Patients receive infusions of stable isotope tracers: [15N]urea IV beginning at -12 hours and continuing over 22 hours to measure protein oxidation; [U-13C]glucose IV beginning at zero hour and continuing over 10 hours to measure glucose appearance rate and gluconeogenesis; [2-13C]glycerol IV over 10 hours to measure lipolysis; and [1-C]leucine IV over 10 hours to measure proteolysis. Blood samples are obtained before the start of the urea tracer infusion; before the start of the glucose, glycerol, and leucine tracer infusions; and at study hours 4, 4.5, 5, 9, 9.5, and 10. Blood glucose is measured hourly, and patients receive glucose IV if blood glucose falls below 40 mg/dL.