Changes in protein turnover after the introduction of parenteral nutrition in premature infants: comparison of breast milk and egg protein-based amino acid solutions.

Rates of protein turnover were measured in 20 infants receiving either Vamin Infant (group A) or Vamin 9 glucose (group B) as the amino acid source in total parenteral nutrition. A constant infusion of L-[1-13C]leucine was used to measure whole body leucine flux, and leucine oxidation rates were derived from measurements of total urinary nitrogen excretion. Infants were first studied when receiving only i.v. glucose and again on each of the next 4 d as total parenteral nutrition was gradually increased to a maximum of 430 mg nitrogen/kg/d and 90 nonprotein kcal/kg/d. Net protein gain and protein synthesis and breakdown rates increased progressively for all infants taken together over the study period as i.v. nutrition was increasing (p less than 0.001). There were no differences between groups in the changes in net protein gain and rates of protein synthesis and breakdown throughout the study period. Nitrogen retention on d 5 for the two groups was similar (60 +/- 16% and 67 +/- 11% in groups A and B, respectively). In a subgroup of infants, measurements were repeated on d 8, when the intake had been constant for 3 d. Protein retention was the same as on d 5, but both synthesis and breakdown were increased. It is concluded that rates of protein turnover increase significantly in response to increasing i.v. nutrition and that this elevation was not influenced by the composition of the amino acid mixture given.     

Whole body protein synthesis and energy expenditure in very low birth weight infants

The aim of the present work was to study whole body protein synthesis and breakdown, as well as energy metabolism, in very low birth weight premature infants (less than 1500 g) during their rapid growth phase. Ten very low birth weight infants were studied during their first and second months of life. They received a mean energy intake of 114 kcal/kg X day and 3 g protein/kg X day as breast milk or milk formula. The average weight gain was 15 g/kg X day. The apparent energy digestibility was 88%, i.e. 99 kcal/kg X day. Their resting postprandial energy expenditure was 58 kcal/kg X day, indicating that 41 kcal/kg X day was retained. The apparent protein digestibility was 89%, i.e. 2.65 g/kg X day. Their rate of protein oxidation was 0.88 g/kg X day so that protein retention was 1.76 g/kg X day. There was a linear relationship between N retention and N intake (r = 0.78, p less than 0.001). The slope of the regression line indicates a net efficiency of N utilization of 67%. Estimates of body composition from the energy balance, coupled with N balance method, showed that 25% of the gain was fat and 75% was lean tissue. Whole body protein synthesis and breakdown were determined using repeated oral administration of 15N glycine for 60-72 h, and 15N enrichment in urinary urea was measured. Protein synthesis averaged 11.2 g/kg X day and protein breakdown 9.4 g/kg X day. Muscular protein breakdown, as estimated by 3-methylhistidine excretion, contributed to 12% of the total protein breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of nutritional rehabilitation on whole body protein metabolism of children with cystic fibrosis

The effect of an increased energy and protein intake was studied on the whole body protein metabolism of 4- to 9-yr-old children with cystic fibrosis. The study was divided into an initial 1-yr period, followed by a treatment period of the same length. Rates of protein turnover were determined using a [15N]glycine label and the Picou and Taylor-Roberts model. The children had significantly higher intakes during the treatment year and grew faster. Nitrogen retention and utilization were significantly increased during the treatment year (p less than 0.01). There were no changes seen in whole body amino nitrogen flux, or protein synthesis rates. However, protein breakdown rates were significantly reduced during the treatment period, with the result that net anabolism (synthesis-breakdown) was increased (p less than 0.05). The protein synthesis rates of about 6 g/kg/day were similar to those reported for healthy children of the same age. It was concluded that the changes in protein turnover seen were in response to the nutritional intervention and not to the children's basic genetic disease process.     

