Inclusion Criteria:  Diagnosis of PKU or associated disorder for example, Tetrahydrobiopterin (BH4) deficiency, requiring a low protein diet and Phe-free L-amino acid supplements (protein substitute). Established on PKU express or an equivalent protein substitute  In the opinion of the study investigator, the participant is able to take the study product and meets at least one of the following criteria:  taking an age inappropriate protein substitute struggling with adherence to current protein substitute delayed in transitioning Aged 3 years and above Willingly given, written, informed consent from patient or parent/guardian. Willingly given, written assent (if appropriate).  Exclusion Criteria:  Diagnosis of persistent hyperphenylalaninaemia, or mild PKU not requiring dietary intervention with a low protein diet and protein substitute. Patients who are tube-fed. Individuals, who in the opinion of the investigator, are unable to comply with the requirements of the protocol. Any co-morbidity/condition which in the opinion of the Investigator, would preclude participation in the study. Patients who are currently participating in, plan to participate in, or have participated in an interventional investigational drug, food or medical device trial within 30 days prior to screening. 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). 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 may, in consultation with their doctor, continue on the study's dietary product if they wish but will not have any investigations that would not normally be carried out during pregnancy.) Those with allergies to fish, milk or soya.