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The gut takes nearly all: threonine kinetics in infants^1,2,3

¹ From the Department of Pediatrics, Division of Neonatology (SRDvdS and JBvG) and the Departments of Internal Medicine (DLW), Clinical Genetics (JH), and Clinical Pharmacy (AV), Erasmus MC–Sophia Children's Hospital, Erasmus University, Rotterdam, Netherlands

² Supported by the Sophia Foundation of Scientific Research (Kröger Foundation) and the Ajinomoto Amino Acid Research Programme.

³ Reprints not available. Address correspondence to JB van Goudoever, Department of Pediatrics, Erasmus MC–Sophia Children's Hospital, Erasmus University, Dr Molewaterplein 60, 3015 GJ, Rotterdam, Netherlands. E-mail: j.vangoudoever{at}erasmusmc.nl.

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Background:Threonine is an essential amino acid that is abundantly present in intestinally produced glycoproteins. Animal studies show that intestinal first-pass threonine metabolism is high, particularly during a restricted enteral protein intake.

Objective:The objective of the study was to quantify intestinal first-pass threonine metabolism in preterm infants during full enteral feeding and during restricted enteral intake.

Design:Eight preterm infants ( ± SD birth weight: 1.1 ± 0.1 kg; gestational age: 29 ± 2 wk) were studied during 2 periods. During period A, 40% of total intake was administered enterally and 60% was administered parenterally. Total threonine intake was 58 ± 6 µmol · kg^–1 · h^–1. During period B, the infants received full enteral feeding, and the total threonine intake was 63 ± 6 µmol · kg^–1 · h^–1. Dual stable-isotope tracer techniques were used to assess splanchnic and whole-body threonine kinetics.

Results:The fractional first-pass threonine uptake by the intestine was remarkably high in both periods: 82 ± 6% during partial enteral feeding and 70 ± 6% during full enteral feeding. Net threonine retention was not affected by the route of feeding.

Conclusion:In preterm infants, the splanchnic tissues extract a very large amount of the dietary threonine intake, which indicates a high obligatory visceral need for threonine, presumably for the purposes of synthesis.

Key Words: Threonine • preterm infants • intestine • stable isotopes • nutrition • splanchnic metabolism

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
During the first few weeks of life, preterm infants are faced with the challenge of doubling their body weight (1). The high growth rate of the newborn infant puts significant pressure on the intestine to efficiently digest and absorb nutrients. This occurs at a time when the neonatal intestine is adapting to the enteral route of nutrition after a prenatal period in which amino acids (AAs) were delivered via the umbilical route. Therefore, it may be speculated that a large quantity of AAs is needed for the growth and maintenance of the premature gut to enable its optimal function and integrity.

Given the key role of the gut in the maintenance of neonatal health, there has been considerable interest in the significance of first-pass intestinal metabolism of dietary AAs (2–4). Enterally absorbed AAs can be used for incorporation into mucosal cellular proteins, for energy production, or for conversion via transamination into other AAs, metabolic substrates, and biosynthetic intermediates. It is known that, in animals, <20% of intestinal AAs are used for constitutive gut growth by the intestinal mucosa (1), and, although some essential AAs (EAAs) are known to be catabolized (3–5), the catabolism of EAAs does not account for their high utilization rate. Therefore, the synthesis of secretory glycoproteins by the enterocytes appears to be a major metabolic fate for EAAs.

Of particular interest is threonine, which is the AA that, in neonatal piglets, is used to the greatest extent by the portal-drained viscera (PDV)—ie, the intestines, pancreas, spleen, and stomach. In neonatal pigs, the splanchnic extraction of threonine ranges from 60% to 80% of the dietary intake (2, 3, 6, 7). This high intestinal requirement for threonine may reflect the use of enterally absorbed threonine for the synthesis of secretory glycoproteins as the major metabolic fate. Indeed, Roberton et al (8) found that the protein cores of secretory mucins contain large amounts of threonine. In addition, Bertolo et al (9) showed that the whole-body threonine requirement in total parenteral nutrition (TPN)–fed piglets is 40% of that observed in enterally fed piglets, which indicates that enteral nutrition itself induces metabolic processes that demand threonine, probably within the intestine. One may postulate that the compromised gut barrier function associated in humans with parenteral nutrition is caused by a sparse threonine availability combined with diminished intestinal mucin production, as has been shown in rats (10).

