Obesity is a major public health problem in the U.S. The major goal of obesity therapy is to decrease body fat. This requires the mobilization of adipose tissue triglycerides (lipolysis) for fuel. Treatment often involves reducing calorie intake and increasing exercise activity. However, the factors that regulate lipolysis and the effect of hypocaloric dieting or exercise training on lipid metabolism in obese persons are not well understood. The studies in this proposal will elucidate: (1) the mechanisms responsible for regulating lipolysis in obese persons; (2) whether lipolytic sensitivity can be improved by endurance exercise training, weight reduction, or both; and (3) the effects of exercise training and weight loss on whole-body, regional, and cellular lipid metabolism. Persons with upper body obesity (UBO) will be studied because this group is at greater risk for medical complications than are persons with lower body obesity. In addition, both men and women will be studied to evaluate potential gender differences in lipid metabolism. Basal resting lipolytic rates, the lipolytic response to exercise, and lipolytic sensitivity to epinephrine infusion will be determined in obese and lean subjects. Whole-body lipolytic rates will be assessed by infusing (2H5]glycerol and (1-13C]palmitic acid. A novel approach will be used to measure regional lipolytic activity by combining stable isotope tracer infusion with abdominal vein catheterization and microdialysis probe insertion. This method permits the determination of substrate isotopic enrichment in abdominal venous blood and abdominal and gluteal adipose tissue extracellular fluid. The regulation of regional lipolysis will be evaluated by local infusion of beta- and alpha- adrenergic blocking agents through the microdialysis probes. Adipose tissue biopsies will be obtained to assess the molecular mechanisms regulating regional lipolysis. The cellular concentrations of GTP-dependent regulatory proteins, Gs and Gi, and hormone-sensitive lipase will be determined in adipocytes taken from subcutaneous and gluteal adipose tissue. After these initial studies are completed, the obese subjects will be randomized to one of three 20-week treatment programs: (1) hypocaloric diet and weight loss, (2) endurance exercise training without weight loss, and (3) hypocaloric diet and weight loss plus endurance exercise training. Repeat isotope infusion studies and adipose tissue biopsies will be performed in those subjects who have successfully completed their treatment program to evaluate the effect of each intervention on lipid metabolism.
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