P Swyer

The Effect of Energy Intake and Expenditure on the Recovery of 13CO2 in the Parenterally Fed Neonate during a 4-Hour Primed Constant Infusion of NAH13CO3

ABSTRACT: The use of 13CO2 excretion to measure the oxidation of 13CO2 labeled substrates is increasing as it is both noninvasive and lacks the radiation exposure associated with the use of 14C. No standards are available for 13CO2 recovery in breath from the bicarbonate pool in the neonate. A primed constant infusion of NaH13CO3 over 4 h was used with open circuit indirect calorimetry in 15 appropriate for gestational age newborn infants (gestational age 28-39 wk; postnatal age 2-52 days), on varying amounts of intravenous feeding (37-114 kcal · kg-1 · day-1). Following a bolus of 6.9 μmol · kg-1 of NaH13CO3, a maintenance infusion of 4.6 μmol · kg-1 · h-1 was started. The I3C enrichment in breath rose rapidly to reach a plateau by 90 min with <5% variation of the plateau. Recovery of the tracer in breath ranged from 69.6-83.5% and was significantly correlated with 1) energy intake (37-114 kcal · kg-1 ·day-1); 2) metabolic rate (34.6-56.1 kcal · kg-1 · day-1); 3) VCO2 (4.86-7.43 ml · kg-1 · min-1). There was no correlation with the level of protein or fat intake. We provide an equation that can be used to calculate the correction factor when doing constant infusion substrate oxidation studies with a 13C label in neonates.

THE EFFECT OF DIET COMPOSITION ON ENERGY METABOLISM, SUBSTRATE UTILISATION AND GROWTH IN THE VLBW INFANT

Medium-chain-triglycerides (MCT) induce a decrease in fat storage in adult animals. To study the effect of MCT on quality of growth in the VLBW infant, oxidation (ox) and accretion (accr) of energy, protein, fat and carbohydrate have been measured in 20 growing VLBW infants, fed either an MCT enriched formula (gp 1; n=8) or an LCT formula (gp 2; n=12). 16 studies were performed in each group, combining indirect calorimetry, macronutrient balance and anthropometry.Weight and length gain simulated intra-uterine growth rate in both groups. Subscapular skinfolds and calculated body fat were similar in both groups, but higher than in the fetus of comparable postconceptional age.CONCLUSIONS: 1) The similar energy expenditure and fat oxidation indicate that MCT are not preferentially oxidised. 2) Consequently, MCT have to be chain elongated before storage. 3) The improvement of protein accretion to a level exceeding the intra-uterine accretion rate and improved absorption for MCT vs LCT indicate that body composition can be manipulated by isocalorically altering diet composition.

SUBSTRATE UTILISATION OF NEWBORN INFANTS FED INTRAVENOUSLY WITH OR WITHOUT A FAT EMULSION

Total parenteral nutrition (TPN) is important in the management of ill newborns. The difference in metabolic rate (MR) and substrate use (SU) between infants receiving TPN with and without fat emulsion have not been fully defined. We compared MR and SU by indirect calorimetry (IC) in infants receiving a glucose/amino acid mixture only (2% Vamin/DIOW), group I, with infants (group II) receiving a fat emulsion (Nutralipid 10%) as well. Birthweight, gest. age, postnatal age and weight were similar. IC was performed for 4±1 h. MR and SU are calculated from the protein-free RQ and VO2.CONCLUSIONS: 1) MR is significantly higher in infants receiving glucose-amino acids alone. 2) VCO2 is higher in group I. 3) This might be caused by a higher conversion of glucose to fat in group I which is energy consuming. 4) We found no effect on protein balance. 5) TPN with lipids has advantages over TPN without lipids.

747 FAT OXIDATION IN PARENTERALLY FED NEWBORN INFANTS

Fat oxidation (FO) has previously been measured by indirect calorimetry (IDC). Due to conversion of glucose into lipid, true fat oxidation may be underestimated. In the present study, we combined IDC with direct measurement of glucose oxidation using U-13C-glucose. IDC was performed for 5 hrs. Simultaneously a primed constant infusion of U-13C-glucose was given. Glucose utilisation was calculated separately from IDC data (ie. protein free RQ + VO2), and from 13CO2 enrichment in breath at plateau. Fat oxidation was then calculated by subtracting the glucose utilisation from the non-protein metabolic rate. Ten AGA parenterally fed infants were studied (x±S.E.) BW 2.7±0.33 kg, gest. age 36.6±1.4 wk, study weight 2.7±0.31 kg, age 14.2±2.8 d., energy intake 86.2±2.35 Kcal/kg.d, protein intake 2.8±0.09 g/kg.d.CONCLUSIONS: Due to lipogenesis, IDC overestimates glucose utilisation and hence underestimates fat oxidation. By measuring glucose utilisation directly, we have been able to demonstrate a significantly higher fat oxidation, approximately 90% of intake.

