Dependence of the utilization of a phenylalanine-free amino acid mixture on different amounts of single dose ingested. A case report

For patients with phenylketonuria the daily ingested phenylalanine-free amino acid mixture is the most important source of nitrogen. It is recommended to ingest one third of the total amount combined with main meals. Some patients, especially the older ones, do not follow this recommendation; they ingest the entire daily amount of amino acid mixture in one portion. This intake mode leads to an increased oxidative utilization of the amino acids. To set up an example for this metabolic phenomenon, a13C-leucine breath test was performed in one female phenylketonuric patient. She ingested a third of her daily amount of the amino acid mixture combined with an oral tracer of 3 mg13C-leucine/kg body weight at breakfast. The breath test was carried out by a standardized time schedule over 5 h. Three days later the breath test was repeated when she ingested the total amount of amino acid mixture in only one portion at breakfast. Total daily caloric intake and food composition were not changed. On both days a 24 h urine was collected to determine total nitrogen loss. The13C-content of expired air was analysed by gas isotope ratio mass spectrometry, the total nitrogen content was determined using a combustion unit. The13C-elimination rate as a percentage of the applied13C-tracer was 9.5% on the first test day as compared to 19.6% on the 2nd day. The corresponding total nitrogen excretion was increased (4.3–6.9 g/24 h). This single case study showed the expected results and confirmed the hypothesis that the ingestion of high amounts of amino acid mixtures is accompanied by a significantly increased oxidative utilization.