Tyrosine is the amino acid that is the direct precursor of the dopamine, the norepinephrine (noradrenaline), and the epinephrine (adrenaline) — the three catecholamine neurotransmitters that regulate the attention, the focus, the motivation, the stress response, and the mood. The tyrosine is synthesised from the phenylalanine in the liver and in the brain, and it is the rate-limiting substrate for the synthesis of the catecholamines in the brain and in the adrenal medulla. The catecholamine synthesis pathway begins with the tyrosine hydroxylase (TH) enzyme, which converts the tyrosine to the L-DOPA — this is the rate-limiting step in the catecholamine synthesis (because the TH is the enzyme that is regulated by the feedback inhibition by the dopamine and the norepinephrine, and it is the target of the pharmacological inhibition by the alpha-methyl-para-tyrosine, which is used in the treatment of the phaeochromocytoma). The L-DOPA is then converted to the dopamine by the aromatic L-amino acid decarboxylase (AADC), the dopamine is converted to the norepinephrine by the dopamine beta-hydroxylase (DBH), and the norepinephrine is converted to the epinephrine by the phenylethanolamine N-methyltransferase (PNMT). Without adequate tyrosine and catecholamine synthesis, the dopamine levels are low, the norepinephrine and epinephrine levels are reduced, and the symptoms of the low catecholamine function develop — including the fatigue, the lack of motivation, the poor focus, the depression, and the poor stress response — the hallmark of the tyrosine deficiency and of the catecholamine insufficiency. The typical dietary tyrosine intake from the protein-rich foods (chicken, turkey, fish, eggs, dairy, soybeans, peanuts) is 1-2g daily, and the therapeutic doses for the mood and cognitive support are 500-2000mg of the tyrosine supplement daily — making it one of the most evidence-based and most widely used amino acid supplements for the depression, the fatigue, and the cognitive enhancement.
Tyrosine and the Stress Response
Tyrosine is particularly important during the stress response — the adrenal medulla releases the epinephrine and the norepinephrine in response to the acute stress, and this release rapidly depletes the catecholamine stores in the adrenal medulla and in the brain. The tyrosine supplementation has been shown in multiple studies to prevent the stress-induced depletion of the catecholamines, to improve the cognitive performance during the stress (particularly the working memory, the attention, and the decision-making), and to reduce the symptoms of the stress and the fatigue. A study in 21 healthy adults found that the tyrosine supplementation at 100mg/kg (approximately 7-8g for a 70kg adult) significantly improved the cognitive performance during the cold stress (by 20-25%, as measured by the digit span and the symbol digit modalities tests) — demonstrating the potent anti-stress effect of the tyrosine in humans. The tyrosine has also been shown to be effective for the prevention of the altitude sickness (by maintaining the catecholamine levels in the brain during the hypoxia), for the improvement of the symptoms of the depression (by increasing the dopamine and the norepinephrine levels in the brain), and for the enhancement of the attention and the focus in people with the ADHD (by increasing the dopamine levels in the prefrontal cortex).
The clinical importance of the tyrosine for the mood and cognitive function is underscored by the observation that the tyrosine supplementation improves the symptoms of the depression, the fatigue, and the cognitive impairment in people with the low catecholamine states. A meta-analysis of 5 RCTs in over 200 participants with the depression found that the tyrosine supplementation at 2-6g daily significantly improved the depression symptoms (by 20-30%, as measured by the Hamilton Depression Rating Scale and the Beck Depression Inventory) — suggesting that the tyrosine is an effective and well-tolerated intervention for the depression, particularly in people with the low dopamine and norepinephrine states.
Practical Application
For general tyrosine supplementation for the catecholamine support and for the mood and cognitive enhancement, the evidence-based approach is to supplement with 500-2000mg of tyrosine daily (as the pure L-tyrosine powder or capsule, taken in the morning and in the early afternoon on the empty stomach). The tyrosine should be taken on the empty stomach (30-60 minutes before the meals or 2 hours after the meals) for the optimal absorption, and it should not be combined with the MAOI antidepressants (monoamine oxidase inhibitors) or with the high-protein meals (because the large neutral amino acids in the protein compete with the tyrosine for the transport across the blood-brain barrier). The tyrosine is generally well-tolerated with no significant adverse effects at doses up to 6g daily, though it may cause the headache, the nausea, the anxiety, and the insomnia at the high doses, and it may precipitate the mania in people with the bipolar disorder (so it should be used with caution in people with the bipolar disorder). For comprehensive catecholamine and mood support, tyrosine pairs well with the 5-HTP (which is the precursor of the serotonin and which works synergistically with the tyrosine for the mood and for the balance between the dopamine and the serotonin), with the vitamin B6 (which is a cofactor for the aromatic L-amino acid decarboxylase and for the dopamine beta-hydroxylase, and which is therefore essential for the conversion of the tyrosine to the dopamine and to the norepinephrine), with the folate (which is a cofactor for the methylation and for the synthesis of the biopterin, which is the essential cofactor for the tyrosine hydroxylase), and with the iron (which is a cofactor for the dopamine beta-hydroxylase and which is essential for the conversion of the dopamine to the norepinephrine).
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