Nutritional management of phenylketonuria. Our research group is conducting a clinical study in subjects with phenylketonuria (PKU) to determine the safety and efficacy of glycomacropeptide from cheese whey in the nutritional management PKU. Individuals with PKU have a deficiency of the liver enzyme phenylalanine hydroxylase which converts the essential amino acid phenylalanine to tyrosine. To prevent brain damage and cognitive impairment they must follow a lifelong, low-phenylalanine diet that is restricted in natural protein and requires ingestion of a bitter tasting amino acid formula. Glycomacropeptide is uniquely suited to the PKU diet as it is the only known dietary protein that is naturally free of phenylalanine. Low-phenylalanine foods and a palatable alternative to the amino acid formulation can be made from glycomacropeptide.