Acetyl-L-tyrosine

Acetyl-L-Tyrosine (N-Acetyl-L-Tyrosine)

1. Identity, Chemical Nomenclature, and Physical Properties

Acetyl-L-Tyrosine, most precisely designated N-Acetyl-L-Tyrosine (NALT), is an acetylated derivative of the conditionally non-essential amino acid L-tyrosine. N-Acetyl-L-tyrosine is a modified form of the amino acid L-tyrosine, where an acetyl group (–COCH₃) is attached to the nitrogen atom of the tyrosine molecule.

Chemical Identifiers

• IUPAC name: (2S)-2-(acetylamino)-3-(4-hydroxyphenyl)propanoic acid
• CAS Registry Number: 537-55-3
• Molecular formula: C₁₁H₁₃NO₄
• Molecular weight: 223.2252 g/mol
• Common synonyms: L-N-acetyl-tyrosine; L-Tyrosine, N-acetyl-; N-Acetyl-L-tyrosin (German); N-acétyl-L-tyrosine (French); Tyrosine, N-acetyl-, L-

Physical Properties

• Appearance: Typically white to off-white crystalline powder.
• Melting point: Generally melts around 140–145°C.
• Solubility: N-Acetyl-L-tyrosine is soluble in water and organic solvents like ethanol and dimethyl sulfoxide (DMSO). This contrasts with free L-tyrosine, which has poor aqueous solubility.
• Stability: It is stable under normal conditions of use and storage.
• Acid/base character: As a derivative of tyrosine, it can behave as a weak base or a weak acid depending on the pH of the solution.

N-acetyl-L-tyrosine is an N-acetyltyrosine in which the chiral centre has L-configuration, and it has a role as an EC 2.1.1.4 (acetylserotonin O-methyltransferase) inhibitor, a biomarker, and a human urinary metabolite.

2. Natural Source and Occurrence

NALT as an isolated chemical entity is not found in appreciable quantities in whole foods or plants; it is a semi-synthetic compound produced by the acetylation of L-tyrosine. However, its parent compound, L-tyrosine, is ubiquitous in nature.

L-Tyrosine is an aromatic amino acid synthesized de novo in plants and microbes. In animals, it must be obtained through their diet or synthesized from L-phenylalanine. In addition to protein synthesis, tyrosine serves as the precursor of neurotransmitters (e.g., dopamine and epinephrine) in animals and of numerous plant natural products, which serve essential functions in both plants and humans (e.g., vitamin E and morphine).

Dietary sources of L-tyrosine — from which the body can derive tyrosine — include:

• Meat: beef, pork, and lamb are all excellent sources of L-tyrosine. The protein in meat contains a relatively high proportion of L-tyrosine.
• Poultry: chicken and turkey are also rich in L-tyrosine.
• Fish: many types of fish, such as salmon, tuna, and mackerel, contain L-tyrosine.
• Dairy products: milk, cheese, and yogurt are sources of L-tyrosine, consumed widely around the world.
• Nuts and seeds: almonds, walnuts, and pumpkin seeds are among the plant-based foods rich in L-tyrosine.

In 1846, German chemist J. von Liebig discovered L-tyrosine in casein obtained from cheese. The name "tyrosine" is derived from the Greek word tyros, meaning cheese. E. Abderhalden and Y. Teruuchi, also in Germany, isolated it from silk waste in 1906.

The NALT molecule itself is produced synthetically. Synthesis of N-Acetyl-L-tyrosine can be accomplished from acetic anhydride and L-tyrosine.

3. Traditional and Historical Use

N-Acetyl-L-Tyrosine as a discrete chemical entity has no documented history of traditional use, as it is a semi-synthetic compound not found in whole foods or historical material medica. Its recorded use is entirely modern.

The use of tyrosine-rich foods for mental and physical resilience does have roots in traditional practices. High-protein foods such as meat, dairy, legumes, and seeds — natural sources of tyrosine — have long been recommended in traditional systems like Ayurveda and Traditional Chinese Medicine (TCM) to nourish the brain, improve strength, and stabilize mood. While they lacked the molecular understanding, ancient practitioners often recommended such foods during periods of convalescence, emotional stress, or mental fatigue.

