Participants will be hospitalized for 2-3 days for the following procedures: history and physical examination; fasting blood test; several urine collections; X-ray studies to determine bone age and amount of body fat and muscle; magnetic resonance imaging (MRI) scan to measure body fat; "hyperglycemic clamp study" to evaluate insulin resistance; food intake testing; nutrition consultation; resting metabolic rate; and a "doubly labeled water" test.