To determine whether increased amino acid availability can reduce proteolysis in premature neonates and to assess the capacity of infants born prematurely to acutely increase the irreversible catabolism of the essential amino acids leucine (via oxidation) and phenylalanine (via hydroxylation to form tyrosine), leucine and phenylalanine kinetics were measured under basal conditions and in response to a graded infusion of intravenous amino acids (1.2 and 2.4 g. kg(-1). day(-1)) in clinically stable premature (approximately 32 wk gestation) infants in the 1st wk of life. In contrast to the dose-dependent suppression of proteolysis seen in healthy full-term neonates, the endogenous rates of appearance of leucine and phenylalanine (reflecting proteolysis) were unchanged in response to amino acids (297 +/- 21, 283 +/- 19, and 284 +/- 31 micromol. kg(-1). h(-1) for leucine and 92 +/- 6, 92 +/- 4, and 84 +/- 7 micromol. kg(-1). h(-1) for phenylalanine). Similar to full-term neonates, leucine oxidation (40 +/- 5, 65 +/- 6, and 99 +/- 7 micromol. kg(-1). h(-1)) and phenylalanine hydroxylation (12 +/- 1, 16 +/- 1, and 20 +/- 2 micromol. kg(-1). h(-1)) increased in a stepwise fashion in response to graded amino acids. This capacity to increase phenylalanine hydroxylation may be crucial to meet tyrosine needs when exogenous supply is limited. Finally, to determine whether amino acids stimulate glucose production in premature neonates, glucose rate of appearance was measured during each study period. In response to amino acid infusion, rates of endogenous glucose production were unchanged (and near zero).

OBJECTIVE:Infants with cyanotic congenital heart disease (CCHD) have previously been shown to have similar resting energy expenditures (REEs) and elevated total energy expenditures (TEEs) compared with age-matched healthy infants. The purpose of this investigation was to re-examine the REE and TEE of the same individuals at 5 years of age, after surgical repair of the heart defect was done, to determine whether metabolic differences persist.STUDY DESIGN:Seven children were studied approximately 2.6 years after they underwent surgical repair of CCHD along with 10 age-matched healthy children. Indirect calorimetry was used to determine REE, and the doubly labeled water method was used to determine TEE and body composition.RESULTS:Results were compared with single-factor repeated measures analysis of variance. No significant differences were found between groups in weight or body composition. No significant differences were found between groups in REE, TEE, or the energy expended in physical activity.CONCLUSION:We conclude that differences in TEE observed during infancy are no longer present in 5-year-old children after they undergo surgical repair of CCHD. Furthermore, the individual components of energy expenditure of children with CCHD after repair are indistinguishable from those of healthy age-matched children.

Objective: To determine the effect of a continuous insulin infusion on protein and glucose metabolism in extremely low birth weight (ELBW) infants. Study design: We measured the rate of appearance (Ra) of the essential amino acids leucine and phenylalanine (reflecting proteolysis), utilization of phenylalanine for protein synthesis, and glucose Ra using stable isotope tracers during a basal infusion of glucose (6 mg/kg/min) and in response to a continuous infusion of insulin (0.05 U/kg/hr) by means of the euglycemic hyperinsulinemic clamp technique. Four clinically stable, euglycemic ELBW infants (26 ± 0 weeks' gestation, 894 ± 44 gm birth weight, 2.8 ± 0.8 days of age) were studied. Results: In response to a greater than tenfold increase in insulin concentration (from 7 ± 2 to 79 ± 13 μU/ml), there was a 20% decrease in leucine Ra (Basal: 272 ± 27 μmol/kg/hr; Insulin: 226 ± 29 μmol/kg/hr; p < 0.01) and in phenylalanine Ra (Basal: 91 ± 5 μmol/kg/hr; Insulin: 72 ± 2 μmol/kg/hr; p < 0.05). Use of phenylalanine for protein synthesis also decreased by a similar magnitude (Basal: 77 ± 4 μmol/kg/hr; Insulin: 62 ± 1 μmol/kg/hr; p < 0.05). Glucose utilization doubled (from 8 ± 0.9 to 15.7 ± 1.1 mg/kg/min; p = 0.0003) and plasma lactate concentrations tripled (from 2.1 ± 0.5 to 5.7 ± 1.0 mmol/L; p < 0.05) during the insulin infusion. Conclusions: During an infusion of glucose alone, pharmacologic concentrations of insulin in ELBW infants produced no net protein anabolic effect. Furthermore, euglycemic hyperinsulinemia was accompanied by development of significant metabolic acidosis. (J Pediatr 1998;132:948-53)

