Effect of enteral versus parenteral feeding on leucine kinetics and fuel utilization in premature newborns

To determine whether the route of nutrient delivery affects whole-body protein kinetics and fuel utilization, eight premature newborns were studied during both a 4-h period of enteral intake and a 4-h period of parenteral nutrition. The kinetics of the essential amino acid leucine were measured using a constant tracer infusion of 1-I3C-leucine, and fuel utilization and energy expenditure were assessed by respiratory calorimetry. All leucine kinetic parameters were similar during enteral or parenteral nutrition (in mean ± SD u.mol/kg/h, flux = 233 ± 51 enteral versus 258 ± 42 parenteral, leucine from protein breakdown = 177 ± 50 enteral versus 200 ± 41 parenteral, leucine oxidation = 57 ± 26 enteral versus 63 ± 20 parenteral, and leucine used for protein synthesis = 176 ± 63 enteral versus 196 ± 50 parenteral). In addition, overall rates of energy expenditure (−52 kcal/kg/d) and pattern of fuel utilization (−70% carbohydrate, 13% fat, 17% protein) were unaltered by the route of feeding. Thus, as reflected by leucine kinetics, overall rates of protein turnover, synthesis, oxidation, and breakdown as well as energy expenditure and fuel utilization are similar when nutrition is provided to premature newborns by either the enteral or parenteral route. These results suggest that short-term provision of parenteral nutrition may be able to substitute appropriately for enteral intake, at least with regard to the utilization of one essential amino acid and the overall pattern of fuel utilization.