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Diurnal variation of phenylalanine and tyrosine concentrations in adult patients with phenylketonuria: subcutaneous microdialysis is no adequate tool for the determination of amino acid concentrations

Sarah C Grünert1, Corinna M Brichta12, Andreas Krebs1, Hans-Willi Clement2, Reinhold Rauh2, Christian Fleischhaker2, Klaus Hennighausen2, Jörn Oliver Sass13 and K Otfried Schwab1*

Author Affiliations

1 Center for Pediatrics and Adolescent Medicine, University Hospital Freiburg, Freiburg, Germany

2 Department of Child and Adolescent Psychiatry and Psychotherapy, University Hospital Freiburg, Freiburg, Germany

3 Clinical Chemistry and Biochemistry, University Children’s Hospital Zürich, Zürich, Switzerland

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Nutrition Journal 2013, 12:60  doi:10.1186/1475-2891-12-60

Published: 14 May 2013



Metabolic control and dietary management of patients with phenylketonuria (PKU) are based on single blood samples obtained at variable intervals. Sampling conditions are often not well-specified and intermittent variation of phenylalanine concentrations between two measurements remains unknown. We determined phenylalanine and tyrosine concentrations in blood over 24 hours. Additionally, the impact of food intake and physical exercise on phenylalanine and tyrosine concentrations was examined. Subcutaneous microdialysis was evaluated as a tool for monitoring phenylalanine and tyrosine concentrations in PKU patients.


Phenylalanine and tyrosine concentrations of eight adult patients with PKU were determined at 60 minute intervals in serum, dried blood and subcutaneous microdialysate and additionally every 30 minutes postprandially in subcutaneous microdialysate. During the study period of 24 hours individually tailored meals with defined phenylalanine and tyrosine contents were served at fixed times and 20 min bicycle-ergometry was performed.


Serum phenylalanine concentrations showed only minor variations while tyrosine concentrations varied significantly more over the 24-hour period. Food intake within the patients’ individual diet had no consistent effect on the mean phenylalanine concentration but the tyrosine concentration increased up to 300% individually. Mean phenylalanine concentration remained stable after short-term bicycle-exercise whereas mean tyrosine concentration declined significantly. Phenylalanine and tyrosine concentrations in dried blood were significantly lower than serum concentrations. No close correlation has been found between serum and microdialysis fluid for phenylalanine and tyrosine concentrations.


Slight diurnal variation of phenylalanine concentrations in serum implicates that a single blood sample does reliably reflect the metabolic control in this group of adult patients. Phenylalanine concentrations determined by subcutaneous microdialysis do not correlate with the patients’ phenylalanine concentrations in serum/blood.

Phenylketonuria; Phenylalanine; Tyrosine; Diurnal variation; Diet; Physical exercise; Microdialysis; Dried blood spot; Ergometry