To evaluate the acceptability, tolerance and effect on metabolic control of PKU Start, a new Phe free protein substitute for the dietary management of PKU in infants from birth.

This is an assessment of ten (10) infants who require a protein restricted diet that is low in Phe. Infants who routinely use a Phe free infant formula as part of their dietary therapy will be recruited for a 28-day assessment of PKU Start, to evaluate tolerance and acceptability.

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 Start based on individual requirements. Parents/carers will be asked to substitute their usual Phe-free infant formula with PKU Start for one (1) month. The sponsor will supply PKU Start free of charge.

Prior to starting PKU Start, parents/carers will be asked to record information about the infant's usual GI tolerance and feeding pattern for a period of three (3) days, to allow for comparison between their existing formula and PKU Start.

They will be asked to record information about the following:

Stools Vomiting and Spit-up Feed / Fluid Intake and Compliance Phenylalanine Levels Final Evaluation about the presentation of the product, ease of preparation and how PKU Start flowed through the teat of a bottle.

Inclusion Criteria:

1. A diagnosis of classical or severe PKU on new-born screening (For the purposes of this study, 'severe' is defined as phe concentrations between 120 and 600 µmol/L at diagnosis)
2. Taking a minimum of one (1) feed of a Phe-free infant formula
3. A minimum period of four (4) weeks from the time of diagnosis to initial approach to parents

Exclusion Criteria:

1. Diagnosed with mild PKU or hyperphenylalaninaemia (For the purposes of this study, 'mild' is defined as phe concentrations between 120 and 600 µmol/L at diagnosis)
2. Diagnosis of a congenital condition