L-Tyrosine
L-tyrosine serves as a precursor for the synthesis of catecholamines—dopamine, norepinephrine, and epinephrine—via the tyrosine hydroxylase (TH) and dopa decarboxylase enzymatic pathways.
Literature Review
The majority of clinical research points to oral L-tyrosine intake as beneficial for cognitive function in stressful environments.
Preliminary studies involving healthy adults indicate that consuming 100-300 mg/kg of tyrosine before short-term exposure to stress from cold or noise can enhance cognitive capabilities like executive function and short-term memory, in comparison to a placebo.[i],[ii],[iii]
Two initial studies involving sleep-deprived healthy individuals suggest that a 150 mg/kg dose of tyrosine may improve certain cognitive metrics, such as alertness, when compared to placebo.[iv],[v]
Research evaluating tyrosine's impact on cognitive control during conflict tasks is limited.
One small-scale study involving healthy subjects implies that consuming 2 grams of tyrosine powder dissolved in orange juice an hour before testing could speed up cognitive reaction times in the Simon task. However, no noticeable improvement was observed in the Flanker task, possibly due to the study's insufficient power to detect an effect.[vi]
In healthy adults, initial studies suggest that a dosage of 150-300 mg/kg of L-tyrosine before acute stress from cold, noise, or multitasking appears to improve memory performance compared to a placebo.3,[vii],[viii],[ix]
Nonetheless, a 150 mg/kg dose of L-tyrosine doesn't seem to enhance memory when individuals engage in less stressful activities, such as completing a single, simple task or testing without exposure to cold.7,9
Oral L-tyrosine intake appears to bolster memory performance in individuals facing stressors like cold environments or having to juggle multiple tasks simultaneously.
Mechanism of Action
General: Tyrosine is a nonessential amino acid endogenously synthesized by the body from phenylalanine.
For those with phenylketonuria (PKU), who can't convert phenylalanine to tyrosine, it becomes an essential amino acid.[x]
L-tyrosine is also found in various dietary proteins, including dairy, meats, fish, eggs, nuts, beans, oats, and wheat. When the dietary intake of tyrosine is insufficient, the body uses phenylalanine for conversion.[xi]
The dietary requirement for tyrosine is influenced by the intake of phenylalanine. If one consumes an adequate amount of phenylalanine (around 9 mg/kg daily), the daily tyrosine requirement is about 7 mg/kg. Tyrosine is a component of all proteins.[xii]
Mood regulatory effects: Tyrosine is potentially significant in enhancing and regulating mood. It serves as a building block for catecholamines, such as norepinephrine, epinephrine, and dopamine.4,11
A deficiency in norepinephrine might be linked to dysregulations of mood and feelings of unhappiness, and dopamine may contribute to the effectiveness of modulating molecules involved.[xiii]
In healthy individuals, a single dose of tyrosine elevates the plasma levels of norepinephrine, epinephrine, and dopamine.[xiv]
Research indicates that consuming an amino acid drink devoid of tyrosine and phenylalanine—key precursors for catecholamines—diminishes the brain's responsiveness to incentives without affecting behavior.[xv]
Anti-stress effects: Some researchers theorize that under stress, the brain might show inefficiency to produce adequate amounts of tyrosine from phenylalanine.11 Catecholamines synthesized from tyrosine may become depleted during stressful situations.[xvi]
Increasing tyrosine availability to the brain could potentially elevate catecholamine production, thereby mitigating the adverse effects of stress, such as physical manifestations of fear.[xvii],[xviii],[xix]
Both animal and human studies provide some evidence that supplemental tyrosine could enhance performance, memory, and learning under various challenging conditions like extreme environments, intense exercise, and psychological stress.4,11,[xx],[xxi],[xxii]
[i]O'Brien, C., Mahoney, C., Tharion, W. J., Sils, I. V., and Castellani, J. W. Dietary tyrosine benefits cognitive and psychomotor performance during body cooling. Physiol Behav. 2-28-2007;90(2-3):301-307.
[ii]Banderet, L. E. and Lieberman, H. R. Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans. Brain Res Bull 1989;22(4):759-762.
[iii]Deijen, J. B. and Orlebeke, J. F. Effect of tyrosine on cognitive function and blood pressure under stress. Brain Res Bull 1994;33(3):319-323.
