Analysis of energy and macronutrient balance in the postoperative infant

The purpose of this study was to better define the energy and macronutrient balance of infants after major surgery. Forty studies were conducted in 29 full-term surgical infants (mean weight, 3.27 +/- 0.19 kg; mean days postsurgery, 8 +/- 1). Nineteen infants were receiving total parenteral nutrition, 13 were orally fed, and eight received a combination of intravenous and oral nutrition (mean caloric intake, 84 +/- 4 kcal/kg/d). Each study was comprised of a three-day nutritional balance measuring energy intake and energy losses in excreta. Metabolizable energy intake (MEI) was determined by subtracting losses from gross intake. Energy expenditure and macronutrient oxidation were determined on the second day of the balance using continuous open-circuit indirect calorimetry in combination with timed urinary nitrogen excretion. Energy and macronutrient storage were calculated by subtracting oxidation and losses from measured intake. The mean global energy expenditure was 57 +/- 1 kcal/kg/d. Linear regression analysis of the nutritional balance data allowed estimates of energy, fat, and protein storage at various MEI. The data suggest that in the surgical infant, protein balance is maintained, even during hypocaloric nutritional regimens. In infants receiving adequate calories to maintain energy balance, maintenance metabolism is supported by the oxidation of endogenous fat. Net fat deposition will predictably only occur at metabolizable intakes of greater than 71 kcal/kg/d.