Energy Substrate Utilization in Infants Receiving Total Parenteral Nutrition with Different Glucose to Fat Ratios

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Abstract
As the fate of glucose and lipids infused during total parenteral nutrition is not well known in infants, we assessed energy substrate oxidation in 36 patients (mean age: 5.8 ± 3.6 mo) on continuous total parenteral nutrition. The infants received isocaloric feeding regimens with nonprotein energy intakes either based on glucose alone (group 1) or on glucose-lipid mixtures providing 15% (group 2), 35% (group 3), 50% (group 4), or 70% (group 5) energy as fat for at least 6 d before glucose and fat oxidation rates were measured by open-circuit indirect calorimetry. Oxidative glucose disposal reached a maximal rate of 12.6 ± 1.2 mg/kg·min (17.9 ± 1.7 g/kg·d) in patients with the higher glucose infusion rates. Glucose infused in excess of maximal oxidative disposal was stored, mainly as fat. The increase in glucose infusion rate was paralleled by an increase in energy expenditure amounting to 16% of the energy value of infused glucose. Net fat oxidation was only observed in group 3 (1.6 ± 0.7 g/kg·d), 4 (3.4 ± 0.6 g/kg·d), and 5 (3.9 ± 0.4 g/kg·d) patients, with glucose infusion rates lower than 18.3 g/kg·d. However, there was no further increase in fat oxidation in group 5 as compared to group 4 patients, despite a further increase in fat intake, which only resulted in increasing fat deposition. Thus, fat infusion aiming at a significant contribution to coverage of energy expenditure requires that glucose oxidation be equal to or lower than maximal oxidative glucose disposal, hence that glucose infusion rates be lower than 18 g/kg·d. In these conditions, excessive fat deposition may be prevented by adapting fat infusion to fat oxidation capacity, which did not exceed 4 g/kg·d in our patients.