The modern, targeted use of NALT as a supplement began in the latter twentieth century. The targeted use of N-acetyl-tyrosine as a supplement began in the late 20th century, particularly among biohackers, athletes, and those seeking enhanced cognitive function under pressure. Its earliest documented clinical applications were in parenteral (intravenous) nutrition, driven by the practical problem of L-tyrosine's poor water solubility making it difficult to incorporate into intravenous amino acid solutions. Due to its significantly higher water solubility, N-acetyl-L-tyrosine is incorporated into total parenteral nutrition (TPN) formulations to provide a stable source of tyrosine for patients with gastrointestinal dysfunction or those unable to consume oral nutrition. This application helps maintain positive nitrogen balance, supports metabolic demands during stress or illness, and is found in commercial products like Aminosyn II and Trophamine.

Inclusion in pre-workout and neuro-enhancement products has grown due to its ability to support neurotransmitter replenishment without directly overstimulating the central nervous system.

4. Active Compounds, Biochemistry, and Mechanisms of Action

4.1 NALT as a Prodrug for L-Tyrosine

NALT acts as a prodrug that undergoes deacetylation in vivo to release L-tyrosine, which is vital for protein synthesis, the production of catecholamine neurotransmitters (such as dopamine, norepinephrine, and epinephrine), and thyroid hormones.

4.2 The Catecholamine Biosynthetic Pathway

The catecholamine metabolic pathway in vivo starts with the amino acid L-phenylalanine, which is converted into L-tyrosine by the enzyme phenylalanine hydroxylase. L-tyrosine is then converted into the compound L-DOPA via tyrosine hydroxylase. L-DOPA is then decarboxylated via aromatic L-amino acid decarboxylase into dopamine, which later turns into noradrenaline via oxidation from the enzyme dopamine-beta-hydroxylase, and is then finally converted to adrenaline via phenylethanolamine-N-methyl-transferase.

Tyrosine is the initial precursor for the biosynthesis of dopa, dopamine, octopamine, norepinephrine, and epinephrine, which are fundamental by functioning as neurotransmitters or hormones for animals and humans.

4.3 Demand-Dependent Mechanism

A critical feature of tyrosine's pharmacology — and central to its evidence base — is that its effect on catecholamine synthesis is demand-dependent. Since tyrosine exerts its effects only when a localized deficiency state exists, its effects appear to be system-specific and present only when needed, such as when local catecholamine stores are expended. The ability of tyrosine supplementation to enhance the synthesis of catecholamines and their release from rapidly firing neurons but not from relatively quiescent cells has been demonstrated by using a variety of experimental manipulations.

L-tyrosine is the precursor of the catecholamines; alterations in the availability of L-tyrosine to the brain can influence the synthesis of both dopamine and norepinephrine in experimental animals and probably in humans. In animals, stress increases the release of catecholamines, which can result in the depletion of their levels, an effect that can be corrected by giving L-tyrosine. L-tyrosine does not seem to enhance the release of catecholamines when neurons are firing at their basal rates, but it does when firing rates are increased by stress.

4.4 Thyroid Hormone Biosynthesis

Tyrosine is also the precursor to thyroid hormones (T3 and T4). The thyroid gland uses iodine and tyrosine to synthesize these hormones. This means NALT, as a tyrosine precursor, participates in the same pathway, though direct clinical evidence linking NALT supplementation to meaningful changes in thyroid hormone levels in healthy individuals is limited.

4.5 Melanin Biosynthesis

Tyrosine is also the precursor to the pigment melanin. This pathway is relevant to skin and hair pigmentation biology, though it is not a primary target of supplemental use.

5. Bioavailability: The Critical NALT vs. L-Tyrosine Question

The rationale for formulating the acetylated derivative was the assumption that greater water solubility would translate to superior bioavailability and better blood-brain barrier penetration. The scientific evidence, however, substantially challenges this assumption and represents a key limitation of NALT compared to its parent compound.

N-Acetyl-L-Tyrosine appears to be able to contribute free L-tyrosine in vivo after IV administration, but is only able to increase L-tyrosine concentration approximately 20% despite much larger increases in serum NALT. 56% of the administered dose of NALT is excreted in 4 hours, and another study suggests that, overall, 35% of the total NALT dose (administered parenterally) is excreted via the urine as NALT and not L-tyrosine.

