Randomised controlled trial with a crossover design. For early and continuously treated patients with phenylketonuria (PKU) that are adherent. Two 12-week periods where patients consume either casein glycomacropeptide (CGMP) based protein substitute or a free amino acid (AA) based protein substitute. 4 week wash out period in between. The protein substitutes will be consumed daily together with the patient's regular low protein diet.

The study is a randomised controlled trial with a crossover design conducted at several international study centres. The study population is early and continuously treated patients with PKU that are adhering to their prescribed diet. The participants will be given both a CGMP based protein substitute and a free AA based protein substitute as their main protein source on two different 12-week periods in a crossover design. The two periods will be separated by a 4 week wash out period. The protein substitutes will be consumed daily together with the patient's regular low protein diet during the intervention periods. Primary Objective The primary objective of the study is to investigate the effects of a CGMP based formula compared to an AA based formula upon mean plasma Phe levels after 12 weeks of daily intake in patients with PKU. Secondary Objectives The secondary objectives of the study are to investigate if a CGMP based formula compared to an AA based formula in PKU patients provides any long-term health benefits concerning gut health, inflammation, oxidative stress and product acceptability.