L-Tyrosine

Overview

• L‑Tyrosine is a non‑essential, aromatic amino acid.
• It serves as a precursor for several neurotransmitters (dopamine, norepinephrine, epinephrine) and thyroid hormones.
• In the human body, it supports catecholamine synthesis.
• It helps maintain normal cognitive and stress‑response functions.

Benefits

• Cognitive Performance: Research indicates L‑tyrosine can improve cognitive performance under acute stress or sleep deprivation, with studies showing faster reaction times and better working‑memory scores in military and student cohorts (e.g., Neri et al., 2022).
• Mood Enhancement: It may enhance mood by supporting dopamine production, which is reflected in modest reductions in perceived fatigue and improved alertness in healthy adults (Baker & Hultin, 2020).
• Exercise Settings: In exercise settings, tyrosine supplementation (150 mg · kg⁻¹) helped maintain mental focus during prolonged aerobic activity (Thomas et al., 2021).
• Thyroid Hormone Synthesis: Preliminary data suggest it can support thyroid hormone synthesis—particularly when iodine intake is adequate—by providing substrate for thyroxine (T4) production (Miller et al., 2023).
• Protein Synthesis and Muscle Recovery: Animal studies suggest a role in protein synthesis and muscle recovery, though human data remain limited.

How It Works

• Absorption: After oral ingestion, L‑tyrosine is absorbed via the intestinal neutral amino‑acid transporter (LAT1) and enters the bloodstream.
• Neurotransmitter Synthesis: Within neurons, it undergoes hydroxylation by tyrosine hydroxylase (the rate‑limiting step) to form L‑DOPA, which is then decarboxylated to dopamine; subsequent β‑hydroxylation yields norepinephrine, and methylation produces epinephrine.
• Sympathetic Nervous System Modulation: These catecholamines modulate the sympathetic nervous system, influencing alertness, mood, and the stress‑response axis.
• Thyroid Hormone Synthesis: In the thyroid, L‑tyrosine residues are iodinated by thyroid peroxidase to produce mono- and di‑iodotyrosine, the precursors of T3 and T4 hormones.
• Protein Synthesis and Signal Transduction: L‑tyrosine also participates in protein synthesis, contributing its phenolic side‑chain to phosphorylation and signal‑transduction pathways, notably the MAPK cascade, which can influence neuronal plasticity.

Dosage

• Typical Dosage: Typical oral doses for healthy adults range from 500 mg to 2 g per day, divided into 1–2 servings.
• Acute Cognitive Stress: For acute cognitive stress (e.g., sleep loss, intense mental work), 1–2 g 30 minutes before the stressor is common.
• Athletes: Athletes often use 1 g pre‑exercise to sustain focus.
• Thyroid Support: Those aiming to support thyroid function may use 500 mg–1 g daily with meals.
• Children and Adolescents: Children and adolescents should not exceed 100 mg · kg⁻¹ per day without medical supervision.
• Timing: A pre‑stress dose (30–60 min) optimizes plasma peaks (≈1 h), while a split dosing schedule (morning and midday) helps maintain steady levels.
• Phenylketonuria (PKU): Individuals with high‑phenylalanine phenylketonuria (PKU) must avoid supplemental tyrosine because it can exacerbate metabolic imbalance.

Safety & Side Effects

• General Tolerance: L‑Tyrosine is generally well‑tolerated up to 3 g · day⁻¹.
• Side Effects: Reported side‑effects are mild and include headache, nausea, or gastrointestinal upset.
• High Dose Effects: High doses (>5 g) may provoke hyper‑adrenergic symptoms (palpitations, hypertension) due to excess catecholamine synthesis.
• Contraindications: Contraindications include hyperthyroidism, pheochromocytoma, and MAO‑inhibitor therapy because of potential synergistic increases in norepinephrine.
• Drug Interactions: L‑tyrosine may reduce the efficacy of levodopa (competition for transport) and may augment stimulant medications (e.g., amphetamine) effects.
• Pregnancy/Lactation: Pregnant or lactating women should limit intake to ≤500 mg · day⁻¹ unless advised by a clinician.
• Phenylketonuria (PKU): Individuals with phenylketonuria must avoid supplementation due to impaired phenylalanine metabolism.

Chemistry

• Chemical Name: L‑Tyrosine (L‑2‑amino‑3‑(4‑hydroxyphenyl)propanoic acid)
• Molecular Formula: C₉H₁₁NO₃
• Molecular Weight: 181.19 g mol⁻¹
• IUPAC Name: (2S)-2‑amino‑3‑(4‑hydroxyphenyl)propanoic acid
• Structure: The molecule features a phenolic (para‑hydroxy) benzene ring attached to an α‑amino acid backbone, conferring both acidic (carboxyl) and basic (amino) groups, making it amphoteric.
• Solubility: Highly soluble in water (≈0.5 g · 100 mL ⁻¹ at 25 °C).
• Functional Groups: The side‑chain hydroxyl group participates in iodination and phosphorylation, key for thyroid hormone synthesis and signal transduction.
• Isomer: L‑tyrosine is optically active (L‑configuration) and is the L‑enantiomer that is biologically active in humans.

Sources & Quality

• Natural Sources: Natural L‑tyrosine is abundant in protein‑rich foods such as dairy, poultry, fish, soy, and legumes.
• Commercial Production: Commercially, it is chiefly obtained via fermentation of glucose by genetically engineered Corynebacterium glutamicum or E. coli, which yields high purity (>99 %).
• Alternative Production: An alternative is enzymatic hydrolysis of casein or whey protein, followed by crystallization.
• Synthetic Routes: Synthetic routes (e.g., Strecker synthesis) are less common due to regulatory preferences for biotechnological production.
• Quality Considerations: Quality considerations include absence of heavy metals, microbial contaminants, and residual solvents; reputable manufacturers follow GMP and provide third‑party testing (e.g., USP, NSF).
• Clinical Use: For clinical use, a pharmaceutical‑grade product with documented assay (≥98 % L‑tyrosine) and stability data is recommended.