabolites like quinolinic acid and 3-hydroxykynurenine.¹² In depression associated with inflammation, the problem is not so much that kynurenic acid is too high or too low, but that the toxic side of the pathway is overproducing relative to the protective side.

Neurodegenerative diseases tend to show the opposite of what is seen in schizophrenia. In Huntington’s disease, postmortem brain tissue shows the ratio of kynurenine to kynurenic acid is doubled compared to healthy controls, indicating that kynurenic acid production is reduced. Cerebrospinal fluid levels of kynurenic acid are also significantly lower in patients with Huntington’s compared to both healthy people and those with other neurological conditions.¹³ A similar deficit has been found in Parkinson’s and Alzheimer’s disease, where cerebrospinal fluid kynurenic acid is significantly reduced compared to controls, and in Alzheimer’s patients, blood kynurenine levels negatively correlate with disease severity.¹⁴ The interpretation is that with less of this natural brake on excitotoxicity, neurons become more vulnerable to damage from quinolinic acid and other excitatory insults.

Exercise and the Muscle-Brain Connection

One of the more striking findings in this field came from a 2014 study in mice that revealed a mechanism by which physical exercise changes kynurenine metabolism in a way that protects against depression. When muscles are trained, they ramp up production of kynurenine aminotransferases, the same enzymes that convert kynurenine into kynurenic acid. This happens in muscle tissue, not the brain. The result is that more circulating kynurenine gets converted to kynurenic acid in the periphery before it ever has a chance to enter the brain and be turned into neurotoxic metabolites. Since kynurenic acid itself crosses the blood-brain barrier poorly, the net effect is that less toxic material reaches the brain.¹⁵ Mice engineered to overexpress the relevant pathway in their muscles were resistant to stress-induced depression.

A follow-up study in humans confirmed that endurance exercise increases the expression of kynurenine aminotransferases in skeletal muscle and raises circulating kynurenic acid levels.¹⁶ This does not mean exercise raises kynurenic acid in the brain (remember the barrier), but it does mean exercise diverts the kynurenine pathway away from its toxic branch systemically. The finding offers a molecular explanation for why physical activity has such robust antidepressant effects, and it is separate from the more commonly cited endorphin and serotonin pathways.

Gut Health, Fat Metabolism, and Immune Modulation

Because GPR35, one of the receptors kynurenic acid activates, is heavily expressed in the gut, researchers have increasingly looked at kynurenic acid’s role outside the brain. In a mouse model of ulcerative colitis, GPR35-mediated sensing of kynurenic acid helped maintain the balance of gut bacteria and reduced colitis severity. The study identified specific bacterial groups whose populations were regulated by this signaling, suggesting kynurenic acid acts as a chemical signal helping the gut immune system keep microbial communities in check.¹⁷

In fat tissue, kynurenic acid appears to boost energy expenditure. Research published in Cell Metabolism showed that kynurenic acid, acting through GPR35, stimulated fat-burning, heat production, and anti-inflammatory gene activity in adipose tissue.¹⁸ These findings connect kynurenic acid to metabolic health in a way that goes well beyond the brain, and they overlap with the exercise story: the same pathway that muscles activate during exercise produces a metabolite that independently promotes healthy fat metabolism.

On the immune side, kynurenic acid plays a double-edged role. It contributes to resolving inflammation, which is beneficial after an infection or injury, but it also helps establish immunosuppressive environments. In the context of cancer, tumors can exploit this immunosuppressive quality to evade immune detection, partly through kynurenic acid signaling via GPR35 and the aryl hydrocarbon receptor.¹⁹ This is an area of active investigation in tumor immunology, and it underscores why simply boosting kynurenic acid levels is not a straightforward therapeutic goal.

Sleep Disruption and Age-Related Memory Loss

Elevated kynurenic acid in the brain does not just impair cognition through NMDA receptor blockade. Recent animal work suggests it also disrupts sleep architecture. When researchers raised brain kynurenic acid levels by injecting its precursor kynurenine, animals showed reduced REM sleep duration, weaker deep-sleep brainwaves (delta power), and fewer sleep spindles, the brief bursts of activity thought to consolidate memories during sleep. Blocking the enzyme that produces kynurenic acid in the brain prevented these sleep disturbances.²⁰ Since conditions that activate the kynurenine pathway, including chronic inflammation and infection, often come with sleep complaints, this finding suggests kynurenic acid may be part of the mechanism linking sickness to poor sleep quality.

