Phenylketonuria (PKU) is a rare inherited metabolic disorder, where subjects are born with a genetic deficiency in the phenylalanine hydroxylase enzyme (PAH), which leaves them unable to convert Phenylalanine (Phe) into Tyrosine (Tyr). PKU patients have specific dietary needs and must follow a restrictive diet in the aim of preventing toxic levels of the amino acid phenylalanine (Phe) accumulation.

The aim of this explorative cross-sectional study is to gain quantitative insights on blood nutrient levels of adult PKU patients on a protein substitute.

- Inclusion Criteria:

Both PKU and Non-PKU comparison subjects:

1. Age ≥18 years

2. Willing and able to provide signed informed consent PKU specific inclusion criteria

3. PKU patients identified by newborn screening and started low Phe diet before 1 month age

4. Usage of at least one Phe-free protein substitute (i.e. an amino acid mixture including micronutrients) on a daily basis for at least 26 consecutive weeks up to Visit 1

   Non-PKU comparison subjects specific inclusion criteria:

5. Same age- (±3 years) and sex as an included PKU subject

   • Exclusion Criteria:

Both PKU and Non-PKU comparison subjects:

1. For women: Currently pregnant or lactating

2. Current psychiatric disorders

3. Current Substance Use Disorders (as described in DSM V)

4. Current use of psychotropic and/or inotropic medication

5. Omega-3, antioxidant, (multi)vitamin and/or (multi)mineral supplement use within six weeks prior to entry in the study

6. Severe hepatic, thyroid or renal dysfunction

7. No acute illnesses like flu, diarrhea, or vomiting (subjects should be symptom free for a week prior to V1)

8. Participation in any other clinical intervention studies involving test products concomitantly or within six weeks prior to entry into the study

9. Other family members taking part in this study

   PKU subject specific exclusion criteria:

10. Use of BH4, or drugs that may interfere with main outcomes

    Non-PKU comparison subjects specific exclusion criteria:

11. Any condition or special diet (e.g. vegan or vegetarian diet) that effects the metabolism and/or normal dietary pattern/intake

12. A first or second degree relative with inborn errors of metabolism

13. Living together with someone with inborn errors of metabolism (e.g. partner, spouse or roommate)

PKU subjects (18 years and olders) and non PKU comparisons.