-Increased availability of lipids may conserve muscle protein during catabolic stress. Our study was designed to define 1) intracellular mechanisms leading to increased lipolysis and 2) whether this scenario is associated with decreased amino acid and urea fluxes, and decreased muscle amino acid release in obese subjects under basal and fasting conditions. We therefore studied nine lean and nine obese subjects twice, after 12 and 72 h of fasting, using measurements of mRNA and protein expression and phosphorylation of lipolytic and protein metabolic signaling molecules in fat and muscle together with whole body and forearm tracer techniques. Obese subjects displayed increased whole body lipolysis, decreased urea production rates, and decreased forearm muscle protein breakdown per 100 ml of forearm tissue, differences that persisted after 72 h of fasting. Lipolysis per fat mass unit was reduced in obese subjects and, correspondingly, adipose tissue hormone-sensitive lipase (HSL) phosphorylation and mRNA and protein levels of the adipose triglyceride lipase (ATGL) coactivator CGI58 were decreased. Fasting resulted in higher HSL phosphorylations and lower protein levels of the ATGL inhibitor G0S2. Muscle protein expressions of mammalian target of rapamycin (mTOR) and 4EBP1 were lower in obese subjects, and MuRf1 mRNA was higher with fasting in lean but not obese subjects. Phosphorylation and signaling of mTOR decreased with fasting in both groups, whereas ULK1 protein and mRNA levels increased. In summary, obese subjects exhibit increased lipolysis due to a large fat mass with blunted prolipolytic signaling, together with decreased urea and amino acid fluxes both in the basal and 72-h fasted state; this is compatible with preservation of muscle and whole body protein.obesity; fasting; protein breakdown; lipolysis; skeletal muscle; subcutaneous adipose tissue; human FASTING AND OBESITY ARE BOTH associated with increased levels of free fatty acids (FFAs) In the fasting state, FFA mobilization is promoted by the combined effects of reduced plasma insulin and increased levels of catecholamines, growth hormone, and cortisol (41) and, remarkably, decreased lipolytic effect of catecholamines in adipose tissue (AT) has consistently been demonstrated in obese subjects (12,31,32). In adipocytes, lipolysis is controlled by the sequential action of adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase that hydrolyze triglycerides to FFA and glycerol (25). It has previously been shown that ATGL protein levels increase and that the ATGL inhibitor G 0 /G 1 switch gene 2 (G0S2) protein and mRNA levels decrease during fasting in humans (26). A number of studies have shown that FFA and ketone bodies are decisive for protein conservation, in particular during fasting (6,22,27), but the underlying mechanisms remain elusive.The metabolic effects of fasting in obese subjects have been studied extensively, but the effects of obesity on amino acid metabolism remain unclear, and data from studies meas...