Effect of energy repletion on dynamic aspects of protein metabolism of malnourished adolescent and young adult patients with cystic fibrosis during the first 12 days of treatment

The short-term effect of nutritional rehabilitation on whole body protein metabolism of six malnourished (energy depleted) adolescent and young adult cystic fibrosis (CF) patients was examined during the first 12 days of treatment. Refeeding was accomplished using continuous nasogastric administration of an elemental diet. Intakes were 1.65 SD 0.3 g protein/kg/day and 85.3 SD 19.2 kcal/kg/day. Turnover measurements were carried out on four occasions, during the control period on a normal diet and days 4, 8, and 12 on the elemental diet. In response to refeeding, whole body amino nitrogen flux (Q) fell consistently over the study (p less than 0.05). Protein synthesis (S) and breakdown (B) also fell but their values did not reach statistical significance. On the other hand, there was a prompt increase in ratios S/Q and B/Q in response to tube feeding discernible on days 4 and 8 (p less than 0.05), but which returned to baseline by day 12. We conclude that malnourished CF patients who are still eating well have increased rates of protein turnover. There is a prompt response of protein metabolism to energy repletion with a progressive decline in rates during the first 12 days of treatment.     

Total parenteral nutrition in sick preterm infants: effects of cysteine supplementation with nitrogen intakes of 240 and 400 mg/kg/day

The effects of supplementing total parenteral nutrition (TPN) solutions with cysteine were assessed at two different levels of nitrogen intake by determining nitrogen retention, sulfate excretion, and sulfur-containing amino acid concentrations. Ten infants received 72 mg/kg/day of cysteine-HCl in a TPN solution for a period of 6 days. Five of these infants received 251 +/- 48 (mean +/- SD) mg/kg/day of nitrogen, and five received 403 +/- 45 mg/kg/day of nitrogen. Two other groups of five infants each received unsupplemented TPN at nitrogen intakes of 235 +/- 48 and 412 +/- 54 mg/kg/day, respectively. Fluid and nonprotein caloric intakes were similar for all four groups. Cysteine supplementation increased plasma and urine free cyst(e)ine concentrations and enhanced total sulfur retention, but did not enhance nitrogen retention. [Cyst(e)ine refers to the mixture in any proportion of the sulfhydryl (cysteine) and the disulfide (cystine) forms of this compound.] Nitrogen retention, sulfate excretion, cyst(e)ine excretion, and plasma taurine concentrations increased as the result of the increase in nitrogen intake.     

Effect of two amino acid solutions on leucine turnover in preterm infants

OBJECTIVE: To assess the effect of two different parenteral amino acid mixtures, Trophamine and Primene, on leucine turnover in preterm infants. METHOD: Leucine kinetics were measured with [5,5,5 D3]leucine tracer in 15 infants receiving Trophamine (group 'T') (mean birth weight 1,263 g) and 22 who received Primene (group 'P') (mean birth weight 1,336 g) during two study periods, within a few hours after birth but before introduction of parenteral amino acid solution, and again at postnatal day 7. The rate of appearance of leucine was calculated from the enrichment of alpha-ketoisocaproic acid in plasma. RESULTS: There were no significant differences in leucine turnover within a few hours after birth in the two groups. In the infants who received Primene leucine turnover on day 7 was significantly lower than in those who received Trophamine (269 +/- 43 vs. 335 +/- 27, p < 0.05). Despite a higher intake of leucine in the Trophamine group (108 +/- 10 vs. 77 +/- 8 micromol.kg(-1).h(-1)), leucine released from proteins at day 7 was higher in this group compared to Primene (227 +/- 27 vs. 192 +/- 42 micromol.kg(-1).h(-1)). CONCLUSIONS: Primene administration results in lower leucine released from proteins, an estimate of protein breakdown, than Trophamine in preterm infants. Increases in whole body leucine turnover in response to administration of i.v. amino acids is influenced by the composition of the amino acid mixture. The factors responsible for this difference remain to be elucidated.     

