The tracer study subjects can commence once catheters in the femoral artery and vein have been placed by the surgical team, if necessary, since the majority of patients wil have pre-existing lines placed for clinical reasons. The catheters will be used for sampling and in a peripheral vein for infusing, as in our previous studies (e.g., 4). Enteral administration of a mixture of carbohydrate and amino acids (Vivonex) will be stopped four hours prior to the study, and will be started immediately following the study.