This is the 5th in a series of physiological studies to determine the amino acid requirements of infants. There have been 4 studies to determine tyrosine, methionine, threonine and lysine requirements in infants when they are fed by parenterally (intravenously). Due to the new requirements of Health Canada for preparation of parenteral solutions, the investigators are starting the phase of the study that determines the enteral (oral) intake of threonine in 1 - 6 mo infants in the interim.