Exogenous (dietary) PHE is an essential amino acid necessary for protein synthesis and is also used as an endogenous alternative energy source during muscle protein catabolism. The initial step in PHE catabolism is an irreversible hydroxylation step to form L-TYR.

TYR, whether derived from PHE or from the diet, is used in anabolic processes (protein synthesis), as a precursor for synthesis of the neurotransmitter dopamine, and of melanin, or is catabolized for energy. In PKU, TYR becomes a conditionally essential amino acid.

Figure 2: PHE Catabolism by PAH

Goals of medical nutrition therapy in PKU are to: reduce and maintain blood PHE between 120-360 µmol/L throughout the lifespan; maintain blood TYR in the normal range; and promote normal growth, development and health maintenance.