PKU UP is a prospective, single-arm, open-label, 26-week acceptability study to evaluate PKU UP for the dietary management of participants with phenylketonuria (PKU). Up to 15 participants aged 1 - 10 years old will be recruited and it is anticipated the study will open in at least three sites in the United Kingdom (UK) to recruit the target number of participants in the required timeframe.

PKU is a rare inborn error of metabolism with a prevalence of 1 in 10,000 in the UK population. The mainstay of treatment for most individuals is dietary therapy with a strict protein-restricted diet. We propose to recruit children with a diagnosis of PKU aged 1-10 years.

The study product, PKU UP, is a food for special medical purposes (FSMP), as defined by the Delegated Act EU 2016/128, used for the dietary management of Phenylketonuria (PKU).

The study will be investigating the acceptability of PKU UP, as defined by the Advisory Committee on Borderline Substances (ACBS). This includes the following:

• participant adherence to recommended intakes
• gastrointestinal symptoms
• evaluations of palatability.

Each participant will be on the study for 26 weeks. This includes evaluation part 1 and evaluation part 2:

• Part 1 consists of a 12 week period where participants introduce the study product into their diet. Gastrointestinal tolerance, metabolic control, compliance, product acceptability and dietary quality will be evaluated.
• Part 2 consists of a 14 week period where participants continue to take the study product for a longer assessment of growth and nutritional status.

Data collection will be performed using paper and/or electronic Case Report Forms completed by the investigators at the baseline, evaluation periods and end of study visits. There will also be three questionnaires completed by the parents/guardians over the course of the study, in relation to protein substitute intake, gastrointestinal tolerance, and product acceptability.

Inclusion Criteria:

• Diagnosis of PKU requiring a low-protein diet and protein substitute.
• Participants who are adherent with their blood phenylalanine (Phe) testing, in the opinion of the investigator.
• Participants aged 1-10 years.
• Participants who in the opinion of the investigator are anticipated to be able to take at least half of their daily protein equivalent requirement from PKU UP (in order to assess this, a taste test (or similar) may be conducted at investigator's discretion as part of the screening process).
• Willingly given, written, informed consent from the parent(s)/guardian(s).
• Willingly given, written assent by the participant (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 hyperphenylalaninemia, or mild PKU not requiring a low protein diet and protein substitute.
• Allergy or intolerance to milk.
• Participants who are currently participating in, plan to participate in or have participated in an interventional investigational drug, food or medical device study within 30 days prior to the screening visit.
• Use of additional micronutrient supplements during the evaluation period, unless clinically indicated and prescribed by the Investigator (must be recorded in both the medical records and case report form).