Inclusion Criteria:  Male or female Over 3 years of age Diagnosed with classical or variant type phenylketonuria, or tyrosinaemia (as appropriate) Have been compliant in taking at least one protein substitute, providing at least 10g protein equivalents, for at least 1 month prior to trial commencement Have a prescribed daily Phe or Phe and Tyr allowance for PKU or TYR respectively Written or electronic informed consent from patient, and/or from parent/caregiver if applicable  Exclusion Criteria:  Currently prescribed Sapropterin or similar tetrahydrobiopterin based medication Pregnant or lactating Requiring parenteral nutrition Major hepatic or renal dysfunction Participation in other studies within 1 month prior to entry of this study Allergy to any of the study product ingredients, including milk and soy Investigator concern around willingness/ability of patient or parent/caregiver to comply with protocol requirements