In view of the central role of the gut in nutrient processing and metabolism, we considered it important to investigate the effect of the amount of enteral intake on splanchnic and whole-body threonine metabolism in preterm infants. Accordingly, we simultaneously used 2 stable isotope–labeled threonine tracers—[U-¹³C]threonine and [¹⁵N]threonine—and administered them via intravenous and intragastric routes to determine the quantitative aspects of threonine metabolism in preterm neonates under both parenteral and enteral feeding conditions. This technique enabled us to measure both the first-pass intestinal threonine uptake and the whole-body threonine kinetics. Previously, our group (7) found in neonatal pigs that the considerable first-pass threonine utilization was not significantly affected by a lower protein intake. Therefore, we hypothesized that the first-pass utilization of threonine by the splanchnic tissues would be substantial in preterm infants and would be independent of the dietary threonine intake. Hence, the present study explores the effect of the amount of enteral formula intake on various components of first-pass and whole-body threonine metabolism in neonates.

	SUBJECTS AND METHODS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Subjects
Splanchnic and whole-body threonine kinetics were quantified in 8 preterm infants during 2 consecutive periods of different enteral and parenteral intakes. Patients eligible for this study were premature infants with a birth weight of 750 to 1250 g that was appropriate for gestational age according to the charts of Usher and McLean (11). Excluded from the study were infants who had major congenital anomalies or gastrointestinal or liver diseases. All infants' Clinical Risk Index for Babies (CRIB; 12) scores on the first day of life were <5. The maximum CRIB score is 23, and the minimum score is 0 (with increasing scores, there is increased morbidity and mortality). Selected relevant clinical variables for the infants studied are shown in Table 1. The infants received a standard nutrient regimen according to our feeding protocol: a combination of the mother's milk or formula (Nenatal; Nutricia, Zoetermeer, Netherlands; 0.024 g/mL protein) and parenteral nutrition containing glucose, AAs (Primene 10%; Clintec Benelux NV, Brussels, Belgium; 0.1 g/mL protein), and lipids (Intralipid 20%; Fresensius Kabi, Den Bosch, Netherlands). Formula feeding was given as the sole enteral nutrition 12 h before the start of the study and during the study days.

Protocol
The study design consisted of 2 periods of one study day (period A: study day 1; period B: study day 2). During period A, the infants received 40% enteral feeding and 60% parenteral feeding; during period B, they received full enteral feeding. A schematic outline of the tracer-infusion studies is shown in Figure 1. During period A, both an arterial and an intravenous catheter were implanted in the infants for the infusion of tracers and withdrawal of blood samples. These catheters were already installed for clinical purposes. During period B, a peripheral intravenous catheter was available for the infusion of tracers, and blood samples were collected by heelstick.

View larger version (23K): [in this window] [in a new window]   FIGURE 1.. Schematic overview of study periods A (study day 1) and B (study day 2). IG, intragastric; IV, intravenous.

