At NIH, 50 African-Americans (25M, 25W, age range 18-49 years) and 14 Caucasians (7M, 7W age range 18-49 years) will be enrolled. Equal numbers of normal weight, overweight and obese subjects will be recruited. As outpatients, participants will be placed on weight maintaining diets for two weeks and then admitted for a two-day hospital stay. On each in-hospital morning, resting energy expenditure will be measured and stable isotopes infused to measure FFA flux. Stable isotopes are naturally occurring forms of elements, which are safe and non-radioactive.