Leucine kinetics during feeding in normal newborns.

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 1-13C 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 mumol/kg/h versus 31 +/- 4 mumol/kg/h) and leucine release from existing protein (156 +/- 16 mumol/kg/h versus 164 +/- 8 mumol/kg/h). In contrast, nonoxidative disposal rates of leucine (a reflection of protein synthesis) were significantly greater in feeding newborns (170 +/- 13 mumol/kg/h versus 129 +/- 9 mumol/kg/h). A significant positive correlation between birth weight and leucine flux was demonstrated in both feeding and fasting newborns. These results suggest that 1) 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 mumol/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.