We have recently demonstrated dose dependent suppression of proteolysis with graded amino acid (aa) infusion in healthy term neonates (Am J Physiol, 1997). Rates of proteolysis are higher in preterm individuals, perhaps related to a greater need for aa supply for tissue remodeling and rapid growth. We hypothesized that preterm neonates would be resistant to suppression of proteolysis in response to aa infusion. To assess this, we measured the endogenous rates of appearance (Ra) of the essential aa's leucine (LEU) and phenylalanine (PHE) (reflecting proteolysis), LEU oxidation (LEU OX) and phenylalanine hydroxylation (PHE OH) (reflecting irreversible losses), and utilization of LEU and PHE for protein synthesis (PS) in 7 clinically stable preterm infants (32±0.5 wks gestation, 1.5±0.1 kg birth wt, 1.4±0.1 kg study wt, 6±1 days of age) during a basal glucose infusion (6mg/kg/min) and in response to a graded infusion of aa's (Aminosyn PF-1.2 & 2.5gm/kg/day).

To determine how increased amino acid availability alters rates of whole body proteolysis and the irreversible catabolism of the essential amino acids leucine and phenylalanine throughout the neonatal period, leucine and phenylalanine kinetics were measured under basal conditions and in response to intravenous amino acids in two separate groups of healthy, full-term newborns (at 3 days and 3 wk of age). The endogenous rates of appearance of leucine and phenylalanine (reflecting proteolysis) were suppressed equally in both groups and in a dose-dependent fashion (by approximately 10% with 1.2 g x kg(-1) x day(-1) and by approximately 20% with 2.4 g x kg(-1) x day(-1)) in response to intravenous amino acid delivery. Insulin concentrations remained unchanged from basal values during amino acid administration. The irreversible catabolism of leucine and phenylalanine increased in a stepwise fashion in response to intravenous amino acids; again, no differences were observed between the two groups. This study clearly demonstrates that the capacity to acutely increase rates of leucine oxidation and phenylalanine hydroxylation is fully present early in the neonatal period in normal newborns. Furthermore, these data suggest that amino acid availability is a primary regulator of proteolysis in normal newborns throughout the neonatal period.

OBJECTIVE Although gestational diabetes affects as many as 3% of all pregnant women, specific aspects of glucose and protein metabolism in this population have not been clearly delineated. We tested the hypothesis that gestational diabetes mellitus (GDM) results in increased glucose production and proteolysis during fasting. RESEARCH DESIGN AND METHODS Using tracer isotope infusions, the rate of appearance (Ra) of glucose, leucine, phenylalanine and tyrosine, phenylalanine hydroxylation, leucine oxidation, and urea nitrogen excretion were determined after an overnight fast in 10 GDM subjects, within 2 weeks of diagnosis and before initiation of treatment, and in a matched control group of nine healthy nondiabetic pregnant women. RESULTS Fasting glucose Ra was similar in GDM patients and control subjects (GDM, 12.8 ± 1.1 vs. control subjects, 12.8 ± 0.9 μumol · kg−1 . min−1). Leucine and phenylalanine Ra (reflecting proteolysis) also were not different between GDM patients and control subjects (GDM leucine Ra, 128 ± 14 vs. control subjects, 124 ± 5; phenylalanine Ra GDM, 35 ± 4 vs. control subjects, 40 ± 2 μumol · kg−1 · h−1). Furthermore, leucine oxidation and phenylalanine hydroxylation were not increased in GDM subjects, urea nitrogen excretion was actually lower in GDM patients. However, fasting insulin concentrations were significantly elevated in GDM subjects (GDM, 165 ± 35 vs. control subjects, 30 ± 5 pmol/l; P < 0.01). CONCLUSIONS Hepatic glucose release and whole-body proteolysis in GDM patients were remarkably similar to matched pregnant control subjects. This was achieved with insulin concentrations three- to fivefold higher than normal, suggesting significant insulin resistance for both glucose and protein metabolism in GDM.

