Changes in serum phenylalanine after overnight fasts in youngsters with phenylketonuria

Background and aims:In patients with phenylketonuria (PKU), overnight fasti ng is associated with rises in serum phenylalanine [PHE) concentration. Thi s 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 substi tute (P/S). This study assessed the effectiveness of this strategy and whet her any additional benefit was conferred by omitting PHE from the bedtime s nack. 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 sub jects 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 b edtime 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 bot h groups; the mean serum PHE level changed very little overnight (Group X: 405 mu mol L-1 post snack, 389 mu mol L-1 prebreakfast; Group Z: 430 mu mol L-1 post snack, 426 mu mol L-1 pre breakfast). Conclusion: Overnight changes in serum PHE levels can be minimized by a bed time snack accompanied by a proportion of the daily protein substitute. Omi ssion of PHE from the snack confers no extra advantage.