Water Deprivation Protocol

Background: \- One area in which obese and lean individuals may be different is how their bodies handle water balance and thirst. Studies done in animals suggest that individuals with greater body fat may tolerate periods without water better than lean animals. Other research has found a link between the ability to tolerate periods without water and increased body weight. Researchers are interested in studying whether the ability to tolerate periods without water and ability to feel thirst might differ in lean versus obese individuals. Objectives: \- To evaluate the effects of water deprivation and feelings of thirst in lean and obese individuals. Eligibility: \- Healthy individuals at least 18 years of age who are either lean (body mass index less than 26 kg/m(2)) or obese (body mass index at least 35 kg/m(2)). Design: * Participants will be screened with a medical history, physical examination, and blood and urine tests. * Participants will spend the entire study (13 days) as inpatients at the National Institutes of Health Clinical Center. * Participants will have the following tests and procedures: * Body composition assessment (using low-level x-rays) to determine the percentage of fat tissue in the body. * Oral glucose tolerance test (similar to that often used to diagnose diabetes). Individuals who are suspected to have diabetes will not be allowed to continue with the study. * High salt infusion test, in which an infusion of saline (salt water) will be given for 2 hours and participants will respond to questions about how hungry and thirsty they feel during the procedure. * Water deprivation test, in which participants will go for 24 hours without water or food and respond to questions about how hungry and thirsty they feel. * 24-hour stay in a metabolic chamber to determine how many calories participants burn in a day. * A series of questionnaires about participants' eating habits, feelings about food, and personal feelings, as well as computer-based