Tyrosine is a precursor of important neurotransmitters in the brain, such as dopamine, epinephrine (adrenaline) and norepinephrine (noradrenaline). The synthesis of these neurotransmitters is limited by the rate of tyrosine entry from plasma into the brain and competing amino acids. L-tyrosine is a conditionally essential amino acid that can be synthesized by the body from the essential amino acid phenylalanine. Therefore, adequate tyrosine production depends on a sufficient supply of phenylalanine in the diet. However, in people that have phenylketonuria (PKU) who cannot synthesize tyrosine from phenylalanine, tyrosine is an essential amino acid. Tyrosine may help support the effects related to stress. Under stressful conditions, it is hypothesized that the brain may not be able to synthesize enough tyrosine from phenylalanine. Catecholamines like epinephrine, norepinephrine, and dopamine that are synthesized from tyrosine may become depleted during stress. Therefore, supplementation with tyrosine may allow increased catecholamine synthesis for the brain, and support healthy response to stress. There is some evidence in animals and humans that supplemental tyrosine might support performance, memory, and learning, under extreme environmental conditions, in addition to alertness following sleep deprivation.