Phenylalanine and tyrosine metabolism in neonates receiving parenteral nutrition differing in pattern of amino acids

In vitro culture and tissue-derived specific expression of melanocytes from ovary of adult Silky Fowl

The presence of a significant number of melanocytes in the ovary and follicular membrane of Silky Fowl suggests their potential involvement in follicle development. Currently, there is a lack of available data regarding to the isolation of primary melanocytes from adult chickens. To date, primary melanocytes and their in vitro culture system have been successfully conducted in the peritoneum of chicken embryos. Herein, melanocytes from silky fowl ovaries were isolated and identified. Silky Fowl ovaries were obtained by mixed digestion of 0.1% collagenase II and 0.25% trypsin-EDTA. Melanocytes could be further purified and cultured up to 5 generations in vitro. RNA-seq analysis was used to investigate whether there were differences in the functional status of melanocytes in different tissues and developmental stages. Consequently, differential gene expressions between peritoneal and ovarian melanocytes were compared. These findings demonstrated that the Silky Fowl ovary had higher expression levels of genes involved in the production of sexual hormones and melanogenesis, while those of melanocytes derived from the peritoneum were involved in amino acid metabolism, lipid synthesis, and overall metabolic rates. This suggests that the role of melanocytes is dependent on the origin tissue and developmental stage, and is tightly connected to the function of the specific source tissue from which the cells were derived. This study provides a method for isolating adult melanocytes and serve as a basis for further investigate the effect of SFOM on germ cells.

The metabolome and bacterial composition of high-moisture Italian ryegrass silage inoculated with lactic acid bacteria during ensiling

BACKGROUND

With its high nutritional value and productivity, Italian ryegrass as a biomass feedstock constantly supplies rumen degradable nitrogen and digestible fiber to ruminants. However, biofuel production is easily reduced during ensiling due to the high-moisture content of Italian ryegrass, leading to economic losses. Lactic acid bacteria inoculants could improve lignocellulosic degradation and fermentation quality and decrease dry matter loss during the bioprocessing of silage. Therefore, this study analyzed the effects of Lactobacillus buchneri TSy1-3 (HE), Lactobacillus rhamnosus BDy3-10 (HO), and the combination of HE and HO (M) on fermentation quality, bacterial community and metabolome in high-moisture Italian ryegrass silage during ensiling.

RESULTS

The results showed that the pH value was significantly lower in the HO groups than in the other treatments at the end of ensiling, and the dry matter and acetic acid contents were significantly higher in the HO group than in the other inoculated groups. All inoculants decreased the diversity of the bacterial community and significantly increased the relative abundance of Lactobacillus. Inoculation with HO significantly improved the concentrations of organic acids, dipeptides, ferulic acid, apigenin, and laricitrin. Compared with Lactobacillus buchneri TSy1-3 (HE), HO significantly upregulated the flavonoid compounds in the flavone and flavonol biosynthesis pathway.

CONCLUSIONS

Overall, these findings suggest that inoculation with HO was beneficial for the development of Italian ryegrass as a biomass feedstock, improving fermentation quality, accelerating changes in bacterial community composition and increasing biofunctional metabolites in high-moisture Italian ryegrass silage.

Integrated Microbiomic and Metabolomic Dynamics of Fermented Corn and Soybean By-Product Mixed Substrate

Microbes and their metabolites produced in fermented food have been considered as critical contributors to the quality of the final products, but the comprehensive understanding of the microbiomic and metabolomic dynamics in plant-based food during solid-state fermentation remains unclear. Here, the probiotics of Bacillus subtilis and Enterococcus faecalis were inoculated into corn and defatted soybean to achieve the two-stage solid-state fermentation. A 16S sequencing and liquid chromatography–tandem mass spectrometry were applied to investigate the dynamics of microbiota, metabolites, and their integrated correlations during fermentation. The results showed that the predominant bacteria changed from Streptophyta and Rickettsiales at 0 h to Bacillus and Pseudomonas in aerobic stage and then to Bacillus, Enterococcus, and Pseudomonas in anaerobic stage. In total, 229 notably different metabolites were identified at different fermentation times, and protein degradation, amino acid synthesis, and carbohydrate metabolism were the main metabolic pathways during the fermentation. Notably, phenylalanine metabolism was the most important metabolic pathway in the fermentation process. Further analysis of the correlations among the microbiota, metabolites, and physicochemical characteristics indicated that Bacillus spp. was significantly correlated with amino acids and carbohydrate metabolism in aerobic stage, and Enterococcus spp. was remarkably associated with amino acids metabolism and lactic acid production in the anaerobic stage. The present study provides new insights into the dynamic changes in the metabolism underlying the metabolic and microbial profiles at different fermentation stages, and are expected to be useful for future studies on the quality of fermented plant-based food.

