USA Surgery Today

Total energy requirement for all patients includes resting energy expenditure, energy allotted to physical activity, and diet-induced thermogenesis. By definition, resting energy expenditure encompasses the body’s energy requirement for growth. Physical activity, which is low in the postoperative period or during severe illness, and diet-induced thermogenesis are of reduced significance in the surgical patient. Knowledge of resting energy expenditure and its changes following critical illness or operation is important because it directly impacts energy requirements.

Resting energy expenditure can be measured in a variety of ways, including direct and indirect calorimetry, as well as stable isotopic techniques. The direct method measures the heat released by a subject at rest and during various activities. This method is based on the principle that all energy is eventually converted to heat. In practice, the subject is placed in a thermally isolated chamber with sensitive temperature measurement over a given period of time (7). A change in temperature of the chamber is used to calculate the energy discharged, and therefore fuel oxidized, during the time period. Although precise, this method is not practical for most pediatric or ill patients (8).

Children and adults rely on the release of energy from ingested nutrients or established body stores to fuel the work of the human body. In this way, the energy derived from the degradation of large substrate molecules is used to synthesize high-energy phosphate bonds in the form of adenosine triphosphate (ATP). The bulk of energy metabolism in the human is performed in the aerobic setting, although anaerobic pathways exist and indeed are accentuated in the setting of stress or disease. The hydrolytic cleavage of a phosphate group from ATP, thereby producing adenosine diphosphate, provides much of the energy needed for cellular reactions.

The major sources of energy to fuel the processes of work are carbohydrate, lipid, and protein. These substrates are degraded by the body into the currency of energy metabolism: glucose, fatty acids, and amino acids. For most tissues, fatty acids are the major source for oxidative energy production. However, glucose is of great importance because it is the preferred energy source for cells in the central nervous system, renal medulla, and erythrocytes. These tissues cannot take up fatty acids from the systemic circulation, and therefore rely on glucose or ketone bodies during starvation. Pathways exist to synthesize glucose from the breakdown of amino acid and lipid substrates, a process that is accelerated in critical illness. In general, the breakdown and synthesis of glucose, lipid, and protein are tightly controlled by neural, hormonal, and allosteric regulation.