To examine how leucine and protein metabolism is affected by feeding, leucine kinetics were determined in 11 normal term newborns during feeding using a prime constant tracer infusion of 1-<sup>13</sup>C leucine combined with respiratory calorimetry. Fed newborns were compared with previously studied fasting newborns. Feeding and fasting newborns had similar rates of leucine oxidation (34 ± 3 μmol/kg/h versus 31 ± 4 μmol/kg/h) and leucine release from existing protein (156 ± 16 μmol/kg/h versus 164 ± 8 μnol/kg/h). In contrast, nonoxidative disposal rates of leucine (a reflection of protein synthesis) were significantly greater in feeding newborns (170 ± 13 μmol/kg/h versus 129 ± 9 μmol/kg/h). A significant positive correlation between birth weight and leucine flux was demonstrated in both feeding and fasting newborns. These results suggest that /) newborns may accomplish protein accretion primarily by increases in protein synthesis rather than suppression of protein breakdown; 2) an estimate can be made of the minimal leucine intake required to replace irreversible leucine oxidative losses (816 μmol/kg/d, 107 mg/kg/d); and 3) the positive correlation between birth weight and leucine flux in both feeding and fasting newborns may be a result of differences in previous protein and energy supplies.
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health