Two parenteral amino acid solutions and plasma levels of amino acids in the neonate: A randomized trial

OBJECTIVE

In neonates on total parenteral nutrition (TPN), amino acids may be a risk factor for developing total parenteral nutrition-associated cholestasis (TPNAC). We aimed, first, to compare methionine, cysteine, and taurine plasma levels between neonates on TPN who were receiving an intravenous amino acid solution based on a breast milk aminogram and those on an intravenous solution of pediatric amino acids based on an umbilical cord aminogram, and second, to determine the frequency of TPNAC.

METHODS

A double-blind randomized controlled trial was conducted. Ninety-four neonates with a birthweight of 1000g or more and a gestational age of 30 wk or older were admitted and enrolled. Blood samples were obtained at 0, 7, and 14 d of TPN, and plasma amino acid concentrations were determined by ultra-high-resolution liquid chromatography. Continuous variables were compared using the Wilcoxon rank-sum test or Student's t test; categorical variables were compared using the Fisher exact test.

RESULTS

Thirty-five neonates completed the study (Primene, n = 14; TrophAmine, n = 21). On day 14, methionine plasma concentrations were significantly lower in the Primene group than in the TrophAmine group (27 µmol/L versus 32.9 µmol/L, P = 0.044); the taurine concentration was significantly higher in the same group (72.4 µmol/L versus 45.3 µmol/L, P < 0.0001). There were no differences in TPNAC incidence.

CONCLUSIONS

Administering an intravenous solution of pediatric amino acids based on the umbilical cord aminogram yielded a higher taurine and lower methionine plasma concentration than did administering a similar solution based on the breast milk aminogram.

Phenylalanine requirements of enterally fed term and preterm neonates

BACKGROUND

Phenylalanine, which is an essential aromatic amino acid, is either used for protein synthesis or irreversibly hydroxylated to tyrosine. The provision of optimal amounts of dietary phenylalanine is not only important for growth and development but might also influence catecholamine synthesis and release rates. The current recommended aromatic amino acid requirement for infants aged 0-6 mo is based on the amino acid content of human milk.

OBJECTIVE

We quantified the requirements for phenylalanine in the presence of excess tyrosine (166 or 177 mg/kg per day for term and preterm infants, respectively) for term and preterm neonates by using the indicator amino acid oxidation method with l-[1-(13)C]lysine 2HCl as an indicator. Hence, we determined the minimum obligatory phenylalanine requirement.

DESIGN

Fully enterally fed term and preterm infants received randomly graded amounts of phenylalanine (5-177 mg/kg per day) as part of an elemental formula. Data are expressed as means ± SDs.

RESULTS

Twenty term (birth weight: 3.19 ± 0.34 kg; gestational age: 38.9 ± 1 wk) and 16 preterm (birth weight: 1.75 ± 0.17 kg; gestational age: 32.5 ± 0.6 wk) Asian infants participated at a postnatal age of 17 ± 8 d. In total, 44 studies were performed. The minimum obligatory phenylalanine requirement was 58 mg/kg per day (95% CI: 38-78 mg/kg per day) and 80 mg/kg per day (95% CI: 40-119 mg/kg per day) for term and preterm infants, respectively.

CONCLUSION

The determined mean phenylalanine-requirement estimates are lower than the contents of term and preterm formulas currently on the market. This trial was registered at www.trialregister.nl as NTR1610.

Effect of hyperalimentation and insulin-treated hyperglycemia on tyrosine levels in very preterm infants receiving parenteral nutrition

BACKGROUND

Hyperalimentation describes the increase in glucose, amino acids (AAs), and lipid intake designed to overcome postnatal growth failure in preterm infants. Preterm infants are dependent on phenylalanine metabolism to maintain tyrosine levels because of tyrosine concentration limits in parenteral nutrition (PN). We hypothesized that hyperalimentation would increase individual AA levels when compared with the control group but avoid high phenylalanine/tyrosine levels.

