PKU Low Calorie Drink Study

Inclusion Criteria: * Male or female * 3 years of age or above * Diagnosed with classical or variant type phenylketonuria (PKU), or hyperphenylalaninemia (HPA) * Have been compliant in taking at least one amino acid-based, Phe-free protein substitute, providing at least 20g protein equivalent daily, for at least 1 month prior to study commencement * Have a prescribed daily Phe allowance * Written informed consent from patient, or from parent / carer if applicable Exclusion Criteria: * Pregnant or lactating * Requiring enteral tube or parenteral nutrition * Major hepatic or renal dysfunction * Participation in other studies within 1 month prior to entry to this study * Allergy to any of the study product ingredients * Investigator concern around the patient being underweight (e.g.: BMI lower than 18.5kg/m2 for adults) and/or having an eating disorder * Investigator concern around willingness/ability of patient or parent/carer to comply with protocol requirements