A clinical study of adult patients receiving the parenteral amino acid product Aminosyn II found that patients receiving Aminosyn II excreted approximately 35% of infused NALT unchanged in their urine.

In animal research published in Pediatric Research, infusion of N-acetyl-[¹⁴C]-L-tyrosine as part of a total parenteral nutrition regimen in rats at a level of 0.5 mmol/kg/day resulted in rapid labeling of tissue tyrosine pools, production of ¹⁴CO₂, incorporation of labeled tyrosine into protein, and modest urinary losses (8.3%). Plasma tyrosine levels, however, remained at fasting values (73.8 ± 5.40 µM). Infusion of N-acetyl-L-tyrosine at 2 mmol/kg/day increased plasma tyrosine above fasting levels (141 ± 16.1 µM), resulted in rapid labeling of tissue tyrosine pools, and incorporation of labeled tyrosine into protein. However, urinary losses were higher (16.8%).

In long-term parenteral nutrition studies, utilization of N-acetyl-L-tyrosine as a source of tyrosine in infusion solutions was tested in rats receiving total parenteral nutrition for 4 weeks, where two-thirds of the phenylalanine was replaced by a corresponding amount of N-acetyl-L-tyrosine. No differences in weight gain or nitrogen balance could be detected as a result of administering N-acetyl-L-tyrosine in place of the solution containing an adequate phenylalanine content.

Research comparing the two forms indicates that oral L-tyrosine supplementation reliably increases plasma tyrosine levels, while NALT has been shown to have a minimal or no effect on increasing these concentrations. Current research does not strongly support the idea that NALT is significantly more bioavailable or better absorbed than L-tyrosine. Studies have actually shown that NALT does not significantly raise tyrosine levels in the blood compared to L-tyrosine when similar doses are taken.

This is a meaningful limitation: the entire hypothesized benefit of NALT over L-tyrosine was improved delivery to the brain. Studies confirm its utilization as a tyrosine precursor during intravenous infusion, though about 35% may be excreted unchanged in urine, indicating incomplete conversion efficiency.

6. Scientific Evidence by Area of Use

Important note on study attribution: The large majority of human clinical studies have used free-form L-tyrosine, not NALT specifically. Because NALT functions as a prodrug for L-tyrosine (after deacetylation), its proposed benefits are mechanistically extrapolated from the L-tyrosine evidence base. Wherever the specific form used in a study is known, it is stated below.

6.1 Cognitive Performance Under Stress and Stress-Induced Cognitive Decline

This is the most robustly studied area for tyrosine supplementation. Consuming the amino acid tyrosine, the precursor of dopamine and norepinephrine, may counteract decrements in neurotransmitter function and cognitive performance. However, reports on the effectiveness of tyrosine supplementation vary considerably, with some studies finding beneficial effects while others do not.

A systematic rapid evidence assessment published in Military Medicine (2015) examined 10 randomized controlled trials and 4 controlled clinical trials. On the basis of the available evidence, no recommendation could be made for the effect of tyrosine on physical performance under stressful physical conditions. However, a weak recommendation in favor of tyrosine was made for cognitive stress as all studies showed a positive effect. The review indicates that the available evidence is insufficient to make confident recommendations on the effectiveness of tyrosine for mitigating stress effects on physical/cognitive performance. However, tyrosine may benefit cognitive performance and is worthy of further study.

A 2015 review in the Journal of Psychiatric Research concluded that tyrosine can enhance dopamine and norepinephrine in the brain, and that tyrosine supplementation reverses cognitive decline under stress or cognitive demands, while disease characteristics likely determine the efficacy of tyrosine supplementation. The review also noted that the potential of using tyrosine supplementation to treat clinical disorders seems limited and its benefits are likely determined by the presence and extent of impaired neurotransmitter function and synthesis.

Sleep Deprivation

A widely cited 1995 study by Neri et al., published in Aviation, Space, and Environmental Medicine, examined the behavioral effects of tyrosine during continuous nighttime work involving one night's sleep loss. Subjects remained awake for more than 24 hours by the end of testing. Six hours after the experiment began, one half of the subjects received 150 mg·kg⁻¹ tyrosine in a split dose while the other half received a cornstarch placebo in a double-blind procedure. Such tests required both cognitive thinking and physical dexterity (psychomotor tasks). One half of the group were given tyrosine, the other half a placebo. Results showed that the tyrosine group experienced less decline of their psychomotor skills and were more able to sustain attention than the placebo group. This benefit lasted for three hours.

