3. AIM 3. Quantification of post-absorptive gluconeogenesis and whole body protein turnover. Total hepatic glucose production (HGP) will be measured using [6,6-2H2]glucose. We will quantify gluconeogenesis by measurement of the incorporation of 2H into the 2nd, 5th and 6th carbons of glucose following 2H20 administration method of Landau). We will determine whole body protein turnover using the stable isotopes [1-13C]leucine and will measure serum amino acid levels. This specific aim tests the hypothesis that gluconeogenesis and whole body protein turnover are disproportionately higher in Hispanic children and adolescents with CF than in Caucasian CF children.