Water Deprivation Protocol

Overview

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 tests involving the performance of various tasks. * Measurement of free-living energy using doubly-labeled water, in which participants will drink a sample of water with extra-heavy atoms of hydrogen and oxygen to evaluate the amount of water in the body. * 24-hour urine collection. * Frequent blood samples, urine collection, and fat tissue biopsies during the various study procedures. * After the end of the 13-day study, participants will return after 1 week for a final urine collection.

Study Description: The percentage of people with overweight / obesity in the United States has reached very high levels, with 65% of adults over the age of 20 being overweight. Recently, there have been a number of advances in our understanding of the underlying causes of obesity, including greater understanding of both the effects of the environment and effects that are hereditary (i.e., genetic). One area in which obese versus 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. Thus, at least in animals, others have found a link between ability to tolerate periods without water and increased body weight. We are studying whether the ability to tolerate periods without water and ability to feel thirst might differ in lean versus obese individuals. To do so, we will ask lean and obese individuals to undergo tests that include a period of approximately 24 hours without drinking any water, and on a separate day a shorter period of an intravenous high salt solution infusion. We will be looking at how the ability to withstand thirst (dehydration), subsequent water intake (rehydration), changes in hormone levels during these periods, and behavioral and physiological responses to thirst and rehydration differ in lean versus obese individuals. We hope that the data gathered from this study will give us more information about important differences in how water balance is regulated in lean versus obese individuals. Understanding these questions may provide new insights into differences between lean and obese individuals. Objectives: Primary Objective: to determine if obese versus lean individuals differ in their response (as measured by objective measurement of water intake) to water deprivation and hyperosmolality (dehydration). Secondary Objective: to determine if during these challenges there are differences in hormonal responses to dehydration and subsequent rehydration, and how these changes are related to adiposity and adipose tissue metabolism.