Phenylketonuria (PKU) is an inherited inborn error of an amino acid phenylalanine (PHE) metabolism affecting 1:15,000 births. It is caused by a decreased activity of an enzyme in the liver called phenylalanine hydroxylase (PAH) which is important to convert PHE into tyrosine, another amino acid. Consequently, PHE accumulates in the blood leading to mental and developmental delays. Nutritional management is the primary choice of treatment that includes providing sufficient protein in the diet and at the same time restricting PHE. However the amount of protein to be given is unknown. A new technique called Indicator Amino Acid Oxidation (IAAO) will be used to determine the protein requirements in children with PKU (5-18y). The study will help treat and manage these children with sufficient protein to ensure proper growth and development. Current dietary recommendations range from 35-65 g/day and is based on factorial calculations.

The investigators hypothesize that the protein requirement in children with PKU will be higher than the current mathematically calculated recommended intake of 35-65 g/day for the 5-18y children.

Inclusion Criteria:

• Children (5-18y) who are diagnosed with Phenylketonuria (PKU), and clinically stable with no acute illness

Exclusion Criteria:

• Children less than 5y of age and who are diagnosed with PKU, as it may be difficult to take breath samples and perform indirect calorimetry in very young children.
• Children diagnosed with PKU, but are currently ill, with a fever, cold, vomiting or diarrhea.