Submitted

Background In patients with Phenylketonuria (PKU) target ranges of blood phenylalanine (Phe) concentrations have become narrow in order to improve long term outcome. Strict dietary treatment necessitates more knowledge about variation of blood Phe concentrations and the effects of changes in Phe intake. Objective We investigated day-to-day and week-to-week variation in blood Phe concentration and the effect of an additional Phe load on blood Phe concentration. Design We performed a longitudinal study in six adult PKU patients. The study was divided in fi ve periods of seven days: one period without any intervention (period I) and four periods with a Phe load on day three equivalent to 100 % (period II-III) and 200 % (period IV-V) of their individual daily Phe intake. Phe loading was given as encapsulated L- Phe. In all periods the diet was similar in energy, natural protein and amino acid supplementation. Bloodspots were taken each morning before breakfast in all periods, to measure blood Phe concentration. Results Blood Phe concentrations varied considerably from day-to-day and week-to