Leptin is a hormone secreted by fat cells under normal conditions and acts in the brain to decrease appetite and increase energy use. Leptin levels usually go down with fasting. This study will evaluate the secretion of an investigational agent called leptin in lean and overweight individuals while fasting and investigate the potential role of leptin as a regulator of immune function and mediator of the neuroendocrine response to food deprivation in humans. Data derived from these studies will provide insights into the mechanisms underlying altered hormone levels and immune function in malnutrition and obesity and thus may provide the basis for future therapeutic interventions for obesity.