Effects of varying protein and energy intakes on growth and metabolic response in low birth weight infants

Growth (weight, length, head circumference, and skinfold thickness), retention of major nutrients (nitrogen, sodium, potassium, chloride, calcium, and phosphorus), and chemical indices of protein adequacy (plasma albumin and transthyretin concentrations) and excess (blood urea nitrogen concentration and acid-base status; plasma amino acid concentrations) were determined serially from the time desired intake was tolerated until discharge weight (2200 gm) was reached in low birth weight infants (birth weight 900 to 1750 gm) fed one of three formulas, which provided protein and energy intakes, respectively, of 2.24 gm/kg/day and 115 kcal/kg/day (group 1), 3.6 gm/kg/day and 115 kcal/kg/day (group 2), and 3.5 gm/kg/day and 149 kcal/kg/day (group 3). Weight gain and rate of increase in length and head circumference were less in group 1 than in groups 2 and 3. Retention of most major nutrients also was less in group 1, as was blood urea nitrogen concentration, plasma albumin and transthyretin concentrations, and plasma concentrations of several amino acids. The rate of weight gain was not significantly greater in group 3 than in group 2, but the rate of increase in skinfold thickness was greater in this group. Neither nutrient retention nor metabolic indices differed between groups 2 and 3. These results suggest that a protein intake of 2.24 gm/kg/day is inadequate for the type of LBW infants studied, that the higher protein intakes are well tolerated, and that an energy intake of 149 vs 115 kcal/kg/day does not enhance utilization of the higher protein intakes studied.     

Parenteral amino acids increase albumin and skeletal muscle protein fractional synthetic rates in premature newborn minipigs

OBJECTIVES: Early administration of parenteral amino acids increases whole body nitrogen retention in premature infants. Tracer kinetic studies suggest an increase in whole body protein synthesis as a possible mechanism for this increase in nitrogen retention. However, the effect of early parenteral amino acids on synthesis of specific proteins remains uncertain. Using premature newborn minipigs as a model for human premature newborns, we investigated the effects of parenterally administered amino acids on albumin and skeletal muscle protein fractional synthetic rates. METHODS: Fifteen Yucatan minipigs were delivered by cesarean section 6 days before the mean expected delivery date (day 106 of gestation; expected gestation, 111-113 days) and randomized to two groups immediately after birth: 7 piglets received a mixture of amino acids (0.4 g. kg. h ) and glucose (0.8 g. kg. h ) for 5 hours, and 8 piglets (control group) received glucose only. All piglets received a continuous primed infusion of 1-[ C]valine. Arterial plasma free C-valine enrichment was measured by gas chromatography/mass spectrometry, and protein synthetic rates were determined by measuring incorporation of C-valine into albumin and skeletal muscle protein using gas chromatography/combustion/isotope ratio mass spectrometry. RESULTS: Administration of amino acids increased albumin (87.0% +/- 42.1% [mean +/- SD] vs. 37.6% +/- 6.8% per 24 hours; < 0.05) and skeletal muscle fractional synthetic rates (11.60% +/- 6.9% vs. 6.5% +/- 1.5% per 24 hours; < 0.05). CONCLUSION: We conclude that parenteral amino acids increase albumin and skeletal muscle fractional synthetic rates in premature piglets on the first day of life.     

Glutamine supplement with parenteral nutrition decreases whole body proteolysis in low birth weight infants

OBJECTIVES: To examine the effect of supplemental glutamine (0.6 g.kg -1 .d -1 ) on whole body protein/nitrogen and glutamine kinetics in low birth weight (LBW) infants receiving parenteral nutrition in the immediate neonatal period. STUDY DESIGN: Premature infants < or =32 weeks gestation with a birth weight from 694 to 1590 g were randomly assigned to either a glutamine-supplemented group (n = 10) or to a control group (n = 10). Tracer isotope studies were performed when the infants were 6 to 7 days old and had been receiving an amino acid intake of approximately 3.0 g.kg -1 .d -1 for at least 3 days. Whole body glutamine and nitrogen kinetics were measured with [5-15N]glutamine, [2H5]phenylalanine, [1-13C, 15 N]leucine, [15N2]urea, and GC-mass spectrometry. RESULTS: Supplemental glutamine was associated with a lower rate of appearance of glutamine ( P = .003), phenylalanine ( P = .001), and leucine C ( P = .003). There was no significant difference in leucine N turnover, urea turnover and plasma cortisol, and C-reactive protein levels in the 2 groups. CONCLUSION: Parenteral glutamine supplement in LBW infants was associated with lower whole-body protein breakdown. Because the decrease in whole body proteolysis is associated with protein accretion, parenteral glutamine supplement may be beneficial in selected populations of LBW infants.     