EXOGENOUS INSULIN SUPPRESSES PROTEOLYSIS AND ENDOGENOUS GLUCOSE PRODUCTION IN EXTREMELY LOW BIRTH WEIGHT (ELBW) INFANTS. ▴ 1886

To determine to what extent intravenous nutrition can reduce proteolysis in very immature and normal newborns, and to assess the capacity of preterm and normal newborns to convert phenylalanine to tyrosine, phenylalanine and leucine kinetics were measured under basal conditions and during parenteral nutrition in clinically stable, extremely premature (approximately 26 wk of gestation) infants and in normal term newborns. In response to parenteral nutrition, there was significantly less suppression (P < 0.001) of endogenous leucine and phenylalanine rate of appearance in extremely premature infants compared with term infants. Phenylalanine utilization for protein synthesis during parenteral nutrition increased significantly (P < 0.01) and by the same magnitude (approximately 15%) in both extremely premature and term infants. Phenylalanine was converted to tyrosine at substantial rates in both extremely premature and term infants; however, this conversion rate was significantly higher (P < 0.05) in extremely premature infants during both the basal and parenteral nutrition periods. These data provide clear evidence that there is no immaturity in the phenylalanine hydroxylation pathway. Furthermore, although parenteral nutrition appears to produce similar increases in protein synthesis in extremely premature and term infants, proteolysis is suppressed much less in extremely premature newborns. The factors responsible for this apparent resistance to suppression of proteolysis in the very immature newborn remain to be elucidated.

To determine to what extent glucose homeostasis, proteolysis, and essential amino acid loss is altered in infants undergoing ECMO for persistent pulmonary hypertension, stable isotope tracer infusions were used to measure endogenous glucose production, phenylalanine (PHE) rates of appearance (Ra), and PHE hydroxylation (PHE-OH) in 7 neonates on ECMO (birth weight 3.5±0.9kg, 40±2 wks gestation, 3±1 days old) and in 10 normal newborn controls (birth weight 3.3±0.5kg, 39±1 wks gestation, 2.4±1.4 days old). ECMO infants and controls were both studied during a 7 mg/kg/min glucose infusion. In addition, to determine the effect of a combined glucose (7 mg/kg/min) and amino acid (AA) infusion (1.5 g/kg/day, 16.2μmol/kg/hr PHE) on proteolysis and PHE balance in ECMO patients, the same measurements were made in 8 ECMO patients receiving the combined infusion.

In order to investigate the effect of dexamethasone (DEX) therapy on energy expenditure and growth of preterm infants with bronchopulmonary dysplasia, we studied 12 infants (26.2±1.4 wks gestation, 916±190 gm birthweight, mean±sd) enrolled in a multicenter blinded trial of the optimum timing of DEX therapy. The 12 infants were randomly assigned to one of two groups. Group 1 infants (n=6, 25.8±1.5 wks gestation, 932±220 gm birthweight) received a 2 week course of DEX followed by two weeks of placebo. Group 2 infants (n=6, 26.6±1.2 wks gestation, 901±175 gm birthweight) had the treatment order reversed. We measured total energy expenditure (TEE) and total body water (TBW) using the doubly labeled water technique (DLW, 2H and 18O labeled water) for 7 days during each of the treatment phases. Daily energy intake (EI) and weight were recorded throughout the study. All infants required intermittent mechanical ventilation during some portion of the study and had Respiratory Index Scores (MAP × FiO2) of 3.4±2.1 at the start of the first DLW period. Results are summarized below (mean ± sd, *p<0.05, DEX vs. placebo). Table

To determine whether nonprotein substrate can suppress proteolysis in normal newborns and to assess the effect of this substrate on glucose production, the rates of appearance (Ra) of leucine (reflecting proteolysis) and glucose were measured in healthy 2-day-old full-term newborns during fasting, an intravenous glucose infusion (5.5 mg.kg-1.min-1), an intravenous lipid infusion (2.5 mg.kg-1.min-1), and a combined glucose plus lipid infusion (5.5 mg.kg-1.min-1 glucose + 2.5 mg.kg-1.min-1 lipid). Leucine RA was not reduced from fasting values during any of the substrate infusions. Intravenous lipid infusion alone neither suppressed nor increased glucose production. In contrast, glucose production was nearly completely suppressed (approximately 90%) during intravenous infusions of glucose provided either alone or in combination with lipid; this suppression was achieved at glucose concentrations of approximately 90 mg/dl and insulin concentrations of approximately 6 microU/ml. Thus normal newborns respond to intravenous glucose with sustained nearly complete suppression of glucose production, even at moderate levels of glycemia and at low insulin concentrations; however, nonprotein substrate infusion does not result in suppression of proteolysis. It remains unclear to what extent any potential regulator can suppress proteolysis in this population.

