Standard treatment of TYR requires a strict, life long, adherence to a special diet, which is very low in phenylalanine and tyrosine. Phenylalanine needs to be restricted along with tyrosine because phenylalanine is converted into tyrosine in the body. Because phenylalanine and tyrosine are present in almost all natural protein sources, the diet is extremely challenging for patients, requiring the elimination of foods that contain protein. Natural protein sources are replaced in the TYR patient with synthetic free amino acid protein substitutes called “metabolic formula” or they are often referred to as “medical foods”. These protein sources are consumed daily in place of natural proteins.