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Leucine Kinetics During Feeding in Normal Newborns

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Leucine Kinetics During Feeding in Normal Newborns 003 1-3998191/3001-0023$03.00/0 PEDIATRIC RESEARCH Vol. 30, No. 1, 1991 Copyright O 1991 International Pediatric Research Foundation, Inc. Printed in (I.S. A. Leucine Kinetics during Feeding in Normal Newborns SCOTT C. DENNE, ELENA M. ROSSI, AND SATISH C. KALHAN Division of Neonatology, Department of Pediatrics, Indiana University School of Medicine, Indianapolis, Indiana 46202-5210 and Case Western Reserve University School of Medicine, Cleveland, Ohio 44106 ABSTRACT. To examine how leucine and protein metab- We have previously determined leucine kinetics in fasting olism is affected by feeding, leucine kinetics were deter- normal term newborns and demonstrated higher rates of leucine mined in 11 normal term newborns during feeding using a flux and oxidation compared to normal fasting adults (8). In prime constant tracer infusion of 1-13C leucine combined addition, a significant correlation between newborn birth weight with respiratory calorimetry. Fed newborns were compared and leucine flux was demonstrated, suggesting a relationship with previously studied fasting newborns. Feeding and between birth weight and fasting protein metabolism. fasting newborns had similar rates of leucine oxidation (34 The purpose of our present study was to determine the effect + 3 pmol/kg/h versus 31 f 4 pmol/kg/h) and leucine release of feeding on leucine kinetics in normal term newborns and to from existing protein (156 + 16 pmol/kg/h versus 164 f 8 examine the relationship between birth weight and leucine ki- pmol/kg/h). In contrast, nonoxidative disposal rates of netics during feeding. By comparing leucine kinetics during leucine (a reflection of protein synthesis) were significantly feeding with those previously measured during fasting, some greater in feeding newborns (170 f 13 ~mol/kg/hversus insight into the normal process of protein accumulation in 129 f 9 pmol/kg/h). A significant positive correlation newborns could be gained. In addition, leucine oxidation meas- between birth weight and leucine flux was demonstrated in urements obtained in both feeding and fasting provide a quan- both feeding and fasting newborns. These results suggest titative basis to estimate leucine requirements in normal new- that 1) newborns may accomplish protein accretion pri- borns. marily by increases in protein synthesis rather than suppression of protein breakdown; 2) an estimate can be MATERIALS AND METHODS made of the minimal leucine intake required to replace irreversible leucine oxidative losses (816 pmol/kg/d, 107 Study protocol. Eleven normal newborns were studied for 7 to mg/kg/d); and 3) the positive correlation between birth 8 h during the first 3 d of life. All babies were full term, had not weight and leucine flux in both feeding and fasting new- experienced birth asphyxia, had not received antibiotics, and had borns may be a result of differences in previous protein normal physical examinations with no apparent clinical prob- and energy supplies. (Pediatr Res 30: 23-27, 1991) lems. Studies were begun 3 to 4 h after the babies' last normal feeding. Clinical characteristics of the study subjects are shown Abbreviations in Table 1. All infants were studied under neurothermal condi- tions, and a skin temperature of 36°C was maintained under a KIC, a-ketoisocaproate servo-controlled radiant warmer. Beginning 3 to 4 h before the MPE, moles percent excess isotope infusion, 4 mL/kg of standard infant formula (Enfamil; Mead Johnson, Evansville, IN) were carefully measured, placed in a small bottle, and fed to the baby orally every hour. This amount of formula corresponds to 1.44 g/kg/d of protein, 64 kcal/kg/d, and 47 pmol/kg/h of leucine. This rate of feeding was In addition to providing specific information about the metab- chosen because it provides a quantity of leucine that approxi- olism of an essential amino acid, leucine kinetic measurements mates estimated infant requirements (-150 mg/kg/d) (9). The may also reflect how different circumstances affect overall pro- bottle was weighed before and after each feeding to determine tein kinetics (1). In particular, these measurements have been the actual intake. After 3 to 4 h of feeding, an indwelling needle used to assess the changes in proteolysis and protein synthesis in was placed in a superficial vein of each arm to infuse the isotopic response to feeding after a period of fasting (2-7). Although the tracer in one and draw blood samples from the other. After a 4- effects of feeding and fasting on leucine kinetics have been pmol/kg priming dose, 1-I3C leucine (99 atom percent excess examined in normal adults (2-7), how feeding affects leucine 13C; Merck Sharp and Dohme, Quebec, Canada) was infused at metabolism in normal newborns has not been determined. a constant rate of 6 pmol/kg/h for the next 5 h. At the end of Adults during feeding must accrete protein