AIM

To compare the plasma AA profiles on days 8-10 of life in preterm infants receiving a hyperalimentation vs a control regimen.

METHODS

Infants <29 weeks' gestation were randomized to receive hyperalimentation (30% more PN macronutrients) or a control regimen. Data were collected to measure macronutrient (including protein) intake and PN intolerance, including hyperglycemia, insulin use, urea, and AA profile. Plasma profiles of 23 individual AA levels were measured on days 8-10 using ion exchange chromatography.

RESULTS

One hundred forty-two infants were randomized with 118 AA profiles obtained on days 8-10. There were no differences in birth weight or gestation between groups. There was an increase (P < .05) in 8 of 23 median individual plasma AA levels when comparing hyperalimentation (n = 57) with controls (n = 61). Only tyrosine levels (median; interquartile range) were lower with hyperalimentation: 27 (15-52) µmol/L vs 43 (24-69) µmol/L (P < .01). Hyperalimentation resulted in more insulin-treated hyperglycemia. No difference between the groups was apparent in tyrosine levels when substratified for insulin-treated hyperglycemia. All insulin vs no insulin comparisons showed lower tyrosine levels with insulin treatment (P < .01).

CONCLUSION

Hyperalimentation can result in paradoxically low plasma tyrosine levels associated with an increase in insulin-treated hyperglycemia.

Rate of phenylalanine hydroxylation in healthy school-aged children

Hydroxylation of phenylalanine to tyrosine is the first and rate-limiting step in phenylalanine catabolism. Currently, there are data on the rate of phenylalanine hydroxylation in infants and adults but not in healthy children. Thus, the aim of the study reported here was to measure the rate of phenylalanine hydroxylation and oxidation in healthy school-aged children both when receiving diets with and without tyrosine. In addition, hydroxylation rates calculated from the isotopic enrichments of amino acids in plasma and in very LDL apoB-100 were compared. Eight healthy 6- to 10-y-old children were studied while receiving a control and again while receiving a tyrosine-free diet. Phenylalanine flux, hydroxylation, and oxidation were determined by a standard tracer protocol using oral administration of ¹³C-phenylalanine and ²H₂-tyrosine for 6 h. Phenylalanine hydroxylation rate of children fed a diet devoid of tyrosine was greater than that of children fed a diet containing tyrosine (40.25 ± 5.48 versus 29.55 ± 5.35 μmol · kg⁻¹ · h⁻¹; p < 0.01). Phenylalanine oxidation was not different from phenylalanine hydroxylation regardless of dietary tyrosine intake, suggesting that phenylalanine converted to tyrosine was mainly oxidized. In conclusion, healthy children are capable of converting phenylalanine to tyrosine, but the need for tyrosine cannot be met by providing extra phenylalanine.

Protein and amino acid metabolism in the human newborn

Birth and adaptation to extrauterine life involve major shifts in the protein and energy metabolism of the human newborn. These include a shift from a state of continuous supply of nutrients including amino acids from the mother to cyclic periodic oral intake, a change in the redox state of organs, thermogenesis, and a significant change in the mobilization and use of oxidative substrates. The development of safe, stable isotopic tracer methods has allowed the study of protein and amino acid metabolism not only in the healthy newborn but also in those born prematurely and of low birth weight. These studies have identified the unique and quantitative aspects of amino acid/protein metabolism in the neonate, thus contributing to rational nutritional care of these babies. The present review summarizes the contemporary data on some of the significant developments in essential and dispensable amino acids and their relationship to overall protein metabolism. Specifically, the recent data of kinetics of leucine, phenylalanine, glutamine, sulfur amino acid, and threonine and their relation to whole-body protein turnover are presented. Finally, the physiological rationale and the impact of nutrient (amino acids) interventions on the dynamics of protein metabolism are discussed.

