Metabolic response to glucose overload in surgical stress: energy disposal in brown adipose tissue

The metabolic response to total parenteral nutrition (TPN) overload was investigated in clinical studies of surgically stressed humans and in experimental studies of surgically stressed animals. In both studies, all non-protein calories were administered as glucose, and subjects were divided into two groups, classified according to the amount of caloric supplementation after surgical stress, i.e., either overfed or appropriate (groups C1 and C2, respectively, in the clinical study, and groups E1 and E2, respectively, in the experimental study). In the clinical study, the postoperative energy expenditure in group C1 increased from the preoperative value, becoming significantly more elevated than that in group C2. The respiratory quotient (RQ) in group C1 exceeded 1.0, representing lipogenesis from carbohydrate, and the plasma norepinephrine concentration was also higher in group C1, indicating that lipolysis was likely to have been enhanced in this milieu. These findings imply that lipogenesis and lipolysis can occur simultaneously, constituting a futile cycle, and that this activated cycle could be a reason for the increased energy expenditure associated with overfeeding after surgical stress. To gather further evidence, we evaluated the rate of lipogenesis and lipolytic activity in white and brown adipose tissue (WAT; BAT) in an experimental study of surgically stressed rats. In BAT, both lipogenesis and lipolysis were activated in group E1. The results of both studies suggest that glucose overload in surgically stressed individuals increases energy expenditure by activating a futile cycle and that BAT is more involved in this cycle than WAT.