[iv]Neri DF, Wiegmann D, Stanny RR, et al. The effects of tyrosine on cognitive performance during extended wakefulness. Aviat Space Environ Med 1995;66:313-9.
[v]Magill, R. A., Waters, W. F., Bray, G. A., Volaufova, J., Smith, S. R., Lieberman, H. R., McNevin, N., and Ryan, D. H. Effects of tyrosine, phe*********, caffeine D-amp********, and placebo on cognitive and motor performance deficits during sleep deprivation. Nutr. Neurosci. 2003;6(4):237-246.
[vi]Stock AK, Colzato L, Beste C. On the effects of tyrosine supplementation on interference control in a randomized, double-blind placebo-control trial. Eur Neuropsychopharmacol. 2018;28(8):933-944.
[vii]Thomas, J. R., Lockwood, P. A., Singh, A., and Deuster, P. A. Tyrosine improves working memory in a multitasking environment. Pharmacol Biochem Behav 1999;64(3):495-500.
[viii]Mahoney, C. R., Castellani, J., Kramer, F. M., Young, A., and Lieberman, H. R. Tyrosine supplementation mitigates working memory decrements during cold exposure. Physiol Behav. 11-23-2007;92(4):575-582.
[ix]Shurtleff, D., Thomas, J. R., Schrot, J., Kowalski, K., and Harford, R. Tyrosine reverses a cold-induced working memory deficit in humans. Pharmacol Biochem Behav 1994;47(4):935-941.
[x]van Spronsen FJ, van Rijn M, Bekhof J. Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets. Am J Clin Nutr 2001;73:153-7.
[xi]Food and Nutrition Board, Institute of Medicine. The Role of Protein and Amino Acids in Sustaining and Enhancing Performance. Washington, DC: National Academy Press, 1999. Available at: http://www.nap.edu/books/0309063469/html/.
[xii]Roberts SA, Thorpe JM, Ball RO, Pencharz PB. Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation. Am J Clin Nutr 2001 Feb;73:276-82.
[xiii]Meyers, S. Use of neurotransmitter precursors for treatment of depression. Altern Med Rev 2000;5(1):64-71.
[xiv]Rasmussen, D. D., Ishizuka, B., Quigley, M. E., and Yen, S. S. Effects of tyrosine and tryptophan ingestion on plasma catecholamine and 3,4-dihydroxyphenylacetic acid concentrations. J.Clin.Endocrinol.Metab 1983;57(4):760-763.
[xv]Bjork JM, Grant SJ, Chen G, Hommer DW. Dietary tyrosine/phenylalanine depletion effects on behavioral and brain signatures of human motivational processing. Neuropsychopharmacology. 2014;39(3):595-604.
[xvi]Reinstein, D. K., Lehnert, H., and Wurtman, R. J. Dietary tyrosine suppresses the rise in plasma corticosterone following acute stress in rats. Life Sci. 12-9-1985;37(23):2157-2163.
[xvii]Colzato LS, Jongkees BJ, Sellaro R, van den Wildenberg WP, Hommel B. Eating to stop: tyrosine supplementation enhances inhibitory control but not response execution. Neuropsychologia. 2014;62:398-402.
[xviii]Steenbergen L, Sellaro R, Hommel B, Colzato LS. Tyrosine promotes cognitive flexibility: evidence from proactive vs. reactive control during task switching performance. Neuropsychologia. 2015;69:50-5.
[xix]Soranzo A, Aquili L. Fear expression is suppressed by tyrosine administration. Sci Rep. 2019;9(1):16073.
[xx] O'Brien, C., Mahoney, C., Tharion, W. J., Sils, I. V., and Castellani, J. W. Dietary tyrosine benefits cognitive and psychomotor performance during body cooling. Physiol Behav. 2-28-2007;90(2-3):301-307.
[xxi]Deijen, J. B. and Orlebeke, J. F. Effect of tyrosine on cognitive function and blood pressure under stress. Brain Res Bull 1994;33(3):319-323.
[xxii]Tumilty L, Davison G, Beckmann M, Thatcher R. Failure of oral tyrosine supplementation to improve exercise performance in the heat. Med Sci Sports Exerc. 2014;46(7):1417-25.