Protein supplementation of human milk for the nutrition of VLBW-infants: human milk protein vs. meat protein hydrolysate

In 25 very low birth weight infants appropriate for gestational age the influences of different human milk (HM) preparations on weight gain, gross indices of nitrogen metabolism and energy balance were studied during the second month of postnatal life. HM was fortified either by HM-protein (HMP) or by an enzymatic meat protein hydrolysate (PH) to protein concentrations between 1.5 and 1.7 g/100 ml. The caloric densities of both HM preparations were similar between 62 and 68 kcal/100 ml. There were no differences in weight gain (MM + HMP: 18.6 +/- 3.4 g/kg/day; HM + PH: 16.5 +/- 4.1 g/kg/day), nitrogen retention (HM + HMP: 31.5 +/- 3.1 mmol/kg/day; HM + PH: 30.0 +/- 3.2 mmol/kg/day), and the preprandial estimated essential amino acid profiles between the both feeding groups. In contrast the serum concentrations of alpha-amino-nitrogen 60 minutes postprandially were elevated in the infants fed HM + PH in comparison to the infants fed HM + HMP. This high postprandial amino acid concentrations in serum in the group fed HM + PH were accompanied by increased bile acids concentrations in serum, higher renal amino acid excretion and increased fecal fat losses. The results suggest that due to the more rapid intestinal absorption of amino acids from PH than from HMP the concentrations of amino acids increase postprandially which results in a detectable increase of the newborn cholestasis in these infants. Nevertheless, the scale of these metabolic responses to feeding protein hydrolysates is small and without detectable influences on nitrogen retention or weight gain.     

Glycogen storage disease: effects of glucose infusions on [15N]glycine kinetics and nitrogen metabolism

The effects of intragastric glucose infusions on [15N]glycine kinetics and whole body protein turnover were investigated in four children with type I and two children with type III glycogen storage disease. Either fasting or the administration of insufficient glucose was associated with a diminution in the glycine pool size relative to values observed when patients received adequate glucose. The cause of the smaller pool size was an increased fractional glycine turnover. Simultaneous determination of whole body protein turnover, using a stochastic model based on [15N]urea excretion, showed higher rates of protein synthesis, nitrogen flux, net tissue protein retention, and reutilization of amino acid nitrogen derived from protein catabolism, in patients receiving sufficient exogenous glucose. Depletion of amino acid pools, presumably because of intensive utilization of these gluconeogenic precursors when inadequate glucose is administered, was associated with a lower rate of whole body protein synthesis.     

Supplementation of pooled human milk with casein hydrolysate: energy and nitrogen balance and weight gain composition in very low birth weight infants

Growth and nitrogen and energy balances were studied with a combined technique of nutrient balance and indirect calorimetry measurement in two groups of eight very low birth weight infants fed pooled pasteurized human milk (HM) or cow's milk casein hydrolysate supplemented HM (HM-Pr). There was no difference in the amount of energy absorbed (91 +/- 17 kcal/kg/day with HM-Pr versus 95 +/- 8 with HM-P) or in the growth rate. The infants fed HM-Pr had a higher nitrogen intake (602 +/- 80 versus 395 +/- 64 mg/kg/day; p less than 0.001), urinary nitrogen excretion (160 +/- 64 versus 78 +/- 16 mg/kg/day; p less than 0.005) and nitrogen retention (326 +/- 32 versus 252 +/- 48 mg/kg/day; p less than 0.01). They also had increased plasma concentrations of essential amino acids, urea nitrogen, and total protein without metabolic imbalance. Energy expenditure was higher (58 versus 49 kcal/kg/day; p less than 0.005) and energy storage lower (33 versus 47 kcal/kg/day; p less than 0.05) with HM-Pr. In percent of weight gain, protein and fat accretion represented 12 and 14% in HM-Pr group versus 10 and 27% in HM group. Very low birth weight infants fed casein hydrolysate supplemented pooled HM achieved a growth rate and a weight gain composition similar to the fetus.     

