Phenylketonuria, Oxidative Stress, and BH4

Oxidative stress will be induced by having participants eat a meal in polyunsaturated fats. The investigators will see how participants respond to the meal challenge over the course of 6 hours. * PKU participants who are currently on BH4 therapy will be evaluated while taking BH4 with the meal at a single study visit. * PKU participants who did not responded to BH4 therapy will be evaluated twice. At the first study visit the investigators will see how you react to the meal challenge when you are not given BH4. You will be given BH4 for two weeks, then return for a second meal challenge study visit while taking BH4. * Healthy controls will be evaluated after consuming the meal only (no BH4) at a single study visit.