Compartmental Model

An overall integration of these autoregulatory control systems in the regulation of body weight and body composition during a cycle of weight loss and weight recovery is discussed with the help of a schematic diagram presented in Figure 4. This diagram embodies the finding that the control of body energy partitioning between protein and fat is an individual characteristic—that is, individuals vary in their partitioning characteristic (Pc) during weight loss and weight recovery—and takes into account the two distinct control systems for adaptive thermogenesis that can operate independently of each other.

During starvation, the control of partitioning determines the relative proportion of protein and fat to be mobilized from the body as fuel—the individual's Pc being dictated primarily by the initial body composition. The functional role of the control of partitioning is to meet the fuel needs of the individual in such a way that the energy reserve component in both the fat and the protein compartments (i.e., the part that can be lost without death or irreversible damage) would reach complete depletion simultaneously—a strategy that ensures the maximum duration of survival for a given individual during long-term food scarcity. Furthermore, the energy conserved resulting from suppressed thermo-genesis is directed at reducing the energy imbalance, with the net result that there is a slowing down in the rate of protein and fat mobilization in the same proportion as defined by the Pc of the individual. Indeed, the fact that the fraction of fuel energy derived from protein (i.e., the P ratio) remains relatively constant during the course of prolonged starvation, albeit in normal-weight humans, implies that neither control system underlying suppressed ther-mogenesis is directed at sparing specifically protein nor specifically fat but, rather, at sparing both protein and fat compartments simultaneously. Therefore, during starvation the functional role of both control systems underlying suppressed thermogen-esis is to reduce the overall rate of fuel utilization (i.e., for energy conservation directed at sparing both lean and fat tissues).

During refeeding, the control of partitioning operates in such a way that protein and fat are deposited in the same relative proportion as determined by the Pc of the individual during starvation, and this serves to reestablish the individual's prestarvation capacity for survival during long-term food scarcity. Furthermore, the increased availability of food leads to the rapid removal of suppression upon the nonspecific (SNS-mediated) control of thermogenesis. In contrast, the suppression of thermogenesis under

Figure 4 Schematic representation of a compartmental model for the regulation of body weight and body composition during a cycle of weight loss (prolonged starvation) and weight recovery (refeeding). In this model, the two distinct control systems underlying adaptive thermogenesis—the nonspecific control and the adipose-specific control—are integrated with the more 'basal' control of partitioning between the body fat and protein compartments as determined by the partitioning characteristic (Pc) of the individual. Adapted from Dulloo AG and Jacquet J (2001) An adipose-specific control of thermogenesis in body weight regulation. International Journal of Obesity 25(supplement 5): S22-S29.

Figure 4 Schematic representation of a compartmental model for the regulation of body weight and body composition during a cycle of weight loss (prolonged starvation) and weight recovery (refeeding). In this model, the two distinct control systems underlying adaptive thermogenesis—the nonspecific control and the adipose-specific control—are integrated with the more 'basal' control of partitioning between the body fat and protein compartments as determined by the partitioning characteristic (Pc) of the individual. Adapted from Dulloo AG and Jacquet J (2001) An adipose-specific control of thermogenesis in body weight regulation. International Journal of Obesity 25(supplement 5): S22-S29.

adipose-specific control is only slowly relieved as a function of fat recovery, such that the energy that continues to be spared is directed specifically at the replenishment of the fat stores. The net effect is that fat is deposited in excess of that determined by the Pc of the individual, thereby contributing to the disproportionate rate of fat relative to lean tissue recovery. This phenomenon of catch-up of fat (rather than catch-up of lean tissue) is often observed, both in adults after severe weight loss due to food unavailability and disease and in infants and children recovering from protein energy malnutrition and growth arrest.

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