Phenylketonuria PKU is an autosomal recessive inherited metabolic disorder that can cause neurocognitive dysfunction and mental retardation if dietary treatment is not initiated within the first weeks of life. The restrictive nature of the phenylalanine- restricted diet makes compliance a challenge for many patients. Exercise promotes muscle protein synthesis and lean body mass. We hypothesized that an acute bout of moderate- intensity exercise would decrease amino acid oxidation increase muscle protein synthesis and promote tissue uptake of indispensable amino acids thereby lowering plasma phenylalanine concentrations in patients with PKU. The purpose of this project was to assess the impact of an acute bout of moderate-intensity exercise on plasma phenylalanine concentrations in adolescent males with PKU.