Three stable-isotope infusions were performed on each study day during both periods. First, a primed, continuous infusion [10.02 µmol/kg priming dose and 10.02 µmol/(kg · h) of [¹³C]sodium bicarbonate (99 mol% ¹³C; Cambridge Isotopes, Woburn, MA) dissolved in sterile saline was administered at a constant rate for 2 h. The ¹³C-labeled bicarbonate infusion was immediately followed by primed, continuous infusion [14.4 µmol/kg priming dose and 14.4 µmol/(kg · h) of [U-¹³C]threonine (97 mol% ¹³C; Cambridge Isotopes) given intravenously and a second primed, continuous infusion [14.7 µmol/kg priming dose and 14.7 µmol/(kg · h) of [¹⁵N]threonine (95 mol% ¹⁵N; Cambridge Isotopes) given enterally for 5 h. This process was designed to assess whole-body and splanchnic threonine kinetics. All isotopes were tested and found to be sterile and pyrogen-free before they were used in our studies. Baseline blood and breath samples were collected at time 0. During the last hour of each tracer infusion, breath samples were collected at 15-min intervals, and blood samples were obtained at 390 and 420 min. The total amount of blood drawn on a study day was 1.5 mL, which is <2% of the blood volume of a 1000-g infant. Blood was centrifuged immediately (2500 x g, 4 °C, 10 min) and stored at –70 °C for further analysis.

Analytic methods
Small aliquots of plasma (100 µL) were taken for the measurement of AA concentrations by using an analyzer (Amino Acid Analyser Biochrom 20; Biochrom Ltd, Cambridge, United Kingdom). Plasma threonine enrichments were determined by gas chromatography–mass spectrometry. Briefly, 50 µL plasma was deproteinized with 50 µL of 0.24 mol sulfosalicylic acid/L. After centrifugation for 8 min at 4 °C and 14 000 x g, the supernatant was passed through an H⁺ column (AG50W-X8; Biorad, Richmond, VA). The column was washed with 3 mL water, and the AAs were eluted with 1.5 mL of 3 mol NH₄OH/L. The eluate was dried at 70 °C under a stream of nitrogen, and, finally, derivatives of the AAs were formed by adding 350 µL acetonitril and 10 µL N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (Pierce Omnilabo, Breda, Netherlands) to the dried AAs (16). Analyses were carried out on a Carlo Erba GC8000 gas chromatograph coupled to a Fisons MD800 mass spectrometer (both: Interscience BV, Breda, Netherlands) by injecting 1 µL with a split ratio of 50:1 on a 25-m x 0.22-mm fused silica capillary column, coated with 0.11 µm HT5 (SGE, Victoria, Australia). Natural threonine, [¹⁵N]-threonine, and [U-¹³C]-threonine were measured by selective ion monitoring of masses 404, 405, and 408, respectively (16). Breath samples were analyzed for enrichment of ¹³CO₂ on an isotope ratio–mass spectrometer (ABCA; Europa Scientific, Van Loenen Instruments, Leiden, Netherlands) (17).

Calculations
The rate of threonine turnover was calculated by measuring the tracer dilution at steady state as modified for stable isotope tracers, as previously described (18, 19). Plasma enrichments of threonine were used to calculate the rate of threonine turnover.

The threonine flux (intravenous or intragastric) was calculated according to the following equation:

(1)

where Q_{iv or ig} is the flux of the intravenous or intragastric threonine tracer [µmol/(kg/h)], i_T is the threonine infusion rate [µmol/(kg/h)], and E_i and E_p are the enrichments [mol percent excess (MPE)] of [U-¹³C or ¹⁵N]threonine in the threonine infusate and in plasma at steady state, respectively.

The first-pass threonine uptake was calculated according to the following equation:

(2)

where U is the first-pass threonine uptake, Q_ig is the flux of the intragastric threonine tracer, and I is the enteral threonine intake [µmol/(kg/h) for all].

In a steady state, the amount of threonine entering the plasma pool should be equal to the amount of threonine leaving the pool. Threonine can enter the pool either by being released from proteins as the result of breakdown or through the diet. Threonine may leave the pool through either oxidative disposal or nonoxidative disposal (threonine used for synthesis). To calculate the amount of threonine leaving the pool, we used the following equation:

(3)

where TRP is the amount of threonine released from protein via protein breakdown [µmol/(kg/h)], Ox is the rate of threonine oxidation [µmol/(kg/h)], and NOTD is the rate of nonoxidative disposal of threonine [a measure of protein synthesis rate, expressed as µmol/(kg/h)].

Net threonine balance, an index of protein deposition, was calculated by using the following equation:

(4)

where TBAL is threonine balance [µmol/(kg/h)].