An approach to defining the upper safe limits of amino acid intake

The existing data on the safe upper limits of amino acid intake in humans is essentially observational; how much do individuals ingest and what side effects do they have? There are numerous studies in humans comparing the effects of high doses of amino acids given as protein bound vs. as free amino acids. These studies have shown that protein-bound amino acids have much less effect on plasma levels of the test amino acid, because protein intake stimulates protein synthesis as another sink for the increased amino acid intake. In practice, the highest amino acid intakes occur with free amino acid supplements that may be ingested by athletes who believe that the amino acids will benefit them in training and/or performance. Previously, in a piglet study, we were able to define the point at which maximal phenylalanine oxidation occurred, above which plasma phenylalanine concentration and body balance rose exponentially. We regard this value of maximal disposal (oxidation) of an amino acid as one metabolic marker of the upper limit of intake. Recently, others have demonstrated a similar maximal oxidation rate for leucine in rats. Based on these experimental data and the paucity of published human data in controlled experiments, we think that a systematic approach needs to be undertaken to define the maximal oxidation rate for all dietary indispensable amino acids and other amino acids that may be ingested in excess by humans. We believe that this will provide a rational basis to begin to define the upper limits of tolerance for dietary amino acids. However, some amino acids, such as threonine and methionine, will be more difficult to study, because they have more than 1 route of disposal or very complex metabolic regulation, in which case defining their upper limits will be more multifaceted.

Total sulfur amino acid requirement and metabolism in parenterally fed postsurgical human neonates

BACKGROUND

Except for tyrosine, the amino acid requirements of human neonates receiving parenteral nutrition (PN) have not been experimentally derived.

OBJECTIVES

The objectives were to determine the total sulfur amino acid (TSAA) requirement (methionine in the absence of cysteine) of postsurgical, PN-fed human neonates by using the indicator amino acid oxidation (IAAO) technique with L-[1-(13)C]phenylalanine as the indicator.

DESIGN

Fifteen postsurgical neonates were randomly assigned to receive 1 of 18 methionine intakes ranging from 10 to 120 mg x kg(-1) x d(-1), delivered in a customized, cysteine-free amino acid solution. Breath and urine samples were collected for the measurement of (13)CO(2) and amino acid enrichment. Blood samples were collected at baseline and after the test methionine infusion for the measurement of plasma methionine, homocysteine, cystathionine, and cysteine concentrations.

RESULTS

Breakpoint analysis determined the mean TSAA requirements to be 47.4 (95% CI: 38.7, 56.1) and 49.0 (95% CI: 39.9, 58.0) mg x kg(-1) x d(-1) with the use of oxidation and F(13)CO(2), respectively.

CONCLUSIONS

This is the first study to report the TSAA requirement of postsurgical, PN-fed human neonates. The estimated methionine requirement expressed as a proportion of the methionine content of current commercial pediatric PN solutions was 90% (range: 48-90%) of that found in the lowest methionine-containing PN solution.

Evidence that phenylalanine may not provide the full needs for aromatic amino acids in children

Phenylalanine is nutritionally classified as an indispensable amino acid and can be converted to tyrosine by phenylalanine hydroxylation. The initial goal of the present study was to determine the aromatic amino acid (phenylalanine plus tyrosine) requirements in healthy children fed a diet without tyrosine by using the indicator amino acid oxidation (IAAO) method using lysine as the indicator amino acid. Healthy school-age children (n = 5) were fed in random order a diet with eight graded intakes of phenylalanine without tyrosine. The requirement was determined by the rate of recovery of CO2 from L-[1-C]lysine oxidation (FCO2). Phenylalanine (total aromatic amino acid) requirement, in the absence of tyrosine, for children was determined to be 28 mg/kg/d, which was only 64% of the adult requirement, which is biologically absurd. A possible reason for the lower estimate of phenylalanine requirement could be lower phenylalanine hydroxylation rate in children, which is supported by the finding of lower urinary tyrosine/phenylalanine ratios in children compared with adults. In conclusion, this study indicates that phenylalanine may not provide the total needs for aromatic amino acids in children fed an amino acid-based diet without tyrosine.

Parenteral amino acid and energy administration to premature infants in early life

After birth, the nutritional supply through the umbilical cord ceases. Premature infants do not immediately tolerate full enteral feedings, yet they retain high nutritional needs for both growth and metabolic maintenance. Parenteral nutrition should therefore be initiated as quickly as possible after premature birth, thereby reducing the dependence on endogenous substrates. Intrauterine studies show very high amino acid uptake, clearly exceeding accretion rates. Studies covering the early neonatal period demonstrate that the initiation of high-dose amino acid administration directly after birth is safe and effective, even at low energy intakes. Future research should reveal whether usage could be improved through better amino acid solutions or by providing more energy via lipids from birth onwards as well.

