Tyrosine is a precursor to neurotransmitters and increases plasma neurotransmitter levels (particularly dopamine and norepinephrine), but has little if any effect on mood in normal subjects. A number of studies have found tyrosine is useful during stress, cold, fatigue (in mice), prolonged work and sleep deprivation, with reductions in stress hormone levels, reductions in stress-induced weight loss seen in animal trials, and improvements in cognitive and physical performance seen in human trials.
