Conduct a prospective, longitudinal study to evaluate nutritional status in adults with phenylketonuria (PKU) before and during treatment with pegvaliase (Palynziq™).

The investigators will compare intra-subject change in adults with PKU who follow a protein-restricted diet (protein intake less than the RDA for protein based on weight (0.8 g/kg/day) with or without a PKU medical food) within 90 days of starting pegvaliase (baseline) and 9 and 15 months later. The investigators will collect laboratory measures of nutrition status, anthropometrics, Dual-Energy X-Ray Absorptiometry (DXA) to measure body composition and bone mineral density (BMD), and indirect calorimetry to measure resting energy expenditure (REE). In addition, three-day food records will be obtained to assess protein intake and overall diet quality.

Inclusion Criteria:

• Diagnosis of phenylketonuria
• Aged 18-65 years
• Following a protein-restricted diet with or without medical food within the last 30 days
• Routine natural protein intake from food is less than the RDA for protein based on weight (0.8 g/kg/day).
• If not consuming animal proteins, recommended protein intake is 115% RDA due to lower bioavailability of plant proteins.
• Within ±90 days of starting treatment with pegvaliase at time of study enrollment

Exclusion Criteria:

• Unable to consent to study
• Under age 18 or over age 65 years
• Routine natural protein intake is greater than the RDA for protein (0.8 g/kg/d) within the last 30 days.
• Females who are currently pregnant or lactating

This study will recruit adults with PKU who are starting therapy with pegvaliase and currently following a protein-restricted diet with or without medical food.