On the day after the tracer infusion the amount of liver and muscle TG and liver glycogen will be determined by MRS. After metal staples are removed, patients will be transported to the clinical MRS facilities at UTMB Dept. of Radiology, where measurements will be performed (see below for details), After obtaining baseline samples, tracer infusions will be started as described in Figure 2. Half the patients with high tissue TG will be given nicotinic acid (500 mg orally) at the start of period 2 to lower FFA levels acutely. In the group given fenofibrate (200 mg/d) or propranolol 0.5mg/kg every 6 hours to lower FFA, half will be infused with heparin (0.5 U/kg•min, 2.8 U/ kg prime IV) at a dose sufficient to activate lipoprotein lipase, thereby elevating plasma FFA, while not affecting blood coagulation. After baseline blood samples from the femoral artery, femoral vein, and peripheral vein are collected, an 8 hour continuous infusion of primed-constant infusions of 6,6-d2-glucose (0.08 mg/kg•min, prime = 6.8 mg/kg) and d5-phenylalanine (0.20 µmol/kg•min, prime = 8.0 µmol/kg) will be given in order to quantify hepatic glucose production and protein synthetic rates, respectively. In addition, 2 hours into the protocol, U-13C16-palmitate (0.16 µmol/kg per minute) will be started with NaH13CO3 prime (150 µmol/kg) in order to quantify hepatic fatty acid uptake and oxidation. These tracer infusions will also be maintained throughout the 8 hour tracer study. Blood samples (2- 12 ml) will be taken from the artery, femoral vein and peripheral vein simultaneously at 120, 180, 210, 225 and 240 minutes (see Appendix 2 for full timeline). Muscle tissue biopsies will be obtained at the start of period 1, and at 4 hours of period 1 to measure protein kinetics and also determine biochemical parameters. Then, period 2 will start. At the start of period 2, a primed, constant infusion of 15N-phenylalanine will be started and maintained throughout period 2. The different tracer of phenylalanine will be used to quantify the plasma protein synthetic rates using the same tracer protocol as in period 1. We have previously shown that the two phenylalanine tracers yield the same results (70). The tracer technique will enable us to measure the primary endpoints of insulin responsiveness of the liver, i.e., endogenous glucose production and synthetic rates of albumin and fibrinogen. At 4 hours, hyperinsulinemia will be initiated by the infusion of insulin at the rate of 1.5 mu/kg•min, which will result in circulating levels of approximately 200 uU/ml (5). This rate of infusion was based on our previous experience with insulin infusion in burned patients (e.g., 1-5). We anticipate a considerable variation in the baseline insulin concentrations, such that if a low rate of infusion were to be used, the resulting "hyperinsulinemia" in some patients would likely be below the baseline concentration in others. Consequently, we have chosen a rate of infusion that will result in a clear-cut difference between the baseline and "hyperinsulinemic" values. Further, although during the insulin infusion we anticipate that insulin concentrations will also be variable, our endpoints will be assessed in terms of the magnitude of change from the baseline value in each subject. This statistical approach should minimize concern regarding subject variability. The dosage was selected because we have previously shown that protein metabolism is responsive to this rate of infusion (5), but that it is below the maximally-effective dose (4). Blood glucose concentration will be monitored throughout the second period, and glucose will be infused (if necessary) to maintain glucose concentrations at the baseline level. Since the baseline concentrations of glucose will vary, this means that during hyperinsulinemia the glucose concentrations will likely differ between subjects, but we have selected this approach because in this way only the insulin concentration will differ between periods 1 and 2, thereby simplifying interpretation of the changes in substrate and protein kinetics from period 1 to 2. The sampling schedule will be the same as in period 1, including the timing of the biopsy (i.e., at 4 h of period 2).