Whole-body carbon dioxide production was estimated by using the following equation:

(5)

where i_B is the infusion rate of NaH¹³CO₃ [µmol/(kg/h)], E_iB is the enrichment (MPE) of [¹³C]bicarbonate in the bicarbonate infusate, and IE_B is the breath ¹³CO₂ enrichment at plateau during the NaH¹³CO₃ infusion (MPE).

As described previously, threonine oxidation was calculated by multiplying the recovery of the [¹³C]label in the expiratory air with the rate of appearance of threonine (20). The fraction of threonine oxidized was measured according to the following equation, assuming a constant rate of CO₂ production during the study, which lasted 5 h (20):

(6)

where IE_T and IE_B are the ¹³CO₂ breath enrichments (MPE) at steady state during the intravenous [U-¹³C]threonine infusion and NaH¹³CO₃ infusion. The denominator is multiplied by a factor of 4 to account for the number of C-atoms that are labeled.

Whole-body threonine oxidation was then calculated by using the following equation:

(7)

Statistical analysis
The data are expressed as the mean ± SD values obtained from samples taken over the last hour of each tracer infusion. We used SPSS statistical software (version 14.0; SPSS Inc, Chicago, IL) to analyze the data. Statistical comparisons were performed by using a paired Student's t test. P < 0.05 was taken as statistically significant.

	RESULTS

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All infants were appropriate for gestational age (mean gestational age: 29 ± 2 wk; Table 1). Seven patients were studied during both periods; in 5 of those infants, whole-body threonine oxidation was determined. During both feeding periods, 1 infant underwent mechanical ventilation, 4 infants received supplemental oxygen via a nasal prong, and 3 infants received nasal continuous positive airway pressure (CPAP), which did not allow us to obtain expired air. All infants received caffeine and were clinically stable at the time of the study. All routine blood chemistry and hematology tests (ie, electrolytes, calcium, glucose, acid base, hematocrit, thrombocyte count, and white blood cell count) were within normal limits; there were no significant changes in these variables in the periods 24 h before and after the study. Intakes of threonine, protein, carbohydrate, fat, and energy are shown in Table 2. We aimed to keep the different macronutrient intakes very close together during both study periods, but this was impossible because different feeds (parenteral nutrition and preterm formula) were used. No differences were found in the intakes of glucose, fat, or energy. The threonine intake was significantly lower during period A, but, in absolute amounts, the difference was only 5 µmol/(kg/d) (ie, 8% less). The protein intake during period B was significantly lower than that during period A, which was inevitable, because we aimed at comparable total threonine intakes.

Figure 2

View larger version (5K): [in this window] [in a new window]   FIGURE 2.. Mean (±SD) isotopic enrichments of plasma threonine at baseline and plateau during period A (A) and period B (B). , [U-13C]threonine; , [15N] threonine.

Figure 3

View larger version (6K): [in this window] [in a new window]   FIGURE 3.. Mean (±SD) isotopic steady state in 13CO2 excretion in expiratory air during the [13C]sodium bicarbonate infusion (0–120 min) and the [U-13C]threonine infusion (120–420 min). A, period A; B, period B; APE, atom percent excess.

Table 3

Figure 4

View larger version (10K): [in this window] [in a new window]   FIGURE 4.. First-pass threonine uptake in 8 preterm infants during periods A and B expressed in absolute amounts [µmol/(kg/h)] and as a fraction of dietary intake. APE, atom percent excess. *,**Significant difference between periods: *P < 0.01, **P < 0.0001.

