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L-Tyrosine 
Background
L-Tyrosine is an amino acid found that is involved in the production of neurotransmitters like dopamine, adrenaline, and noradrenaline (see figure 1).
Figure 1
Scientific effects
Tyrosine has numerous studies in humans showing a variety of effects. One study by the U.S. Army Research Institute of Environmental Medicine found that tyrosine improved the mood and performance of individuals under stressful conditions (1).
Figure 2
As you can see in figure 2, supplementation with tyrosine resulted in a statistically significant effect on a reduction of distress, muscular discomfort (a pretty cool finding), fatigue, and sleepiness when compared to the placebo group.
Another study in humans investigated the effects of tyrosine in 21 cadets. They found that the tyrosine group (compared to placebo) improved working memory (see figure 3), and reduced the effects of stress and fatigue on cognitive exhausting tasks, They also found that it reduced blood pressure (2).
Figure 3
As you can see in figure 3, tyrosine (compared to placebo) resulted in a 50% decrease in mistakes in the reaction task (2).
Curiously enough, a study in 22 humans on tyrosine in a stop-signal test found that tyrosine helped inhibit impulses (see figure 4). It also improves your ability to stop overt responses (see figure 5) (3).
Figure 4
Figure 5
As you can see above in figure 5, there was a statistically significant effect of tyrosine supplementation on response inhibition but response execution. The way I like to look at it is that tyrosine can help you prevent yourself from responding incorrectly, but still lets you respond just as quickly as placebo (i.e. the increase in response inhibition did not lead to a deficit in response execution).
These studies indicate that tyrosine may benefit healthy individuals exposed to demanding situational conditions (everyday life, strenuous exercise, etc.). Through some studies on humans, Tyrosine has evidence showing it can improve the mood and performance of individuals under stressful conditions, improve working memory, and reduce the effects of stress and fatigue on cognitive exhausting tasks.
References
1. Banderet, L. E., & Lieberman, H. R. (1989). Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans. Brain Research Bulletin, 22(4), 759–762.
2. Deijen, J. B., Wientjes, C. J. E., Vullinghs, H. F. M., Cloin, P. A., & Langefeld, J. J. (1999). Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course. Brain Research Bulletin, 48(2), 203–209.
3. Colzato, L. S., Jongkees, B. J., Sellaro, R., van den Wildenberg, W. P. M., & Hommel, B. (2014). Eating to stop: Tyrosine supplementation enhances inhibitory control but not response execution. Neuropsychologia, 62, 398–402.
