Participants will receive carbon-13 labeled [U-13C]propionate (three 400 mg doses given by mouth over 1 hour). During the study, subjects will also be given 0.5% heavy water ad libitum. Subjects will undergo measurement of their respiratory quotient (RQ) using an indirect calorimeter just prior to the infusion protocol. Subjects will then have an intravenous catheter placed and an infusion of the stable isotopes carbon-13 labeled [3,4-13C2]glucose, carbon-13 labeled [1,2-13C]acetoacetate and carbon-13 labeled [3,4-13C]3-hydroxybutyrate initiated over two hours. During this period, 5 ml. blood draws will occur every hour to determine plasma non-esterified fatty acids (NEFA), ketones, glucose, insulin, and glucagon levels. At two hrs after initiation of the infusion, a 50 ml. blood draw will be performed to obtain samples for spectroscopic determination of the rate of ketogenesis and citrate oxidation in the citric acid (TCA) cycle. At the completion of study procedures, the stable isotope infusion will be stopped and participants will be sent to the CTRC outpatient clinic for Dual Energy x-Ray Absorptiometry (DEXA) to determine estimated total body fat measurement. After the DEXA, healthy subjects will be discharged to home; NAFLD subjects will be admitted to CTRC for liver biopsy, if needed.