Table 4

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

Therefore, the most striking observation to emerge from this study was the very high fractional first-pass threonine uptake by the intestine in the first week of life in preterm infants during a restricted enteral intake. This observation indicates a high obligatory visceral need for threonine in neonates. Although the gastrointestinal tissues represent only 5% of body weight, because of their high rates of metabolism, they account for 15%–35% of whole-body oxygen consumption and protein turnover (22–24). Of the many factors that affect neonatal gut growth and adaptation, probably the most physiologically significant stimulus is enteral nutrition (25, 26). Enteral feeding acts directly by supplying nutrients for the growth and mucosal metabolism of epithelial cells. In the present study, we show that up to 82% of specific nutrients are utilized in first-pass uptake. We hypothesize that this mainly represents intestinal tissue utilization. We measured first-pass splanchnic uptake, which includes hepatic uptake. However, neonatal animal studies suggest that the intestine is the major site of utilization.

The route of administration of nutrition is a major issue in the clinical care of preterm infants, because of these infants' intolerance of enteral feeding and the associated morbidity (27). To reduce the complications of TPN and to accelerate the adaptation to full enteral feeding, many neonatologists often provide small volumes of enteral nutrition—ie, minimal enteral feeding—in combination with TPN to preterm infants in the first weeks of life (28). Studies in neonatal piglets showed that an enteral intake of 20% is necessary to prevent gut protein loss, whereas an intake of 40% is needed to maintain normal growth (29, 30). In the present study, preterm infants were enterally fed 40% of their total nutrient intake during period A, and the results show the fractional first-pass threonine requirements were significantly higher (82% of dietary intake) than those of fully fed infants (70% of dietary intake). Because the first-pass uptake is upregulated, even at 40% of enteral intake, this finding indicates that the enteral requirement is not yet reached.

The high enteral threonine uptake observed during both partial and full enteral feeding reflects the use of absorbed threonine for the synthesis of secretory glycoproteins, for the synthesis of mucosal cellular proteins, or for oxidative purposes (2–4). In neonatal piglets, we did not find substantial first-pass threonine oxidation, which indicates that enterally absorbed threonine is mostly used for the other 2 metabolic pathways. Although the intestinal mucosa is highly secretory and proliferatory tissue, dietary threonine is not incorporated into constitutive mucosal proteins to a great extent (4). However, the intestinal mucosa is protected by a complex network of glycoproteins, and the core proteins of the highly glycosylated domains of intestinal mucins contain large amounts of threonine (31). It is likely that a significant proportion of the utilized threonine is channeled toward mucin production. These secretory mucins play a key role in the defense of the mucosa. In fact, there is evidence that mucin production is impaired in piglets fed threonine-deficient diets, and supplying threonine parenterally cannot restore normal mucin production (10). Moreover, recent studies suggested that the restriction of dietary threonine significantly and specifically impairs intestinal mucin synthesis and consequently reduces gut barrier function (32, 33). Especially in preterm infants who are vulnerable to infections during the first weeks of life, mucus would be an important aspect of defense.

A second aim of this study was to determine whole-body threonine kinetics in neonates under 2 different feeding circumstances. During both partial and full enteral feeding, whole-body threonine oxidation accounted for 6% of the threonine flux, which is comparable to the fractional oxidation rates found by Darling et al (34) in breastfed preterm infants. However, Parimi et al (35) recently reported a higher fractional oxidation rate of threonine in newborn infants, although the total threonine intake was substantially lower.

In conclusion, the present study showed that the splanchnic tissues of preterm infants use the dietary threonine intake to a substantial degree—ie, more than three-quarters—irrespective of the amount of enteral threonine delivery. Furthermore, our data show that <10% of the threonine flux is oxidized, and the route of feeding does not affect this whole-body threonine oxidation. Overall, we suggest that the major metabolic fate of intestinal utilized threonine is mucosal glycoprotein synthesis.

	ACKNOWLEDGMENTS

 
We thank Professor Doctors Dick Tibboel and Hans A Büller of Erasmus-MC Sophia Children's Hospital for helpful comments and review of the manuscript and Peter Reeds (since deceased), who inspired us to undertake the mission. We especially thank the parents who provided consent for their infants to participate in the study.

