Glucose utilization in the surgical newborn infant receiving total parenteral nutrition

Abstract

Glucose is the main source of nonprotein calories in total parenteral nutrition (TPN). However, its use has been associated with various nutritional, metabolic, and respiratory complications. The aim of this study was to determine, in the stable surgical newborn infant, the characteristics of carbohydrate metabolism, in particular the maximum oxidative threshold for intravenous glucose and the thermogenic effect of glucose. Twenty-one studies were done on 11 infants (weight 2.82 ± 0.19 kg) receiving TPN containing constant amounts of amino acids (2.5 g/kg/d) and fat (3.0 g/kg/d), and different amounts of glucose (range, 10 to 25 g/kg/d). Oxygen consumption (V̇O 2), carbon-dioxide production (V̇O 2), and resting energy expenditure (REE) were measured by indirect calorimetry. Urinary nitrogen excretion rate was measured and substrate utilization calculated from the non-protein respiratory quotient (NPRQ). There was a positive correlation between the predictor variable glucose intake and the dependent variables V̇O 2 ( r = .55; P < .05), V̇CO 2 ( r = .83; P < .0001), REE ( r = .65; P < .005), NPRQ ( r = .94; P < .0001), respiratory rate ( r = .46; P < .06), and plasma triglycerides level ( r = .67; P < .01). When glucose intake exceeded 18 g/kg/d the NPRQ was greater than 1.0, indicating glucose conversion to fat. Above this level of intake, the gradient of the correlation between the predictor variable glucose intake and the dependent variables V̇CO 2 and REE increased. From this study we conclude that: (1) Glucose intake is the principal determinant of glucose utilization and exerts an influence on the metabolism of exogenous fat. (2) Once the maximum oxidative capacity for glucose (18 g/kg/d) is reached: net fat oxidation ceases and net fat synthesis occurs; the thermogenic effect of glucose increases and the efficiency with which glucose is metabolized decreases; carbon dioxide production increases and is associated with an increased respiratory rate; and endogenous triglycerides are released into the circulation.