The aim of this study is to demonstrate that a new protein substitute is acceptable and well tolerated in children with PKU.

Inclusion Criteria:

• Diagnosis of PKU or PKU variant requiring a protein substitute.
• Subjects who were already taking a phenylalanine-free/low phenylalanine protein substitute and were willing to try the study product for 7 days.
• Patients aged 3 years of age to adulthood.
• Written informed consent obtained from participant or parental caregiver (as appropriate).

Exclusion Criteria:

• Presence of serious concurrent illness
• Investigator's uncertainty about the willingness or ability of the patient to comply with the protocol requirements
• Participation in any other studies involving investigational or marketed products concomitantly or within two weeks prior to entry into the study.
• Any patients having taken antibiotics over the previous 2 weeks leading up to the study.
• Patients less than 3 years of age.