Effect of the energy source on changes in energy expenditure, respiratory quotient, and nitrogen balance during total parenteral nutrition in children

The effects of three isocaloric intravenous nutritional regimens were studied in seven infants and children, ages 2 months to 9 yr, with congenital gastrointestinal anomalies (four patients) or with prior history of malignant disease admitted in remission for bone marrow transplantation (three patients). Energy metabolism, as measured by the basal metabolic rate (BMR), and substrate utilization, as measured by the respiratory quotient (RQ), were studied to determine the effect of different levels of carbohydrate and fat on nitrogen retention in each patient. Solution A provided 8% of energy as amino acids, 87% as carbohydrate, and 5% as fat. Solution B provided 8% of energy as amino acids, 60% as carbohydrate, and 32% as fat. Solution C provided 8% of energy as amino acids, 34% as carbohydrate, and 58% as fat. Administration of solution A (high carbohydrate, low fat) was associated with moderately increased mean (+/- SD) BMR and RQ and with low nitrogen retention (19.1 +/- 12.7%, 1.06 +/- 0.14, and 98 +/- 28 mg N/kg/day). Both the BMR and the RQ decreased when less carbohydrate and more lipid was given: BMR 4.3 +/- 11.6% (p less than 0.005), RQ 0.92 +/- 0.09 (p less than 0.001) for solution B; BMR 3.94 +/- 10.6% (p less than 0.005), RQ 0.86 +/- 0.09 (p less than 0.001) for solution C. Among the solutions tested, optimal nitrogen retention [163 +/- 60 mg N/kg/day (p less than 0.01)] was noted with solution B. Our data support the conclusion that a physiologic balance of fat and carbohydrate results in optimal nitrogen retention.     

Protein quality in feeding low birth weight infants: a comparison of whey-predominant versus casein-predominant formulas

Growth (delta weight, delta length, delta head circumference, and delta skinfold thickness), nitrogen retention, and chemical indices of metabolic tolerance (BUN concentration and acid-base status; plasma amino acid concentrations including free and bound cyst(e)ine; urinary excretion of sulfur amino acids) were determined serially in low birth weight infants (900 to 1,750 g) fed formulas differing only in protein quality. One contained unmodified bovine milk protein (a ratio of whey proteins to caseins of 18:82); the other contained modified bovine milk protein (a ratio of whey proteins to caseins of 60:40). Both provided protein and energy intakes, respectively, of approximately 3.4 g/kg/d and 120 kcal/kg/d. Neither weight gain nor the rate of increase in length, head circumference, and skinfold thickness differed between the two groups. Nitrogen retention of the two groups also did not differ. Although BUN concentration and blood acid-base status did not differ, there were differences in the plasma concentrations of some amino acids. Plasma tyrosine concentration was higher in infants fed the casein-predominant protein, and plasma threonine concentration was higher in infants fed the whey-predominant protein. Neither plasma-free nor bound cyst(e)ine concentration differed between the two groups, but the greater cyst(e)ine intake of the whey-predominant group resulted in greater cyst(e)ine retention; this was accompanied by greater urinary taurine excretion, a reflection of greater taurine stores.     

Whole body protein turnover measured with 13C-leucine and energy expenditure in preterm infants