Cold Stress

A double-blind, within-subjects study (Mahoney et al., published in Physiology & Behavior) investigated severe cold water immersion in 19 volunteers. Volunteers completed three test sessions on different days (35°C control/placebo, approximately 10°C/placebo, approximately 10°C/tyrosine) using a double-blind, within-subjects design. During each session, volunteers completed two 90-minute water immersions and consumed a food bar (150 mg/kg tyrosine or placebo) before each immersion (total tyrosine 300 mg/kg). When volunteers consumed tyrosine, correct responses increased on a Match-to-Sample memory measure and study time for the sample was shorter, indicative of more rapid and accurate information processing. Reaction time on the memory measure revealed a similar pattern across immersions for the tyrosine and thermoneutral conditions, but not the cold/placebo condition.

A study of combined cold and hypoxia stress found that Banderet and Lieberman found supplemental tyrosine was effective at decreasing symptoms such as headaches, cold sensation, and fatigue during a combined stress of cold and hypoxia. Performance on cognitive tests, including addition, coding, map/compass, pattern recognition, and reaction time was also better with tyrosine compared to placebo.

Working Memory and Cognitive Flexibility

The supply of tyrosine was found to reduce stress-induced impairments of working memory and attentional tasks, but more so in individuals who were particularly sensitive to the stressors. Even without exposure to stress, administration of tyrosine has been shown to have an acute beneficial effect on task performance thought to be related to dopamine, e.g., simultaneously performing multiple tasks, the updating and monitoring of working memory, and inhibitory control.

A PLOS Computational Biology study (2022) found that on a physiological level, tyrosine intake reduced participants' arousal as revealed by increases in pupil dilation variability and reductions in heart rate compared with placebo. The same study noted that in addition to counteracting decrements of cognitive performance in working memory tasks, a few studies indicated that a single dose of tyrosine administration may improve a wider range of cognitive functions, including cognitive flexibility, inhibitory control, working memory, and reasoning. There is also evidence for positive effects of long-term tyrosine intake on cognitive performance, reflected in associations between daily tyrosine intake and working memory, episodic memory, and fluid intelligence.

Evidence Strength: Cognition Under Stress

The overall evidence for L-tyrosine's (and by extension, NALT's) benefit on stress-induced cognitive decline is moderate, particularly for conditions such as sleep deprivation, cold stress, and combined environmental stressors. The effect appears most reliable when catecholamine depletion is occurring (i.e., under genuine stress) and is weak to absent in rested, non-stressed individuals. The main effects of L-tyrosine that have been reported are acute effects in preventing a decline in cognitive function in response to physical stress. The physical stressors include those of interest to the military, such as cold stress, the combination of cold stress and high-altitude stress (mild hypoxia), extended wakefulness, and lower body negative pressure.

6.2 Phenylketonuria (PKU)

Phenylketonuria is a metabolic disorder in which phenylalanine cannot be hydroxylated to tyrosine. Tyrosine is used in protein supplements to treat the inherited disorder phenylketonuria. People who have this problem cannot process phenylalanine properly, so as a result they cannot make tyrosine. To meet their bodies' needs, supplemental tyrosine is given.

However, the Cochrane Collaboration review of tyrosine supplementation for PKU found negative results. The review included three trials with 56 people with phenylketonuria aged between six and 28 years. Trials compared adding tyrosine or placebo to a phenylalanine-restricted diet with random allocation. The length of the treatment and control arms were short in all three trials and some of the outcomes considered important were not measured. Although the amount of tyrosine measured in the blood of those taking the supplement was higher, there were no differences noted in any other outcome measures. There is no evidence to suggest that tyrosine should be routinely added to the diet of people with phenylketonuria.

6.3 Parenteral Nutrition

The most clearly supported clinical use of NALT as a distinct compound (rather than as an oral supplement extrapolated from L-tyrosine data) is in parenteral (intravenous) nutrition. The tyrosine content of parenteral solutions is limited by poor tyrosine solubility. N-acetyl-L-tyrosine has excellent solubility and is a potential source of intravenous tyrosine.