The authors' responsibilities were as follows—SRDvdS: recruitment of the participants, blood and breath sample collection, preparation and analysis of the data, and writing of the manuscript; DLW and JH: data analysis; JBvG (principal investigator): study design and supervision; and AV: preparation of stable isotopes. None of the authors had a personal or financial conflict of interest.

	REFERENCES

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 

1. Ehrenkranz RA, Younes N, Lemons JA, et al. Longitudinal growth of hospitalized very low birth weight infants. Pediatrics 1999;104:280–9.[Abstract/Free Full Text]
2. Stoll B, Henry JF, Reeds PJ, Yu H, Yahoor F, Burrin DG. Catabolism dominates the first-pass intestinal metabolism of dietary essential amino acids in milk protein fed piglets. J Nutr 1998;128:606–14.[Abstract/Free Full Text]
3. Van Goudoever JB, Stoll B, Henry JF, Burrin DG, Reeds PJ. Adaptive regulation of intestinal lysine metabolism. Proc Natl Acad Sci U S A 2000;97:11620–5.[Abstract/Free Full Text]
4. Van der Schoor SRD, Van Goudoever JB, Stoll B, et al. The pattern of intestinal substrate oxidation is altered by protein restriction in pigs. Gastroenterology 2001;121:1167–75.[Medline]
5. Benevenga NJ, Radcliffe BC, Egan AR. Tissue metabolism of methionine in sheep. Aust J Biol Sci 1983;36:475–85.[Medline]
6. Van der Schoor SRD, Reeds PJ, Stoll B, et al. The high metabolic cost of a functional gut. Gastroenterology 2002;123:1931–40.[Medline]
7. Schaart MW, Schierbeek H, Van der Schoor SR, et al. Threonine utilization is high in the intestine of piglets. J Nutr 2005;135:765–70.[Abstract/Free Full Text]
8. Roberton AM, Rabel B, Harding CA, Tasman-Jones C, Harris PJ, Lee SP. Use of the ileal conduit as a model for studying human small intestinal mucus glycoprotein secretion. Am J Physiol 1991;261:G728–34.[Medline]
9. Bertolo RF, Chen CZ, Law G, Pencharz PB, Ball RO. Threonine requirement of neonatal piglets receiving total parenteral nutrition is considerably lower than that of piglets receiving an identical diet intragastrically. J Nutr 1998;128:1752–9.[Abstract/Free Full Text]
10. Iiboshi Y, Nezu R, Kennedy M, et al. Total parenteral nutrition decreases luminal mucous gel and increases permeability of small intestine. JPEN J Parenter Enteral Nutr 1994;18:346–50.[Abstract/Free Full Text]
11. Usher RH, McLean F. Intrauterine growth of live-born caucasian infants at sea level: standards obtained from measurements in 7 dimensions of infants born between 25 and 44 weeks of gestation. J Pediatr 1969;74:901–10.[Medline]
12. Rautonen J, Makela A, Boyd H, Apajasalo M, Pohjavuori M. CRIB and SNAP: assessing the risk of death for preterm neonates. Lancet 1994;343:1272–3.[Medline]
13. Perman JA, Barr RG, Watkins JB. Sucrose malabsorption in children; a noninvasive diagnosis by interval breath hydrogen determination. J Pediatr 1978;93:17–22.[Medline]
14. Van der Schoor SRD, De Koning BA, Wattimena DL, Tibboel D, Van Goudoever JB. Validation of the direct nasopharyngeal sampling method for collection of expired air in preterm neonates. Pediatr Res 2004;55:50–4.[Medline]
15. Riedijk MA, Voortman G, Van Goudoever JB. Use of [¹³C]bicarbonate for metabolic studies in preterm infants: intragastric versus intravenous administration. Pediatr Res 2005;58:861–4.[Medline]