in sufliicient quantity each study, the actual rate of infusion was measured using the to replenish protein losses during fasting; in contrast, newborns same syringe, i.v. tubing, and butterfly needle used with that must not only replace fasting losses but accumulate additional particular study. The infusate concentration was also measured. protein to accomplish growth. Therefore, feeding may result in Blood samples were obtained at 0.5- to 1.0-h intervals. Steady different protein kinetic responses in newborns than in adults. state determinations of I3C enrichment of bicarbonate and Received August 28, 1990; accepted February 22, 199 1. plasma leucine, obtained in the last 2 h of the study, were used Correspondenceand reprint requests: Scott C. Denne, M.D., Indiana University for calculation of leucine kinetics. We have previously deter- School of Medicine, Department of Pediatrics, Section of Neonatal-Perinatal Med- mined that I3C enrichment measurements in bicarbonate and icine, 702 Barnhill Dr., RR208, Indianapolis, IN 46202-5210. Supported in part by Diabetes Research Training Center NIH Grant PHS breath C02yield equivalent results (10). P60DK20542-13 and NIH Grant K08HD840-03, HD 11089, and by The James The plasma enrichment of leucine was used in these studies to Whitcomb Riley Memorial Association. calculate rates of leucine flux and oxidation. Plasma enrichment 2:3 24 DENNE ET AL Table 1. Clinical characteristics Age at Gestation Birth wt Study wt study end Apgar Subject M/F (wk) (kg) Percentile (kg) (h) Delivery at 5 min - - 1 Vaginal 2 Vaginal 3 Vaginal 4 CS* 5 Vaginal 6 Vaginal 7 Vaginal 8 Vaginal 9 Vaginal 10 Vaginal 11 Vaginal Mean SEM * CS, cesarean section. of KIC instead of leucine has been suggested for the calculation of oxidation (1 I), and some authors have advocated the use of KIC enrichment for the determination of leucine flux as an index of intracellular leucine enrichment (12). Because KIC enrich- where Q is leucine turnover or flux (pmol/kg/h), Ep is I3C ment is lower than leucine enrichment, the use of plasma leucine enrichment of leucine in plasma at steady state (MPE), and I is enrichment may underestimate total body leucine flux. However, rate of tracer infusion (pmol/kg/h). the constant infusion of 1-I3Cleucine over a wide range of dietary Oxygen consumption and C02production were calculated by intake and clinical circumstances has been shown to result in a multiplying the gradients and the flow rates of air across the face constant ratio of KIC to leucine enrichment of approximately using Haldane transformation and corrected to the stp. The basal 0.77 (1 1, 13). Most pertinently, the KIC to leucine enrichment caloric expenditure was calculated from the RQ and oxygen ratio was similar in adult humans studied during feeding and consumption data (20). fasting (1 1). However, in studies of starved and fed rats, the ratio The fraction of leucine oxidized (F) was assessed by determin- of plasma KIC to leucine enrichment was slightly (but not ing the amount of infused label that appeared in CO2. The statistically) lower in the fed state (0.72 versus 0.76) (14). Relative following equation was used (16): to fasting, this would slightly increase leucine oxidation in the fed state if based on KIC rather than leucine enrichment. Never- theless, the use of KIC instead of leucine for calculating flux and oxidation in the present study would probably result-in a pro- where VCO~is the carbon dioxide production and the constant portional increase in values but would likely alter none of the 0.8 represents the fractional recovery of I3CO2(2 1, 22). Leucine relative findings. oxidation was then calculated by multiplying the leucine flux by Resting oxygen consumption and C02production were meas- the fraction of leucine flux oxidized. ured using open circuit indirect respiratory calorimetry as pre- A single value for the fractional I3CO2recovery has been used viously described (8, 10). combustion of absolute ethyl alcohol here. However, in parenterally fed premature newborns, a range in the system gave results within 5% of expected values. Respi- of recovery of labeled C02has been obtained (0.70-0.84) and a ratory calorimetry was performed throughout the study, and correlation observed between 13C02recovery rates and energy values obtained in the last 2 h were used for analyses. Subjects intake. If the same relationship exists for fed and fasted normal were quiet or sleeping for all measurements. The protocol was term newborns, it is possible that using a constant value for approved by the hospital committee on human investigation, recovery may slightly underestimate leucine oxidation in the and written informed consent was obtained from the babies' fasted state and overestimate oxidation in the fed state. If these parents after fully explaining the procedure. errors exist, protein synthesis estimates would be undervalued Analytical procedures. I3C enrichment of plasma leucine was during feeding and overvalued during fasting.
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