All subjects will be admitted to the GCRC in the afternoon before the tests (Day 1) and will undergo a thorough physical examination. Routine admission lab tests (including a 2-h 75g OGTT) and body composition measurements will be obtained. A standardized dinner will be served at ~ 6PM. The next (Day 2) after an overnight fast, IV lines will be placed (arms will be wrapped with a heating blanket and kept at 70ºF to arterialize venous blood).