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Food Nutr Res. 2015 Apr 29;59:26407. doi: 10.3402/fnr.v59.26407. eCollection 2015.

Effects of dietary tryptophan and phenylalanine-tyrosine depletion on phasic alertness in healthy adults - A pilot study.

Author information

Clinic for Child and Adolescent Psychiatry, Psychosomatics and Psychotherapy, JARA Brain, Medical Faculty, RWTH Aachen University, Aachen, Germany.
Institute of Neuroscience and Medicine (INM-3, -4, -5), Research Centre Jülich, Jülich, Germany.
Department of Nuclear Medicine, RWTH Aachen University Hospital, Aachen, Germany.
Department of Neurology, University of Cologne, Cologne, Germany.
Department of Child and Adolescent Psychiatry, School of Psychiatry and Clinical Neurosciences & School of Paediatrics and Child Health, Faculty of Medicine, Dentistry and Health Sciences, The University of Western Australia, Perth, Australia.
Specialised Child and Adolescent Mental Health Services (CAHMS), Department of Health in Western Australia, Perth, WA, Australia;



The synthesis of the neurotransmitters serotonin (5-HT) and dopamine (DA) in the brain can be directly altered by dietary manipulation of their relevant precursor amino acids (AA). There is evidence that altered serotonergic and dopaminergic neurotransmission are both associated with impaired attentional control. Specifically, phasic alertness is one specific aspect of attention that has been linked to changes in 5-HT and DA availability in different neurocircuitries related to attentional processes. The present study investigated the impact of short-term reductions in central nervous system 5-HT and DA synthesis, which was achieved by dietary depletion of the relevant precursor AA, on phasic alertness in healthy adult volunteers; body weight-adapted dietary tryptophan and phenylalanine-tyrosine depletion (PTD) techniques were used.


The study employed a double-blind between-subject design. Fifty healthy male and female subjects were allocated to three groups in a randomized and counterbalanced manner and received three different dietary challenge conditions: acute tryptophan depletion (ATD, for the depletion of 5-HT; N=16), PTD (for the depletion of DA; N=17), and a balanced AA load (BAL; N=17), which served as a control condition. Three hours after challenge intake (ATD/PTD/BAL), phasic alertness was assessed using a standardized test battery for attentional performance (TAP). Blood samples for AA level analyses were obtained at baseline and 360 min after the challenge intake.


Overall, there were no significant differences in phasic alertness for the different challenge conditions. Regarding PTD administration, a positive correlation between the reaction times and the DA-related depletion magnitude was detected via the lower plasma tyrosine levels and the slow reaction times of the first run of the task. In contrast, higher tryptophan concentrations were associated with slower reaction times in the fourth run of the task in the same challenge group.


The present study is the first to demonstrate preliminary data that support an association between decreased central nervous system DA synthesis, which was achieved by dietary depletion strategies, and slower reaction times in specific runs of a task designed to assess phasic alertness in healthy adult volunteers; these findings are consistent with previous evidence that links phasic alertness with dopaminergic neurotransmission. A lack of significant differences between the three groups could be due to compensatory mechanisms and the limited sample size, as well as the dietary challenge procedures administered to healthy participants and the strict exclusion criteria used. The potential underlying neurochemical processes related to phasic alertness should be the subject of further investigations.


acute tryptophan depletion; amino acids; dietary challenge procedures; dopamine; phasic alertness; phenylalanine–tyrosine depletion; serotonin

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