16. Chaves Das Neves HJ, Vasconcelos AM. Capillary gas chromatography of amino acids, including asparagine and glutamine: sensitive gas chromatographic-mass spectrometric and selected ion monitoring gas chromatographic-mass spectrometric detection of the N,O(S)-tert. -butyldimethylsilyl derivatives. J Chromatogr 1987;392:249–58.
17. Zuijdgeest-van Leeuwen SD, Van den Berg JW, Wattimena JL, et al. Lipolysis and lipid oxidation in weight-losing cancer patients and healthy subjects. Metabolism 2000;49:931–6.[Medline]
18. Steele R. Influence of glucose loading and of injected insulin on hepatic glucose output. Proc N Y Acad Sci 1959;82:420–30.
19. Tserng K, Kalhan SC. Calculation of substrate turnover rate in stable isotope tracer studies. Am J Physiol Endocrinol Metab 1983;245:E308–11.[Abstract/Free Full Text]
20. Van Goudoever JB, Sulkers EJ, Chapman TE, et al. Glucose kinetics and glucoregulatory hormone levels in ventilated, preterm infants on the first day of life. Pediatr Res 1993;33:553–9.
21. Wu PYK, Edwards N, Storm MC. Plasma amino acid pattern in normal breast fed infants. J Pediatr 1986;109:347–9.[Medline]
22. Edelstone DI, Holzman IR. Oxygen consumption by the gastrointestinal tract and liver in conscious newborn lambs. Am J Physiol 1981;241:G289–93.[Medline]
23. Ebner S, Schoknecht P, Reeds P, Burrin D. Growth and metabolism of gastrointestinal and skeletal muscle tissues in protein-malnourished neonatal pigs. Am J Physiol 1994;266:R1736–43.[Medline]
24. McNurlan MA, Garlick PJ. Contribution of rat liver and gastrointestinal tract to whole-body protein synthesis in the rat. Biochem J 1980;186:381–3.[Medline]
25. Berseth CL, Nordyke C. Enteral nutrients promote postnatal maturation of intestinal motor activity in preterm infants. Am J Physiol 1993;264:G1046–51.[Medline]
26. Rojahn A, Lindgren CG. Enteral feeding in infants <1250 g starting within 24 h post-partum. Eur J Pediatr 2001;160:629–32.[Medline]
27. Berseth CL. Gut motility and the pathogenesis of necrotizing enterocolitis. Clin Perinatol 1994;21:263–70.[Medline]
28. Thureen PJ, Hay WW Jr. Early aggressive nutrition in preterm infants. Semin Neonatol 2001;6:403–15.[Medline]
29. Stoll B, Chang X, Fan MZ, Reeds PJ, Burrin DG. Enteral nutrient intake level determines intestinal protein synthesis and accretion rates in neonatal pigs. Am J Physiol Gastrointest Liver Physiol 2000;279:G288–94.[Abstract/Free Full Text]
30. Burrin DG, Stoll B, Jiang R, et al. Minimal enteral nutrient requirements for intestinal growth in neonatal piglets: how much is enough? Am J Clin Nutr 2000;71:1603–10.[Abstract/Free Full Text]
31. Bengmark S, Jeppsson B. Gastrointestinal surface protection and mucosa reconditioning. JPEN J Parenter Enteral Nutr 1995;19:410–5.[Abstract/Free Full Text]
32. Faure M, Moennoz D, Montignon F, Mettraux C, Breuille D, Ballevre O. Dietary threonine restriction specifically reduces intestinal mucin synthesis in rats. J Nutr 2005;135:486–91.[Abstract/Free Full Text]
33. Law GK, Bertolo RF, Adjiri-Awere A, Pencharz PB, Ball RO. Adequate oral threonine is critical for mucin production and gut function in neonatal piglets. Am J Physiol Gastrointest Liver Physiol 2007;292:G1293–301.[Abstract/Free Full Text]
34. Darling PB, Dunn M, Sarwar G, Brookes S, Ball RO, Pencharz PB. Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein. Am J Clin Nutr 1999;69:105–14.[Abstract/Free Full Text]
35. Parimi PS, Gruca LL, Kalhan SC. Metabolism of threonine in newborn infants. Am J Physiol Endocrinol Metab 2005;289:E981–5.[Abstract/Free Full Text]

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