L-tyrosine is an amino acid that is a precursor for many important compounds in your body, including thyroid hormone and catecholamines like dopamine and adrenaline. Dopamine is your body's reward neurotransmitter that can mediate motivational salience (incentive for doing something to yield pleasure). L-tyrosine has been studied as a stress alleviator, as well as a mood and cognitive enhancer. In a cognitive function study, subjects given tyrosine 1hr prior had significantly higher accuracy vs the placebo on a battery of simultaneous cognitive testing for working memory, arithmetic skills, and visual and auditory monitoring (multi-task testing).12 Two other studies demonstrated a cognitive enhancing effect from supplementing with 2g of L-tyrosine, including improving convergent(deep) thinking, which may require more macrolevel cognitive control.13,14
One last thing I would like to mention is about L-tyrosine vs N-acetyl-L-Tyrosine(NALT). There are claims that NALT is more soluble and bioavilable compared to L-tyrosine. While that is true, supplementing with NALT doesn’t seem to raise tyrosine plasma levels as much as L-tyrosine. In fact, 5g of NALT injected only increased tyrosine levels by 25% vs a 100mg oral dose of L-tyrosine raises levels by 140%. Almost 56% of NALT ingested is excreted as NALT without being metabolized.15
12. Thomas, John R., et al. "Tyrosine improves working memory in a multitasking environment." Pharmacology Biochemistry and Behavior3 (1999): 495-500.
13. Colzato, Lorenza S., Annelies M. de Haan, and Bernhard Hommel. "Food for creativity: Tyrosine promotes deep thinking." Psychological research5 (2015): 709-714.
14. Jongkees, Bryant J., et al. "Effect of tyrosine supplementation on clinical and healthy populations under stress or cognitive demands—A review." Journal of psychiatric research 70 (2015): 50-57.
15. Magnusson, Inger, et al. "N-acetyl-L-tyrosine and N-acetyl-L-cysteine as tyrosine and cysteine precursors during intravenous infusion in humans." Metabolism10 (1989): 957-961.