ABSTRACT: 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 1–13C-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 μmol/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.

To ascertain whether the inability to suppress glucose production and increase glucose utilization in response to glucose infusion is an inherent characteristic of immature individuals, we determined glucose rate of appearance (R(a)) in minimally stressed, clinically stable, extremely premature infants (approximately 26-wk gestation) at two glucose infusion rates (6.2 +/- 0.4 and 9.5 +/- 0.5 mg/kg per min). We also assessed whether an increase in glucose delivery suppresses proteolysis by measuring the R(a) of phenylalanine and leucine. Glucose R(a) (and utilization) increased significantly at the higher glucose infusion rate (7.9 +/- 0.5 vs. 9.8 +/- 0.6 mg/kg per min). Glucose production persisted at the lower glucose infusion rate but was suppressed to nearly zero at the higher rate (1.7 +/- 0.5 vs. 0.3 +/- 0.1 mg/kg per min). Proteolysis was unaffected by the higher glucose infusion rate as reflected by no change in the rates of appearance of either phenylalanine (96 +/- 5 vs. 95 +/- 3 mumol/kg per h) or leucine (285 +/- 20 vs. 283 +/- 14 mumol/kg per h). Thus, clinically stable, extremely premature infants suppress glucose production and increase glucose utilization in response to increased glucose infusion, demonstrating no inherent immaturity of these processes. In contrast, increasing the rate of glucose delivery results in no change in whole body proteolysis in these infants. The regulation of proteolysis in this population remains to be defined.

To examine how feeding affects changes in leucine and protein metabolism, leucine kinetics were determined in nine preterm infants (32 +/- 2 wk gestation; mean +/- SD) after a brief fast and again during hourly feedings. Rates of leucine oxidation were similar during the fasting and feeding periods (31 +/- 4 vs 37 +/- 6 mumol.kg-1.h-1; mean +/- SE). The nonoxidative disposal rates of leucine (a reflection of protein synthesis) were also similar during both periods (228 +/- 20 vs 205 +/- 10 mumol.kg-1.h-1; mean +/- SE). In contrast, the rates of leucine release from endogenous protein (an indication of protein breakdown) were significantly reduced by feeding (259 +/- 23 vs 185 +/- 11 mumol.kg-1.h-1; mean +/- SE, P = 0.02). A significant positive correlation was demonstrated between the fasting rate of leucine release from endogenous protein and the degree of suppression produced by feeding (r2 = 0.796, P = 0.001). Conversely, a significant inverse correlation was shown between the nonoxidative disposal rate of leucine during fasting and the increase in response to feeding (r2 = 0.848, P < 0.001). These data suggest that premature infants respond to feeding after a brief fast by suppressing protein breakdown, rather than by increasing protein synthesis, and changes in protein metabolism produced by feeding in premature newborns may be influenced by the prevailing rates of protein breakdown and synthesis during fasting.

Infants with cyanotic congenital heart disease (CCHD) often have reduced weight gain compared with infants in control groups. Our purpose was to conduct a longitudinal study of energy intake, resting energy expenditure (REE), and total energy expenditure (TEE) of a group of infants with CCHD. We hypothesized that increased REE and TEE and decreased energy intake in these infants would lead to reduced growth. Ten infants with uncorrected CCHD and 12 infants in a control group were studied at 2 weeks of age and again at 3 months. Indirect calorimetry was used to determine REE; the doubly labeled water method was used to determine TEE and intake. At 2 weeks and 3 months of age, infants with CCHD weighed significantly less than infants in the control group. No significant difference was seen in energy intake or REE between groups during either period. TEE was slightly but not statistically increased in the CCHD group at 2 weeks (72.6 +/- 17.4 vs 59.8 +/- 10.9 kcal/kg/d) and significantly increased at 3 months (93.6 +/- 23.3 vs 72.2 +/- 13.2 kcal/kg/d, P =.03). We conclude that increased TEE but not increased REE is a primary factor in the reduced growth in infants with CCHD.