'TRUE' RATES OF GLUCOSE OXIDATION IN NEONATES

We have previously studied substrate utilization in neonates using indirect calorimetry (IC). As IC might overestimate glucose oxidation rate (GOR) due to conversion of glucose to fat, we compared GORs measured by IC and U-13C-glucose utilization. 7 AGA neonates fed only glucose-amino acids IV were studied. Glucose intake was 15.1±3.0g/kg/d (mean SD), protein intake 2.6±1.3g/kg/d BW 2.4±0.4kg, gestational age 37±2 wks, study weight 2.4±0.4kg, age 4-30d. IC was performed for 6h metabolic rate (MR) and GORs were calculated from protein free VO2 and RQ. Simultaneously a primed constant infusion of U-13C-glucose was given. GOR was calculated from the increase in 13CO2 excretion above baseline. A plateau was obtained after ≅2h.CONCLUSIONS: 1) IC shows a significantly higher glucose oxidation rate than 13C-glucose; 2) glucose oxidation rate of neonates is lower and endogenous fat oxidation higher than previously measured; 3) a significant amount of ingested glucose presumably is converted into fat.

GLUCOSE OXIDATION IN NEONATES, INDIRECT CALORIMETRY OR STABLE ISOTOPES|[quest]|

Intravenous glucose is used frequently in newborn infants to cover their energy needs. Whether this infused glucose is directly oxidised is questionable. Indirect calorimetry (IC) may overestimate the glucose oxidation rate (GOR) due to the conversion of glucose into fat. We therefore compared GOR measured by IC and 13CO2 production from U-13C-glucose. IC was performed for 6 hr, metabolic rate (MR), and GOR was calculated from the protein-free RQ and VO2. Simultaneously a primed constant infusion of U-13C-glucose was given, GOR calculated from the increase in 13CO2 excretion above baseline.A plateau was obtained after ~2 hr. 10 AGA infants were studied. BW 2.4±0.4 kg, gestational age 37±2 wks, age 9±8 days, weight 2.3±0.4 kg. Energy intake 70±14 kcal/kg/d, glucose intake 15±2.5 g/kg/d, protein intake 2.7±1.1 g/kg/d. Results: n=10, Mean±SE.CONCLUSIONS: 1) IC shows a significantly higher glucose oxidation than U-13C methodology. 2) This difference represents glucose converted into fat with concomitant fat oxidation. 3) GOR measured by IC increased with glucose intake, but GOR calculated from U-13C-gluc showed no correlation with glucose intake. 4) GOR calculated from U-13C-gluc is 4.8 mg/kg/min, which approximates the endogenous glucose production found in previous studies.

324 CAN INTRAUTERINE WEIGHT GAIN AND BODY COMPOSITION BE SIMULATED IN THE FORMULA FED VERY LOW BIRTHWEIGHT (VLBW) INFANT

Until now no formula has been able to simulate both intra-uter-ine weight gain and body composition. In order to evaluate a long-chain-triglyceride formula at an energy intake recommended by the Am. Acad. Fed. Nutr. Committee, we compared 12 very-low-birth-weight (VLBW) infants with the fetus of the same postconceptional age. Twenty-three studies, combining open-circuit indirect calori-metry, macronutrient balance and anthropometry were performed in 12 growing VLBW infants (x±S.E.: B.W. 986±36 g; Gest age 27.8±0.4wks; study wt. 1307±51 g; age 38±3 d).Weight gain 15.7±0.85 g/kg.d and length gain 1.04±0.09 cm/wk were equal to intra-uterine growth rates (weight 13.9 g/kg.d; length 1.24 cm/wk). The accretion rate of protein was comparable to that of the fetus, fat accretion was 1.4 times higher.CONCLUSIONS: (1) At a global energy intake of 120 Kcal/kg.d, the intra-uterine weight and length gain could be simulated but body composition not. (2) The higher fat deposition occurred at the expense of water space indicating that caloric intake and/or composition of feeds may have to be modified to attain the intra-uterine accretion for macronutrients.

