To evaluate the acceptability, tolerance and effect on metabolic control of PKU Explore, a renovated Phe free protein substitute for the dietary management of PKU in children from 6 months to 5 years.

This is a prospective, acceptability study to evaluate the gastrointestinal tolerance, palatability and adherence of PKU explore in a four-week period for 15 participants aged between 6 months and 5 years for the dietary management of PKU.

The outcome of this assessment will be used in a submission to the regulatory authorities, Advisory Committee on Borderline Substances (ACBS), for PKU Start to become reimbursable on prescription in the UK.

The participant's dietitian will advise on an appropriate amount of PKU Explore based on individual requirements. Parents/carers will be asked to substitute their usual Phe-free protein substitute with PKU Explore for one (1) month. The sponsor will supply PKU Explore free of charge.

Parents/guardians will be asked to record information about the following:

Daily PKU Explore intake Stools Vomiting and Spit-up Feed / Fluid Intake Compliance Phenylalanine Levels (from their routine blood spot testing)

Final Evaluation about the presentation of the product, ease of preparation and use.

Inclusion Criteria:

i. A diagnosis of PKU on new-born screening requiring a low protein diet and Phe-free L-amino acid supplements.

ii. Aged between 6 months and 5 years.

iii. Already taking a second stage concentrated protein substitute as part of their PKU management OR is at the stage in their PKU management when a second stage concentrated protein substitute is recommended to commence.

iv. Willingly given, written, informed consent from parent/guardian.

v. Willingly given, written assent (if appropriate).

Exclusion Criteria:

i. Diagnosis of persistent hyperphenylalaninaemia, or mild PKU not requiring dietary treatment with a low protein diet and Phe-free L-amino acid supplements.

ii. Diagnosis of a concurrent condition which may adversely affect developmental progression and feeding ability.

iii. Participation in any other clinical trial/acceptability study.

iv. Any serious medical precluding the study intervention.

v. Use of additional macro/micronutrient supplements during the study period, unless clinically indicated and prescribed by the investigator (must be recorded in patient case record file).