PAH and phenylketonuria: The addition of extra tyrosine to AA mixtures used for dietary treatment of PKU is critical to compensate the lack of endogenous tyrosine production due to low or missing phenylalanine hydroxylase activity; however, plasma tyrosine concentrations can fluctuate greatly over the day in patients with PKU, and this may be exacerbated by supplementation with free AA mixtures [28,29,30].