Conduct a cross-sectional analysis of the nutritional status of individuals with PKU treated with pegvaliase for one or more years or who have followed an unrestricted diet for 6 or more months while on pegvaliase treatment. We will evaluate laboratory markers of nutritional status, including markers of protein and micronutrient nutriture. In addition, a three-day food record and food frequency questionnaire will be obtained to assess protein intake and overall diet quality.

Inclusion Criteria:

1. Diagnosis of phenylketonuria
2. Aged 18-65 years
3. Currently treated with pegvaliase
4. Have either 1) received treatment with pegvaliase for one or more years or 2) followed an unrestricted diet for 6 or more months while on pegvaliase
5. Currently following an unrestricted diet that meets or exceeds the RDA for protein (0.8 g/kg/day)

Exclusion Criteria:

1. Unable to consent to study
2. Under age 18 years or over age 65
3. Routine food intake provides less than the RDA for protein (0.8 g/kg/d)
4. Consuming any PKU medical foods

This cross-sectional study will recruit adults with PKU who have been treated with pegvaliase for one or more years or who have followed an unrestricted diet for 6 or more months while on pegvaliase treatment. This may include individuals who participated in the phase III clinical trials for pegvaliase.