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.

Whole-body and hindlimb protein breakdown are differentially altered by feeding in neonatal piglets

The high rate of muscle protein accretion in neonates is sustained by the marked increase in muscle protein synthesis in response to feeding. Little is known about the role of proteolysis in the regulation of protein accretion in response to feeding during the neonatal period. To determine the feeding-induced response of protein breakdown at the whole-body level and in the hindlimb of neonates, 10- and 28-d-old piglets that had been food deprived overnight were infused (7 h) with [1-13C]phenylalanine and [ring-2H4]tyrosine during an initial food deprivation period (3 h), followed by a feeding period (4 h). During feeding, endogenous flux of phenylalanine decreased (P < 0.01) in both the whole body and the hindlimb. Feeding reduced (P < 0.01) whole-body proteolysis but increased hindlimb proteolysis (P = 0.04), suggesting that tissues other than the hindlimb are involved in the reduction in whole-body proteolysis during feeding. Overnight food deprivation resulted in a net mobilization of phenylalanine from whole-body proteins (P < 0.01) but not hindlimb proteins. These responses were unaffected by age. The results suggest that the hindlimb requires a continuous supply of free amino acids to sustain the high rate of muscle protein turnover in neonates and that adaptive mechanisms provide free amino acids to sustain skeletal muscle protein accretion in early postnatal life when the amino acid supply is limited.

Measurement of hepatic phenylalanine metabolism in children using the [13C]-phenylalanine breath test and gas chromatography–mass spectrometry

The essential amino acid, phenylalanine (PA), is known to be metabolized mainly in the liver of human adults. Because the liver is still in the developmental phase, the PA-related metabolic events in infants remain unsolved. In this study, evaluations of development in hepatic PA metabolism in 37 children and 16 adults were attempted using the (13)C -PA breath test (PBT). The subjects were categorized into four groups according to their ages in years and months: 2 years and 0 month to 3 years and 5 months (group I; n = 12); 3 years and 6 months to 4 years and 11 months (group II, n = 12); 5 years and 0 month to 6 years and 11 months (group III, n = 13); and healthy adults (group IV; n = 16). Changes in CO(2) level of exhaled gas at various time intervals after oral administration of (13)C -PA were monitored using gas chromatography-mass spectrometry to derive the (13)C excretion rate, cumulative excretion curve and time maximum [(13)C excretion rate (T(MAX)). In the present investigation involving children, significant increases of maximum(13)C excretion rate and cumulative excretion at 120 min after administration were established in group III. Furthermore, differences in PBT were not established between groups III and IV. The index for first-pass effect, T(MAX), did not change with time. From the above findings, the (13)C excretion rate increased with time although hepatic PA metabolism in infants remained underdeveloped, and children at the age of 5-7 years manifested PA metabolism similar to that of adults.

Amino acid needs for early growth and development

Few data exist on amino acid needs in infants and children, mainly because until recently, amino acid requirements were determined using nitrogen balance. The advent of the indicator amino acid oxidation (IAAO) method permits studies to be conducted with minimal adaptation to the test amino acid. In light of the very limited data available for human infants, toddlers, and children, it was proposed that a factorial approach should be used to estimate their essential amino acid requirements. Using amino acid oxidation techniques, dietary essential amino acid requirements in adults have been nearly completed. Data on changes in total body potassium are now available for infants and children. From these data it is possible to calculate protein deposition during growth, and hence, it is now possible to estimate the amino acid requirements in children using a factorial model. However, there has been no independent verification of the model. Recently we determined total branched chain-amino acid requirements for young adults and children, and we can provide data to support the validity of the factorial model. IAAO has been used on children with liver disease as young as 3 y. The minimally invasive IAAO model opens the door for determination of dietary essential amino acid requirements in infants and children during health and disease. For study of preterm neonates, we used a piglet model to show that the amino acid needs for parenteral feeding are markedly reduced for several essential amino acids; this suggests that current commercial total parenteral nutrition amino acid solutions are less than ideal.