In neonatal parenteral nutrition solutions, tyrosine solubility limitations are overcome by adding N-acetyltyrosine (NAT), along with tyrosine. NAT is found in parenteral nutrition solutions prescribed to NICU neonates in the U.S., and it is not present in adult parenteral nutrition solutions.

Tyrosine's low solubility in water limits tyrosine to less than 1% of total amino acids in parenteral nutrition solutions, while aromatic amino acid requirements in neonates were estimated at 3.1% to 3.9% of the total. It was hypothesized that tyrosine deficiency in parenteral nutrition solutions could trigger low thyroxine since both thyroxine and tyrosine were similarly low in a parenteral nutrition randomized clinical trial.

For low birth weight and preterm infants requiring total parenteral nutrition, NAT serves as a key tyrosine source in specialized amino acid mixtures, with clinical studies demonstrating its role in promoting growth and preventing deficiencies in conditions like sepsis or small for gestational age status.

6.4 Physical Exercise Performance

Evidence for benefit on athletic or exercise performance is weak. A double-blind crossover study in eight healthy males exercising in the heat at a dose of 150 mg/kg found that no significant difference was observed between the tyrosine and placebo conditions in any of the cognitive tests measured. Furthermore, no significant difference was observed in time-trial completion time. Clinical trial data for other conditions is limited and does not support tyrosine supplementation, including for the enhancement of sports performance.

6.5 Mood and Depression

People take tyrosine for depression, attention deficit disorder (ADD), attention deficit-hyperactivity disorder (ADHD), the inability to stay awake (narcolepsy), and improving alertness following sleep deprivation. However, these applications are largely investigational or anecdotal. Tyrosine hydroxylase is the rate-limiting step in the production of L-dopa and dopamine, forming the basis for the use of supplemental tyrosine. However, clinical trial data supporting a place in therapy are lacking.

7. Body Systems and Health Areas Associated with NALT

• Central nervous system / Neurotransmitter support: L-tyrosine is an amino acid used as a precursor for the synthesis of the catecholamines dopamine and norepinephrine. These are depleted under stressful conditions, which can compromise cognitive function. L-tyrosine supplementation may help alleviate acute stress-induced cognitive decline by restoring catecholamine levels in the brain.
• Endocrine system / Thyroid hormone synthesis: NALT acts as a prodrug that undergoes deacetylation in vivo to release L-tyrosine, which is vital for the production of thyroid hormones.
• Skin and pigmentation: Tyrosine is the precursor for melanin synthesis in most organisms including humans and animals.
• Metabolic/nutritional support: Via its role in parenteral nutrition, NALT supports positive nitrogen balance and overall protein metabolism in critically ill or peri-operative patients who cannot receive oral nutrition.
• Cardiovascular/adrenal: Through epinephrine and norepinephrine synthesis, tyrosine participates in the adrenal stress response, though supplementation effects on the cardiovascular system specifically have not been well characterized in clinical trials.

8. Dosage Forms and Dosages Reported in Studies

NALT is commercially available in the following dosage forms: capsules, tablets, and loose powder (for inclusion in stacks and pre-workout formulations). In the parenteral nutrition setting, it is a component of sterile intravenous amino acid solutions.

Dosages Reported in Research

• Parenteral nutrition (intravenous), animal research: Infusion at 0.5 mmol/kg/day and 2 mmol/kg/day in rats as part of total parenteral nutrition regimens.
• Sleep deprivation study (L-tyrosine, human): 150 mg·kg⁻¹ in a split dose.
• Cold stress study (L-tyrosine, human): 150 mg/kg consumed before each of two 90-minute immersions (total 300 mg/kg).
• Military combat training study (L-tyrosine, human): L-tyrosine was given at a dosage of 2 g per day for 5 days during a demanding military combat training course.
• Exercise in heat study (L-tyrosine, human): 150 mg·kg⁻¹ tyrosine ingested 1 hour pre-exercise.
• General supplemental doses cited in clinical literature: Limited clinical studies use 100 to 150 mg/kg per day. Manufacturers commonly recommend 500 to 1,500 mg/day, and dosages of more than 12 g/day are not recommended.
• Alertness after sleep deprivation (cited dosage): 150 mg/kg/day of tyrosine for improving alertness after being without sleep for a long time.

