Inclusion Criteria:  Diagnosis of PKU requiring a low protein diet and protein substitute. Aged three (3) years and over Already taking a minimum of one GMP based protein substitute per day or have previously taken one Able to comply with the study protocol and take the study product, according to the opinion of the PI. Willingly given, written, informed consent from patient or parent/guardian. Willingly given, written assent (if appropriate).  Exclusion Criteria:  Inability to comply with the study protocol, in the opinion of the investigator. Any co-morbidity, which, in the opinion of the investigator, would preclude participation in the study. Diagnosis of persistent hyperphenylalaninaemia, or mild PKU not requiring dietary intervention with a low protein diet and protein supplement. Unwilling/unable to take a GMP based protein substitute. Women who are pregnant / breastfeeding at the start of the study or planning to become pregnant during the study period.  N.B.: Women who become pregnant unexpectedly during this study will be immediately withdrawn from the study.