Patients will be infused with stable (non-radioactive) isotope tracers of glucose, phenylalanine and palmitate for up to 8 hours. After 4 hours, without interruption of the tracer infusion, an infusion of insulin will be started and maintained at the rate of 1.5 mU/kg•min for the final 4 h. Blood glucose concentrations will be measured throughout the insulin infusion and glucose infused as necessary to maintain the basal plasma glucose concentration.