On the morning before the study subjects will report to the Clinical Research Center and will be asked to stay at the Clinical Research Center for up to 72 hours. You will be asked not to eat anything after midnight. Small plastic catheters will be placed in two veins in subjects arms for infusion of glucose and for measuring blood glucose levels every 15-30 minutes. While subjects are in the study they will continuously receive a small dose of glucose enriched with 13C label either through the catheter or by drinking a small amount of a sugar water solution every 15-60 minutes. Subjects will be asked to transfer to the Center for Magnetic Resonance Research every 6-8 hours (except at night) where they will be asked each time to conduct an MRI scan as follows: Subjects will be placed on the patient bed in the magnet room for one-to-two hours. A nurse will stay with the subject at all times during this study. During some of the scans subjects may be asked to perform a visual task such as watching a changing checkerboard pattern through a mirror placed in front of their eyes. After each scan subjects will receive a low carbohydrate meal for weight-maintenance.