Group I: Patients are randomized to one of two study arms. Arm I: Patients receive standard total parenteral nutrition (TPN), except the [U-13C]glucose is substituted for a portion of the glucose. Arm II: The infusions of lipids (Intralipid) and amino acids (TrophAmine) are discontinued at study hour zero. The infusion rate of natural glucose will be reduced during the first hour, and thereafter discontinued.