Phenylketonuria: Plasma phenylalanine responses to different distributions of the daily phenylalanine allowance over the day

Objective. To achieve smooth control of plasma phenylalanine concentrations in phenylketonuric patients, it is advocated to divide the daily intake of natural protein and amino acid supplements equally over the meals. However, this may be quite an encumbrance for the patient. We, therefore, investigated whether a breakfast with an unequal daily distribution results in an undue rise in the plasma phenylalanine concentration. Design. Plasma phenylalanine concentrations were measured in seven patients with phenylketonuria in response to three tests with breakfast and lunch, representing an equally or unequally divided daily distribution of the individually tailored phenylalanine intake. Breakfast contained 25%, 50%, or 75%, whereas lunch contained 30% or 10% of the individual daily phenylalanine allowance, respectively. Results. Plasma phenylalanine concentrations showed postprandial increases of up to 26% above baseline. Generally, phenylalanine returned to baseline during the test and remained within the target range if baseline phenylalanine was within that range. Two patients having baseline values in the upper target range showed a rise just above the target range for 60 minutes on an unequal daily distribution of phenylalanine. In another patient treated similarly, plasma phenylalanine did not return to baseline during the test. Conclusions. Unequal distributions of the daily phenylalanine allowance are justified, provided that the patient is in good clinical condition, adjusted to the diet adequately, and the daily allowance is not exceeded. At this time, however, we cannot recommend this unequal daily distribution for daily practice.