Enteral tryptophan requirement determined by oxidation of gastrically or intravenously infused phenylalanine is not different from the parenteral requirement in neonatal piglets

We have recently shown that the requirements of several amino acids differ substantially when neonates are fed parenterally as opposed to enterally. Our first objective was to determine whether the tryptophan requirement was different in parenterally fed (IV(fed)/IV(dose)) versus enterally fed (IG(fed)/IV(dose)) piglets. Because of the extensive extraction of amino acids by the gut, our other objective was to determine whether the route of isotope administration [i.e. intragastric (IG(fed)/IG(dose)) versus i.v. (IG(fed)/IV(dose)) dose] affects the estimate of tryptophan requirement in enterally fed piglets. We used the indicator amino acid oxidation technique in piglets (10 +/- 0.5 d old, 2.79 +/- 0.28 kg) receiving a complete elemental diet for 6 d either intragastrically or intravenously. Piglets were randomly assigned to receive test diets containing one of seven levels of tryptophan. All animals received a primed, constant infusion of l-[1-(14)C]phenylalanine either parenterally (IV(fed)/IV(dose) and IG(fed)/IV(dose)) or enterally (IG(fed)/IG(dose)). The mean tryptophan requirements for IV(fed)/IV(dose) (0.145 +/- 0.023 g/kg/d), IG(fed)/IV(dose) (0.127 +/- 0.022 g/kg/d), and IG(fed)/IG(dose) (0.113 +/- 0.024 g/kg/d) were similar as were the safe intakes (upper 95% confidence interval) (0.185, 0.164, 0.154 g/kg/d, respectively). These data indicate that tryptophan is not extensively used by the gut, in contrast to all the other amino acids we have studied. Furthermore, in spite of a splanchnic extraction of 27% of the phenylalanine dose, the route of isotope infusion does not affect the tryptophan requirement as determined by indicator amino acid oxidation.

Effect of parenteral glutamine supplementation on plasma amino acid concentrations in extremely low-birth-weight infants

BACKGROUND

Glutamine is one of the most abundant amino acids in both plasma and human milk and may be conditionally essential in premature infants. However, glutamine is not provided by standard intravenous amino acid solutions.

OBJECTIVE

We assessed the effect of parenteral glutamine supplementation on plasma amino acid concentrations in extremely low-birth-weight infants receiving parenteral nutrition (PN).

DESIGN

A total of 141 infants with birth weights of 401-1000 g were randomly assigned to receive a standard intravenous amino acid solution that did not contain glutamine or an isonitrogenous amino acid solution with 20% of the total amino acids as glutamine. Blood samples were obtained just before initiation of study PN and again after the infants had received study PN (mean intake: 2.3 +/- 1.0 g amino acids x kg(-1) x d(-1)) for approximately 10 d.

RESULTS

Infants randomly assigned to receive glutamine had mean plasma glutamine concentrations that increased significantly and were approximately 30% higher than those in the control group in response to PN (425 +/- 182 and 332 +/- 148 micromol/L for the glutamine and control groups, respectively). There was no significant difference between the 2 groups in the relative change in plasma glutamate concentration between the baseline and PN samples. In both groups, there were significant decreases in plasma phenylalanine and tyrosine between the baseline and PN samples; the decrease in tyrosine was greater in the group that received glutamine.

CONCLUSIONS

In extremely low-birth-weight infants, parenteral glutamine supplementation can increase plasma glutamine concentrations without apparent biochemical risk. Currently available amino acid solutions are likely to be suboptimal in their supply of phenylalanine, tyrosine, or both for these infants.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Protein/amino acid metabolism and nutrition in very low birth weight infants

A large number of studies in recent years have described protein and nitrogen metabolism in the neonate. However, the majority of these data are difficult to interpret because of a number of confounding variables, particularly in very low birth weight (VLBW) infants. In contrast, application of state-of-the-art tracer isotopic and molecular biology methods in isolated cell system and whole animals has resulted in major advances in our understanding of the regulation of protein breakdown, synthesis, and protein accretion. The following workshop summary reviews the recent developments in basic physiology of protein metabolism in cellular and animal models in relation to human preterm infants, and identifies the important areas toward which future basic and clinical research should be directed to provide for optimal nitrogen accretion and growth of the VLBW infant.

