Before each study trial If a subject is a regular exerciser, they will be asked to come to the Substrate Metabolism Laboratory in the afternoon before each of the 2 study trials to exercise in the lab for 45 min at a moderate intensity. The evening before the study trial, all subjects will need to eat a standardized dinner meal and snack. They must eat the meal and snack at specific times as indicated by the research staff. After eating the snack, subjects will need to fast (no food or beverages - besides water) overnight.