Scientists investigate why some people resist weight gain

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

* Inclusion Criteria: * Age: 18-55 years, to exclude effects of aging on energy expenditure and weight change. For the PET-CT substudy, women will be limited to ages 18-40 years because there is a high likelihood that for women greater than 40 years, their personal physician may recommend an annual routine mammogram, thereby increasing their annual radiation exposure. Men will also be limited to this age range to prevent age discrepancies between men and women for this substudy. * Premenopausal * Weight: less than or equal to 450 lb (maximum weight allowed on the DXA scanning tables by the manufacturer). Exclusion Criteria * History or clinical manifestation of: * Current smoking * Impaired glucose tolerance (IGT), type 1 and type 2 diabetes * Endocrine disorders, such as Cushing s disease, pituitary disorders, and hypo- and hyperthyroidism * Pulmonary disorders, including chronic obstructive pulmonary disease, which would limit ability to follow the protocol (investigator judgment) * Cardiovascular diseases, including coronary heart disease, heart failure, arrhythmias, and peripheral artery disease * Hypertension, as diagnosed and treated by an outside physician or by sitting blood pressure measurement, using an appropriate cuff, higher than 140/90 mmHg on two or more occasions * Liver disease, including cirrhosis, active hepatitis B or C, and AST or ALT greater than or equal to 3 times normal * Renal disease, as defined by serum creatinine concentrations greater than or equal 1.5 mg/dl and/or proteinuria greater than 300 mg/day (200 (Micro)g/min) * Central nervous system disease, including previous history of cerebrovascular accidents, dementia, and neurodegenerative disorders * Cancer requiring treatment in the past five years, except for non-melanoma skin cancers or cancers that have clearly been cured or in the opinion of the investigator carry an excellent prognosis (e.g., Stage 1 cervical cancer). * Infectious disease such as active tuberculosis, HIV (by self-report), chronic coccidiomycoses or other chronic infections that might influence weight. * Conditions not specifically mentioned above may serve as criteria for exclusion at the discretion of the investigators * Alcohol and/or drug abuse (more than 3 drinks per day and use of drugs, such as amphetamines, cocaine, heroin, or marijuana). * Pregnancy or lactation * For the PET-CT substudy, radiation exposure to the torso for research or medical purposes within the past 12 months * Positive Urinalysis Drug Screen Prior to beginning any of the study procedures, all subjects will be fully informed of the aim, nature, and risks of the study prior to giving written informed consent. The study s informed consent will be obtained by a principal or associate investigator, research physician or physician assistant working in the clinical research unit.