Phenylalanine and
tyrosine constitute the two initial steps in the biosynthesis of
dopamine, which, in its turn, is the metabolic precursor of
noradrenaline and
adrenaline. The extracellular
phenylalanine concentration influences brain function in
phenylalanine deficiency (PHD) by decreased
dopamine synthesis. It has been shown to induce EEG slowing, and prolonged the performance time on neuropsychological tests. The
tyrosine concentration in the CNS is reduced in PHD, possibly implying insufficient substrate (=
tyrosine) for
catecholamine synthesis due to competition inhibition, for instance across the blood brain barrier. In experimental studies it has been shown that the synthesis and release of
dopamine can be influenced by an increase in the availability of
tyrosine. In PHD an extra dietary intake of three doses of
tyrosine (160 mg/kg/24h) induced a shortening of reaction time and decreased variability, and in a double-blind crossover study a similar dose has been reported to induce an improvement on psychological tests. In a study with lower doses of
tyrosine (110 mg/kg/24 h) no effect was found on reaction time tests. These findings need to be substantiated, and more detailed information should be obtained.