Phenylalanine is an essential amino acid that must be obtained through the diet. Found primarily in high protein sources such as meat and eggs, it plays an important role in regulating the nervous system. L-Phenylalanine can be used to metabolize two distinct chemicals, phenylethylamine and tyrosine. Phenylethylamine is a neuroamine which may have regulating effects in mood and feelings of well-being. Tyrosine is an important amino acid used to produce certain neurotransmitters, including dopamine, epinephrine (adrenaline) and norepinephrine (noradrenaline). The synthesis of these neurotransmitters is limited by the rate of tyrosine entry from plasma into the brain. L-tyrosine is a conditionally essential amino acid that can be manufactured by the body from the essential amino acid phenylalanine. Therefore, adequate tyrosine production depends on a sufficient supply of phenylalanine in the diet.