Our study was undertaken in preterm infants to examine the relationship of whole body protein kinetics with protein intake and energy expenditure. Leucine kinetics were determined in seven low birth wt preterm infants fed human milk or human milk enriched with protein (2.5 to 4.3 g protein/kg.d). The infants received a short (4-h) constant infusion of L-[1-13C]leucine and leucine turnover and oxidation were calculated from 13C-plasma leucine and expired 13CO2 enrichments measured by mass spectrometry. Energy expenditure was measured by indirect calorimetry. Nonoxidative leucine disposal (an estimate of protein synthesis) and leucine derived from protein (an estimate of protein breakdown) were, respectively, 2.98 +/- 0.82 and 2.06 +/- 0.74 mumol/kg.min. Whole body protein turnover and deposition, derived from leucine kinetics, were 8.22 +/- 2.31 and 2.17 +/- 0.50 g/kg.d, whereas energy expenditure was 56.3 kcal/kg.day. Protein turnover was correlated with protein intake but not with protein deposition. Energy expenditure was correlated with protein turnover, synthesis, and breakdown but not with protein deposition. These data are in agreement with the fact that protein deposition depends upon protein intake, but they also suggest that an elevated protein deposition is not necessarily the result of a rapid protein turnover or associated with an elevated energy expenditure.     

Whole-body protein parameters in premature infants: a comparison of different 15N tracer substances and different methods.

[15N]glycine, [15N]leucine, and [15N]yeast protein thermitase hydrolysate (YPTH) as tracers for investigating the protein turnover rates in premature infants were studied in nine human milk-fed neonates (born after 32 to 34 wk of gestation) by paired comparison of the tracers. The 15N enrichment of total urinary nitrogen and ammonia after administration of a single oral dose of 15N was measured by emission spectrometry. Flux rates were calculated using a three-compartment model and the ammonia end product method. The mean whole-body protein synthesis rates, as determined by the three-compartment model derived from the three 15N tracers, differed significantly (p less than 0.01) among [15N]glycine (15.9 g/kg/d), [15N] leucine (9.1 g/kg/d), and 15N-YPTH (5.9 g/kg/d). When the corresponding rates were determined from the excretion of label in ammonia, the results showed the opposite tendency; the lowest apparent synthesis rates were found after [15N]glycine (7.5 g/kg/d), followed by [15N]leucine (14.4 g/kg/d), and the highest figure resulted after [15N] YPTH (16.7 g/kg/d). The results of this comparison substantiate the assumption that there are methodologic errors in connection with the use of different tracers and models for the calculation of whole-body protein parameters in preterm infants, with respect to the main requirement for tracer kinetic studies; the tracer nitrogen must be representative of total amino acid nitrogen. Seen in this light, mixtures of completely labeled amino acids such as YPTH may represent the most reliable tracer substance.(ABSTRACT TRUNCATED AT 250 WORDS)     

15N-tracer investigations into the nitrogen metabolism of preterm infants fed mother's milk and a formula diet

Protein synthesis, protein breakdown, protein-N turnover, and other parameters describing the nitrogen metabolism were measured in five male preterm infants. The weight of the subjects at birth was 2,064 +/- 107 g and the measurements were performed at age 16.0 +/- 4.5 days in the case of the mother's milk diet and 27.4 +/- 6.8 days in the case of the formula diet containing 1.8% protein. The parameters were measured by means of the 15N-tracer technique using [15N]glycine (95 atom %) applied in a single oral dose of 20 mg/kg as a tracer. The three-pool model proposed by Winkler and Faust was used to calculate the whole body protein parameters. No difference in net protein gain, protein synthesis, protein breakdown, or the other protein metabolism parameters were recorded despite the different protein inputs. Renal nitrogen excretion and the rate of endogenous urea N excretion were significantly higher for the formula diet than for the mother's milk diet. The protein synthesis rate of 7.9 g X kg-1 X day-1 was, as has previously been observed, higher than in other age groups. The protein metabolism of the preterm infant older than 33 weeks of gestational age does not benefit from a formula diet based on cow's milk that is richer in protein than mother's milk.     

