Changes in serum phenylalanine after overnight fasts in youngsters with phenylketonuria

Background and aims: In patients with phenylketonuria (PKU), overnight fasting is associated with rises in serum phenylalanine (PHE) concentration. This is thought to result from catabolism of endogenous protein, which can be reduced by a bedtime snack accompanied by a dose of PHE-free protein substitute (P/S). This study assessed the effectiveness of this strategy and whether any additional benefit was conferred by omitting PHE from the bedtime snack. Methods: Data are presented for 18 PKU children (aged 9–16 years), studied over a 48-h period whilst following diets of known composition. All the subjects had a bedtime snack accompanied by 25% of their P/S daily requirement. Half the subjects (Group X) had 25% of their daily PHE allowance in the bedtime snack, whilst the others (Group Z) ate only ‘PHE-free’ foods at this time. Results: Overnight changes in serum PHE levels were generally small for both groups; the mean serum PHE level changed very little overnight (Group X: 405 μmol L^?1 post snack, 389 μmol L^?1 prebreakfast; Group Z: 430 μmol L^?1 post snack, 426 μmol L^?1 pre breakfast). Conclusion: Overnight changes in serum PHE levels can be minimized by a bedtime snack accompanied by a proportion of the daily protein substitute. Omission of PHE from the snack confers no extra advantage.