The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirement

Although tyrosine is considered indispensable during the neonatal period, its poor solubility has limited its inclusion in parenteral amino acid solutions to less than 1% of total amino acids. Dipeptides of tyrosine are highly soluble, have been shown to be well used and safe in animal models and humans, and, therefore, may be used as an effective means of providing tyrosine in the parenterally fed neonate. The goal of the present study was to determine the tyrosine requirement of the parenterally fed neonate receiving graded intakes of glycyl-L-tyrosine as a source of tyrosine. Thirteen infants receiving adequate energy (340 +/- 38 kJ. kg(-1).d(-1)) and protein (2.4 +/- 0.4 g.kg(-1).d(-1)) were randomized to receive parenteral nutrition with one of five graded levels of glycyl-L-tyrosine. The mean requirement and safe level of intake were estimated using a 1-(13)C-phenylalanine tracer and linear regression cross-over analysis that identified a break point in the response of label appearance in breath CO(2) (F(13)CO(2)) and phenylalanine oxidation to graded tyrosine intake. Based on the mean estimates of whole-body phenylalanine oxidation, the tyrosine mean requirement and safe level of intake were found to be 74 mg.kg(-1). d(-1) and 94 mg.kg(-1).d(-1), respectively. This represents 3.1 and 3.9% of total amino acids, respectively, considerably higher than levels found in present commercially available pediatric amino acid solutions. These data raise concern regarding the adequacy of aromatic amino acid intake in the parenterally fed neonate.

Steady state is not achieved for most plasma amino acids during 12 hours of fasting in the neonatal piglet

Kinetics studies in neonates are important to establish the requirement for amino acids and to understand the mechanisms of normal and altered metabolism. During kinetics experiments, plasma amino acid concentrations should be in steady state. Our objective was to determine whether 12 h of fasting, after parenteral or enteral feeding, resulted in a steady state in concentrations of amino acids. Two-day-old piglets were implanted with catheters (d 0), and randomly assigned to either intragastric (i.g., n = 6) or i.v. (n = 6) feeding. On d 5, piglets were fasted for 12 h. During the first 2 h, plasma concentrations of almost all amino acids declined except asparagine (i.g. and i.v.), tyrosine (i.v.), and glycine (i.v.), which increased. Only i.g. glycine did not change. Between 2 and 12 h, the only indispensable amino acids that did not change were phenylalanine (i.v.) and histidine (i.g. and i.v.). The branched-chain amino acids increased during this period (i.v. and i.g.). The greatest change was tyrosine, increasing 13% (i.v.) and 32% (i.g.) per hour. After 12 h of refeeding, glycine, serine, threonine, and asparagine concentrations were lower than baseline (p<0.05) in the i.v. group. In i.g. fed piglets, only threonine remained below baseline (p<0.05), and arginine was greater than baseline (p<0.05). Differences between i.v. and i.g. may be the result of impaired small intestinal metabolism secondary to parenteral feeding. In neonatal pigs, most plasma amino acids were unstable during 12 h of fasting. Thus, kinetics studies that require a steady state must be conducted in the fed state.

Current total parenteral nutrition solutions for the neonate are inadequate

The amino acid requirements of the parenterally fed neonate are poorly defined. Newborn infants are at risk for amino acid deficiency and toxicity, due to lack of small intestinal metabolism and metabolic immaturity. We discuss recent evidence that identifies inadequacies of commercial amino acid solutions with respect to the balance and quantity of aromatic amino acids, and sulphur amino acids. We present data demonstrating that impaired small intestinal metabolism (or lack of first pass metabolism) alters the whole body requirement for methionine, threonine, and arginine, and discuss the potential adverse effects of excess or inadequate parenteral amino acid intake.

Protein metabolism in the extremely low-birth weight infant

Although extensive data are available on the impact of nutrient and protein administration on growth, plasma amino acids, and nitrogen balance in the newborn and growing infants, relatively few studies have carefully examined the dynamic aspects of protein metabolism in vivo and particularly in the micropremie or ELBW infant. These studies show that the very preterm infants, either because of immaturity or because of the intercurrent illness, have high rates of protein turnover and protein breakdown. This high rate of proteolysis is not as responsive to nutrient administration. Intervention strategies aimed at promoting nitrogen accretion, such as insulin, human growth hormone, or glutamine, have not thus far resulted in enhanced protein accretion and growth. This may be, in part, due to limitations in delivery of adequate calorie and nitrogen.