This study is about understanding the protein needs of adults with **Phenylketonuria (PKU)**. PKU is a condition where the body can't break down an amino acid called phenylalanine, found in protein foods. It's important because it can help people with PKU have better health by knowing exactly how much protein they need. The study uses the **Indicator Amino Acid Oxidation (IAAO) method**, a safe technique to find out how much protein is needed. The study takes place at Emory Genetics Clinic and needs participants to visit 8 times over 4-6 months.

• Participants will attend one preliminary visit and 7 study days. Each study day is 8 hours long.
• The study involves surveys, body measurements, and collecting blood, urine, and breath samples.
• Participants can choose to donate plasma and urine samples for future research.

To join, you must be 18 or older, diagnosed with PKU, and able to consent. You can't join if you have a recent illness, are pregnant, or have certain medical devices.