This proposal ties together four projects and three core laboratories that collectively share the hypothesis that application of advanced, or creation of new, body composition measurement methods will provide new and clinically important biological insights. The third in this series of Program Project Grants, the present PPG builds upon the success of PPG II with over 700 subject cross-sectional and 500 longitudinal evaluations using state-of-the art measurement methods, four new or improved facilities, four new or improved facilities within the core units (i.e., new in vivo Neutron Activation Analysis (IVNA), Magnetic Resonance Imaging (MRI) Analysis, Bioimpedance Analysis, and Regional 40K Counter; upgrade delayed-gamma, prompt-gamma, and inelastic scattering neutron activation systems), advanced method development (gamma nuclear resonance absorptiometry), important insights into the aging process, HIV-related conditions and obesity, and over 150 published original reports and reviews. The third phase of the PPG is formulated on the interactive projects that are focused on: 1. Advanced skeletal muscle method and method development with a focus on children, adolescents, and adults either gaining or losing body weight; 2. The newly reported HIV-lipodystrophy syndrome and its relationship to phenotypically similar subjects without viral illnesses, both before and after interventions designed to alter metabolic status; 3. The bone and skeletal muscle effects of gastric obesity surgery in type II diabetic subjects with BMI >35 less than or equal too 40 kg/m2 compared to those observed in medically-managed comparable patients; and 4. The lowering of resting energy expenditure observed with aging, even after controlling for conventional body composition measures. The projects will be linked and interact with two continued cores (A, Columbia, Laboratories, and B, Brookhaven National Laboratory) and a new core (C, Biostatistics and Data Management) emerging to fully utilize the PPG's expanding database and to chart new areas of interest including body composition and image modeling. A close tie with Columbia's new Department of Biomedical Engineering will be developed with planning imaging and nuclear studies combined with use of the Medical School's high-field strength Hatch MRI and positron emission facilities. The large number of developing and planned interactions extend the program's boundaries outside of the four- hypothesis driven projects. This PPG aims to broadly extend clinical and research capabilities for measuring body compartments to answer important and prevailing clinical questions and to bridge the expanding interface between biology and engineering.