It is therefore our hypothesis that a high-leucine essential amino acids mixture specifically designed to stimulate protein anabolism will target the metabolic alterations of pediatric subjects with CF. In the present proposal, the acute metabolic effects of this high leucine essential amino acids mixture will be examined in pediatric subjects with CF and compared to that of a regular balanced total mixture of essential and non-essential amino acids. The principal endpoints will be the extent of stimulation of whole body protein synthesis as this is the principal mechanism by which either amino acid or protein intake causes muscle anabolism, and the reduction in endogenous protein breakdown. Both endpoints will be assessed by isotope methodology which is thought to be the reference method.