 Each participant will take part in two x 3 day study periods. The first two days are called adaptation days where the subjects will consume a liquid diet specially designed for him. The diet will be consumed at home. It contains all vitamins, minerals, protein and all other nutrients required. On the third day, the participant will come to the Hospital for Sick Children in Toronto. Subjects will consume hourly meals for a total of 8 meals and a stable isotope 13C-phenylalanine. Breath and urine samples will be collected to measure the oxidation of phenylalanine from which the threonine requirement will be determined. 