In this study the feeding patterns and practices at the introduction of a second stage phe-free protein substitute will be assessed in young children with PKU. This will be compared with the normal weaning process in healthy non-PKU age-matched children. In addition, tolerability and acceptability of the study product will be evaluated in a smaller group of subjects who have already been transferred onto a second stage protein substitute. The study is performed in 3 centers in the United Kingdom.

Part 1:

For subjects with PKU, the study product will be incorporated in the diet as the second stage protein substitute until the subjects are 2 years of age.

The healthy non-PKU age-matched subjects will not have the study product incorporated in their diet in order to be able to compare the weaning experience and nutritional intake of PKU subjects with non-PKU subjects. The healthy subjects will also be part of the study until the subjects are 2 years of age.

Part 2:

All PKU subjects will take the study product for 7 days.

Inclusion Criteria:

• Diagnosis of PKU identified on screening during the newborn period and requiring a phenylalanine free protein substitute
• Subjects who are about to commence weaning and require a concentrated phe-free protein substitute to meet their total protein requirements and are aged between 4 and 10 months (Part 1)
• Subjects aged under 5 years who have already been fully transferred onto a concentrated phe-free protein substitute and are willing to try the study product for 7 days (Part 2)
• Written informed consent obtained from subject or parents / caregiver
• May or may not have commenced weaning solids (i.e. using normal foods)

Exclusion Criteria:

• Presence of serious concurrent illness
• Investigator's uncertainty about the willingness or ability of the patient to comply with the protocol requirements
• Any child who has not commenced a second stage protein substitute and is over the age of 10 months
• Participation in any other studies involving investigational or marketed products concomitantly or within two weeks prior to entry into the study
• Any children having taken antibiotics over the previous 2 weeks leading up to the study