Note: The majority of these dosages pertain to free-form L-tyrosine, not to NALT specifically. Dose equivalence between NALT and L-tyrosine cannot be assumed on a gram-for-gram basis, given the lower conversion efficiency of NALT to free tyrosine demonstrated in pharmacokinetic studies.

9. Safety Considerations and Drug Interactions

9.1 General Safety Status

L-tyrosine supplements are recognized as generally safe by the U.S. Food and Drug Administration (FDA). There are no known adverse effects from tyrosine administration. Also, because tyrosine is normally present in substantial quantities in ordinary foods and is rapidly metabolized, its administration is unlikely to have long-term toxicity or unwanted side effects.

9.2 Reported Adverse Effects

Some side effects of N-Acetyl-L-tyrosine include nausea, headache, fatigue, and heartburn. These appear to be uncommon and generally mild, but side effects of tyrosine are not well known.

9.3 Drug and Supplement Interactions

• Thyroid hormones: Tyrosine is contraindicated in hyperthyroidism or Graves' disease because it may increase levels of thyroid hormone. Tyrosine may change the amount of thyroid hormone in your body. Healthcare providers should be informed of thyroid problems before taking tyrosine.
• Monoamine oxidase inhibitors (MAOIs): Coadministration of tyrosine with monoamine oxidase inhibitors (MAOIs) is contraindicated. MAOIs inhibit the breakdown of catecholamines, and combining them with a catecholamine precursor could potentiate hypertensive or serotonergic effects.
• Levodopa (L-DOPA): Because tyrosine and L-DOPA share the same large neutral amino acid transporter for absorption into the gut and across the blood-brain barrier, competition for transport is possible. This interaction has been studied primarily in the context of pharmacokinetics, and patients on levodopa for Parkinson's disease should be aware.

9.4 Special Populations

• Liver disease: The liver helps break down and process tyrosine. People with liver problems might have trouble with this process, which could lead to higher amounts of tyrosine in the blood.
• Phenylketonuria: While tyrosine supplementation is often prescribed to PKU patients because they cannot hydroxylate phenylalanine to tyrosine, both high tyrosine enrichment of protein substitutes and extra free tyrosine supplementation may not be as safe as considered at present, especially to the fetus of a woman with PKU.
• Pregnancy and lactation: Information regarding safety and efficacy in pregnancy and lactation is lacking.
• NALT-specific urinary excretion concern: About 35% of NALT may be excreted unchanged in urine, indicating incomplete conversion efficiency. The long-term renal implications of chronic high-dose urinary NALT excretion have not been formally evaluated in humans.

9.5 Animal Toxicology

Rats were given doses of 0, 200, 600, or 2,000 mg/kg/day to examine toxicity of L-tyrosine. Edema of the cornified layer of the forestomach was seen in 600 mg/kg supplementation in female rats and in both sexes at 2,000 mg/kg/day. These doses are substantially higher than those used in human supplementation.

10. Summary of Evidence Strength

• Parenteral nutrition (NALT specifically): Moderate–strong evidence that NALT can serve as a functional tyrosine source in TPN, though conversion efficiency is incomplete (~35% excreted unchanged). This is the most direct application of NALT as a distinct compound, supported by multiple published animal and human clinical studies.
• Cognitive performance under acute environmental or physiological stress (L-tyrosine evidence, extrapolated to NALT): Moderate evidence, based on multiple randomized controlled trials showing attenuation of cognitive decline under sleep deprivation, cold exposure, and combined stressors. Effects are not reliably observed in well-rested, non-stressed individuals.
• Phenylketonuria: Negative to neutral — Cochrane review found no clinical benefit beyond raising blood tyrosine levels.
• Athletic performance: Weak/unsupported — available RCTs do not demonstrate a consistent benefit.
• Depression, ADHD, mood disorders: Preliminary/investigational only — clinical evidence is insufficient to support formal recommendations.
• NALT vs. L-tyrosine bioavailability: Current evidence suggests L-tyrosine is superior for oral supplementation in terms of reliably raising plasma tyrosine levels, meaning NALT's purported advantage over free L-tyrosine is not supported by pharmacokinetic data.

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