SUBSTRATE UTILISATION OF NEWBORN INFANTS RECEIVING TOTAL PARENTERAL NUTRITION (TPN)

Substrate utilisation, previously measured indirectly with Indirect Calorimetry (IDC), can now be measured directly using stable isotopes. IDC may underestimate fat utilisation due to the conversion of glucose into fat. We combined IDC with direct measurement of glucose oxidatior using U-13C-glucose. Glucose and fat utilisation was calculated from IDC (non-protein VO2 and RQ) over 5 hr. Simultaneously, a primed constant infusion of U-13C-glucose was given. Glucose oxidation was calculated from the 13CO2 excretion in breath at plateau, fat oxidation by subtracting the glucose oxidation from the non-protein metabolic rate. Sixteen AGA TPN fed infants were studied (x±SE). BW 2.7±0.2 kg, gest. age 36.6±1 wk, study weight 2.7±0.2 kg, age 13.7±0.3 d. Energy intake 87.0±1.6 kcal/kg/d, protein int. 3.2±0.2 g/kg/d, glucose int. 13.8±0.3 g/kq/d, fat int. 2.0±0.1 g/kq/d.Conclusions: 1. IDC shows a significantly higher glucose and lower fat utilisation compared with 13C-data. 2. The 13C-data show that 92% of the fat intake is being utilised. 3. Fat oxidation is an important contribution to the energy expenditure during TPN with lipids.

THE EFFECT OF ADDING LIPIDS TO THE INTRAVENOUS FEEDING OF NEWBORN INFANTS

Previously we showed a decrease in metabolic rate in infants receiving Total Parenteral Nutrition (TPN) containing glucose + lipids vs glucose only. In the present study we measured substrate utilisation by Indirect Calorimetry (IDC) over 5 hr and from the excretion of 13CO2 during a primed constant infusion of U-13C-glucose. Glucose oxidation (GO) was measured from the 13CO2 enrichtment in breath at plateau and fat oxidation (FO) as the difference of non-protein metabolic rate and glucose oxidation. We compared metabolic rate (MR) and substrate utilisation in two groups of AGA infants on an isocaloric intake. Group I received glucose/aminoacids only, Group II received a lipid emulsion as well, fat intake 2.0±0.1 g/kg/d. Birthweight (2.5±0.2 vs 2.7±0.2 kg), gest. age (35.9±1.0 vs 36.9±1.0 wk), study weight (2.5±0.2 vs 2.7±0.2 kg) and postnatal age (18.7±2.4 vs 13.2±2.0 d) was not different between groups.Conclusions: 1. The replacement of glucose by fat results in a lower metabolic rate, presumably as a result of a decreased conversion of glucose into fat. 2. Fat intake is almost equal to fat oxidation at an intake of 2 g/kg/d. 3. AGA newborn infants oxidize fat efficiently. 4. TPN with lipids has advantages above glucose/amino acids only.

725 IMPROVEMENT OF PROTEIN UTILISATION BY HIGHER ENERGY INTAKE IS NOT ACCOMPANIED BY A CHANGE IN NON-PROTEIN SUBSTRATE OXIDATION

Nitrogen utilisation is enhanced in parenterally fed neonates by increasing energy intake. In the present study we examine the change in fuel utilisation caused by the addition of lipid to a glucose/amino acid regimen. Metabolic rate (MR) was measured by 5 hr indirect calorimetry (IDC). Glucose oxidation (GO) was determined by measuring 13CO2 enrichment at plateau during a 5 hr primed constant infusion of U-13C-glucose. Fat oxidation (FO) was calculated as non-protein MR minus GO. Protein utilisation was expressed as the percentage of infused nitrogen that was retained. Sixteen neonates of comparable birthweight, gestational and postnatal age were divided into two equal groups; I-glucose only. II-glucose +lipid. Protein (2.8 g/Kg.d) and glucose (14 g/Kg.d) intakes were equal; but fat (2.1 g/Kg.d in group II) + energy (64 kcal gpl, cf gpII 86 kcal/kg.d) were different. No difference in MR (47.5 cf 45.5 kcal/Kg.d) were detected.CONCLUSIONS: The addition of energy as lipid enhances protein utilisation without influencing the contribution of fat and carbohydrate to the resting energy expenditure. Hence, resulting in net fat accretion, as well as improved protein accretion.