Growth and metabolic responses in low-birth-weight infants fed human milk fortified with human milk protein or with a bovine milk protein preparation

Unfortified human milk does not normally provide enough protein to secure maximal growth in low-body-weight (LBW) infants. Due to the practical difficulties in obtaining human milk protein (HMP), a bovine milk protein preparation (BMP) was designed by computer calculation to contain as close as possible the amino acid composition of the nutritionally available human milk proteins. Twenty-one AGA, LBW infants (BW of 1,180 to 1,600 g, GA of 27 to 33 weeks) were randomly assigned to be fed HM enriched either with HMP (9 infants) or BMP (12 infants). When full volume intake (170 ml/kg/day) was reached, the protein intakes were 3.6 +/- 0.5 and 3.3 +/- 0.3 g/kg/day, respectively, in the two diet groups. During the study period of 24 days, the infants achieved intrauterine or better weight gains: 32.9 +/- 3.3 g/day (17.7 +/- 1.9 g/kg/day) in the HMP group and 34.7 +/- 7.3 g/day (18.3 +/- 3.5 g/kg/day) in the BMP group. Serum urea nitrogen, acid-base status, and albumin values were normal and similar in both groups of infants. Plasma concentrations of total essential and total amino acids at the end of the study were 3,999 and 1,539 mumol/L and 3,899 and 1,422 mumol/L in the HMP and the BMP groups, respectively. The concentrations of all individual plasma amino acids were similar in both feeding groups. These results show that feeding human milk fortified with a modified bovine milk protein preparation produces satisfactory growth and a plasma amino acid profile similar to that found in LBW infants fed exclusively human milk protein at similar intakes.     

Effect of low versus high intravenous amino acid intake on very low birth weight infants in the early neonatal period

Greater protein intakes are required than have been commonly used to achieve fetal in utero protein accretion rates in preterm neonates. To study the efficacy and safety of more aggressive amino acid intake, we performed a prospective randomized study in 28 infants [mean wt, 946 +/- 40 g (SEM)] of 1 (low amino acid intake, LAA) versus 3 g.kg(-1).d(-1) (high amino acid intake, HAA) at 52.0 +/- 3.0 h of life. After a minimum of 12 h of parenteral nutrition, efficacy was determined by protein balance and was significantly lower in the LAA versus HAA groups by both nitrogen balance (-0.26 +/- 0.11 versus 1.16 +/- 0.15 g.kg(-1).d(-1), p < 0.00005) and leucine stable isotope (0.184 +/- 0.17 versus 1.63 +/- 0.20 g.kg(-1).d(-1), p < 0.0005) methods. Leucine flux and oxidation and nonoxidative leucine disposal rates were all significantly higher in the HAA versus LAA groups (249 +/- 13 versus 164 +/- 8, 69 +/- 5 versus 32 +/- 3, and 180 +/- 10 versus 132 +/- 8 micro mol.kg(-1).h(-1), respectively, p < 0.005), but leucine appearance from protein breakdown was not (140 +/- 15 in HAA versus 128 +/- 8 micro mol.kg(-1).h(-1)). In terms of possible toxicity with HAA, there were no significant differences between groups in the amount of sodium bicarbonate administered, degree of acidosis as determined by base deficit, or blood urea nitrogen concentration. Parenteral HAA versus LAA intake resulted in increased protein accretion, primarily by increasing protein synthesis versus suppressing protein breakdown, and appeared to be well tolerated by very preterm infants in the first days of life.     

Evaluation of different 15N-tracer substances for calculation of whole body protein parameters in infants

The validity of using different 15N-tracer substances to measure whole body protein parameters, i.e., protein synthesis, protein breakdown, net protein gain, protein turnover, metabolic pool, and reutilization, was assessed by comparing the results obtained with: [15N]glycine, a mixture of 10 15N-labeled amino acids, and a 15N-labeled chicken egg protein in two infants, 9 and 12 weeks old, who were fed human milk. The tracer substances were fed orally as a single dose corresponding to a 15N-excess quantity of 0.2 mmol X kg-1 body weight. 15N Excretion in the urine was measured cumulatively by emission spectrometry, and the data on the protein metabolism were calculated by means of a three-pool model. All three tests yielded consistent net protein gains. The protein synthesis, protein breakdown, protein turnover, and nitrogen reutilization values produced by the [15N]glycine tracer study were higher than those produced by application of the 15N-amino acid mixture and the 15N-labeled egg protein. However, in our opinion, this discrepancy does not justify the replacement of [15N]glycine by expensive 15N-amino acid mixtures as tracer substances.