A 2015 review evaluated the available cognitive and behavioral studies on tyrosine to determine whether and when tyrosine supplementation can be beneficial for performance. A number of studies have shown an effect on tyrosine to improve cognitive performance in healthy humans prior to stress exposure. Typically in these studies, hypothermia is used as the stressor, and tyrosine has repeatedly been shown to reverse cognitive impairments. Tyrosine has also been shown to improve working memory while participants were exposed to an auditory stressor. Additionally, two studies showed enhancement of working memory performance with tyrosine in the absence of stress exposure. Improvement has also been shown in inhibitory control without influencing response execution. Doses ranged significantly from 500 mg to 12 g per day. For depressed, schizophrenic, and phenylketonuria patients, tyrosine alone was not found to be effective. Patients with ADHD and Parkinson’s disease had mixed results, with some studies showing benefit and others showing no effect. It is hypothesized that tyrosine supplementation seems to replete neurotransmitter levels in the brain and reverse the stress-induced degradation of a variety of cognitive functions. Of note, most literature reports results on short-term rather than long-term settings. [1]
A 1984 article reviewed the few trials observing tyrosine with mixed results. One study did not find benefit for depression but was limited in descriptions. A case report mentioned of a 30-year-old woman with unipolar depression who could not tolerate antidepressants benefitting from tyrosine 100 mg/kg/day divided into 3 daily doses. Further non-blinded trials found tyrosine same dose to double tyrosine plasma and cerebrospinal fluid concentration. At the time this article was written, tyrosine and its effects on human pathology was still vaguely understood. [2]
An animal study conducted in 1980 found that tyrosine administration increased brain neuron synthesis of both dopamine and adrenaline. A single dose of 100 to 150 mg/kg in healthy volunteers found a dose-related increase in plasma tyrosine concentration and ratio of concentration of tyrosine. [3]
A small 2018 study (N= 24) investigated the effects of tyrosine on reactive and proactive response inhibition and signaling in dopaminergically innervated fronto-striatal regions. The study incorporated a double-blind, counterbalanced, placebo-controlled, within-subject design. Patients received a dose of 150 mg/kg tyrosine or placebo, adjusted to body weight. Effects were measured based on various neurological measures (e.g., verbal IQ, Baratt impulsiveness scale, hospital anxiety, and depression scale), as well as tasks to be performed during test sessions.Tyrosine was not found to alter reactive (stopping response inhibition) or proactive (anticipatory response slowing) inhibition behaviorally. However, it did increase fronto-parietal proactive inhibition-related activation. Ultimately, it was suggested that tyrosine resulted in detrimental effects on brain and cognition, with impact becoming worse with increasing age. [4]