Aging itself drives kynurenic acid accumulation. A study in the nematode C. elegans found that a significant portion of the decline in learning and memory that comes with aging is attributable to accumulating kynurenic acid. Worms engineered to produce more kynurenic acid showed worse age-related learning impairment, while worms engineered to produce less were partially protected against it. The researchers noted that altering kynurenic acid levels did not change the worms’ lifespan, so the memory benefits were not simply a side effect of living longer.²¹ While worm biology is far from human biology, the NMDA receptor system is conserved across species, and age-related increases in kynurenine pathway metabolites have been observed in human serum and cerebrospinal fluid as well.¹⁴

Can You Get Kynurenic Acid from Food?

Kynurenic acid is present in a variety of foods, though in small amounts. A survey of 37 food samples found it in every one tested. Honeybee products had the highest concentrations, with propolis leading the pack, followed by honey and bee pollen. Among common vegetables, broccoli and potato contained measurable quantities. When rats were given kynurenic acid by mouth, it was absorbed from the intestine into the bloodstream and transported to the liver and kidneys.²² Kynurenine pathway metabolites from food can be absorbed in the digestive tract.²³

However, there is a critical caveat: the amounts in food are tiny compared to what the body produces endogenously, and dietary kynurenic acid ends up in the bloodstream, not the brain. Given how poorly kynurenic acid crosses the blood-brain barrier, eating more broccoli or honey is not going to meaningfully change what is happening at your NMDA receptors. The dietary research is interesting for understanding the gut and peripheral immune effects discussed earlier, but anyone marketing a food or supplement as a way to boost brain kynurenic acid is ignoring the barrier problem.

Measuring Kynurenic Acid as a Biomarker

Because kynurenic acid levels shift in characteristic ways across different diseases, researchers are exploring whether measuring it could help with diagnosis or treatment monitoring. In epileptic spasms, a severe form of infant epilepsy, cerebrospinal fluid kynurenic acid is decreased, and the degree of decrease may predict which patients will respond to steroid treatment.²⁴ In Alzheimer’s disease, new rapid-detection tools are being developed to measure kynurenic acid and the kynurenic acid-to-tryptophan ratio in cerebrospinal fluid, with elevated levels in Alzheimer’s patients compared to controls, consistent with earlier findings.²⁵

There is a subtle wrinkle worth noting. Earlier sources describe reduced cerebrospinal fluid kynurenic acid in Alzheimer’s patients, while this newer biosensor study reports elevated levels. The discrepancy likely reflects differences in disease stage, patient populations, or the specific brain region and fluid compartment being measured. It also highlights that the field has not fully settled the question of whether Alzheimer’s involves too much or too little kynurenic acid at every stage. Biomarker research here is promising but still evolving.

Drug Development Targeting Kynurenic Acid

The clearest pharmaceutical angle involves lowering brain kynurenic acid in conditions where it is too high. Researchers have developed inhibitors of kynurenine aminotransferase II, the primary enzyme that makes kynurenic acid in the brain. One such compound, PF-04859989, was shown to restore glutamate release events in the prefrontal cortex of rats that had artificially elevated kynurenic acid levels. The drug is considered a promising candidate for treating cognitive symptoms in schizophrenia and other disorders characterized by excess brain kynurenic acid.²⁶ The same compound was used in the sleep study mentioned earlier, where it prevented kynurenic acid-driven sleep disturbances.²⁰

Raising kynurenic acid in the periphery is the other side of the coin, relevant for neurodegenerative diseases where brain levels are too low and for metabolic or inflammatory conditions where peripheral GPR35 activation could help. Exercise achieves this naturally. Pharmaceutical approaches might target the same muscle enzyme pathway, though no drugs have reached clinical use for this purpose yet. The challenge is always tissue specificity: ideally you would raise kynurenic acid in the periphery while leaving brain levels alone, or vice versa, and the kynurenine pathway’s presence in nearly every tissue makes that tricky.

One thing that makes this entire therapeutic landscape complicated is the immune modulation angle. Any intervention that broadly shifts the kynurenine pathway risks affecting immune surveillance, and as the tumor immune-escape research suggests, that is not a risk to take lightly. The molecules being developed are precise for a reason: they target a single enzyme step, in a specific tissue context, to nudge the balance rather than overhaul it. That is also why you will not find a credible scientist recommending a kynurenic acid supplement for brain health. The biology is too compartmentalized, too context-dependent, and too tightly linked to immune function for a blanket “more is better” approach to make sense.