Nutrition Status of Adults With PKU Before and During Treatment With Pegvaliase

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.