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,**,
,,3
* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada T6G 2P5;
The Research Institute, The Hospital for Sick Children, Toronto, ON, Canada; Departments of
** Paediatrics and
Nutritional Sciences, University of Toronto, Toronto, ON, Canada M5G 1X8
3To whom correspondence should be addressed. E-mail: ron.ball{at}ualberta.ca.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • branched-chain amino acids • ratio • piglets • total parenteral nutrition • indicator amino acid oxidation
Previously, we determined the total branched-chain amino acid (BCAA3; isoleucine, leucine and valine) requirement during parenteral and enteral routes of feeding in neonatal piglets (1). The mean total BCAA requirement was determined to be 1.53 g/(kg · d) during parenteral feeding, compared with 2.64 g/(kg · d) for enteral feeding, when a fixed ratio of BCAA (1:1.8:1.2; isoleucine/leucine/valine) was provided (1). The ratio among BCAA used in the previous study was based on the NRC requirements for swine (2), for piglets weighing between 1 and 5 kg. The NRC recommendations were based on oral feeding studies and therefore may not be appropriate for different routes of feeding.
Feeding by total parenteral nutrition (TPN) by-passes first-pass metabolism by the small intestine and liver; therefore, nutrients are provided to the peripheral organs in different concentrations compared with enteral feeding. Thus, the route of feeding alters whole-body nitrogen metabolism (3), and organ and plasma amino acid concentrations (4). Premature neonates and low-birth-weight infants often receive TPN as an essential component of nutritional support, and evidence is accumulating that current TPN solutions are inadequate (5). The previous study (1), which showed that total BCAA requirement were significantly different between parenteral and enteral nutrition, further strengthens this evidence. In addition, the optimum ratio of BCAA might also be different and specific for each route of nutrient supply; however, this has not been investigated to the authors’ knowledge.
BCAA-enriched TPN and enteral nutrition have been used as therapeutic agents in various catabolic states such as hepatic encephalopathy, chronic renal failure, muscle protein wasting, trauma and sepsis (6). In premature neonates, BCAA-enriched TPN was shown to decrease apnea and improve respiratory pattern and function (7). In spite of the numerous studies conducted on BCAA supplementation, clinical results have remained inconclusive and controversial (8,9). One of the reasons for such varied outcomes of BCAA supplementation could be an inappropriate ratio among BCAA being used in TPN and enteral nutrition.
BCAA have been shown to exhibit antagonism whereby excessive intakes of any one BCAA, especially leucine, will have an effect on the fate and utilization of the other two BCAA, resulting in altered blood and tissue amino acid concentrations (10). Antagonism among BCAA results in severe growth impairment and food intake depression in young growing animals (11). Thus, determining the optimum ratio among BCAA is crucial to achieve our long-term goal of defining the "optimal" profile for neonatal TPN, which provides all amino acids in the balance necessary to maximize protein synthesis and with minimal excess.
In the present study, our objective was to determine whether the optimum ratio among BCAA would differ between the parenteral and enteral routes of feeding in neonatal piglets using the indicator amino acid oxidation (IAAO) method. The change in oxidation of phenylalanine was measured after supplementation of isoleucine, leucine or valine to meet 100% of requirement to a diet that provided 75% of total BCAA requirement for parenteral and enteral routes, respectively, as determined previously (1). The addition of the most limiting amino acid among the three BCAA was hypothesized to decrease significantly the oxidation of the indicator amino acid, phenylalanine.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1.5 kg and, 1–2 d old were transferred to the Metabolic Research Facility at the University of Alberta. The piglets were weighed and anesthetized (0.8% halothane) for the surgical implantation of catheters (Ed-Art, Don Mills, Canada). Venous catheters were placed following modified procedures of Wykes et al. (12) and Rombeau et al. (13) for gastric catheters. The surgical procedures were recently described by Bertolo et al. (14). After surgery, the pigs were fitted with adjustable cotton jackets, which prevent tangling and occlusion of the catheters. The laboratory conditions and piglet housing were described previously (12).
Diet regimen.
The composition of the elemental and complete diet was based on the initial formulation by Wykes et al. (12), with modifications. Diet was infused (continuous, 24 h) using infusion pumps via a tether-swivel system (Alice King Chatham Medical Arts, Los Angeles, CA) intravenously (Expt. 1, parenteral BCAA ratio) or intragastrically (Expt. 2, enteral BCAA ratio). At full infusion rate [272 mL/(kg · d)], the complete diet provided 1.1 MJ available energy/(kg · d) and 15.3, 27.4, 9.4 g of amino acids, glucose and fat/(kg · d), respectively. The base amino acid profile of the complete diet was (mg/g total L-amino acids): alanine, 104; arginine, 78; aspartate, 60; cysteine, 14; glutamate, 103; glycine, 24; histidine, 31; isoleucine, 45; leucine, 103; lysine, 56; methionine, 19; phenylalanine, 40; proline, 82; serine, 32; taurine, 5; threonine, 52; tryptophan, 21; tyrosine, 27; valine, 52. The addition of individual BCAA to make the test diets is described below. The phenylalanine concentration of the diet was increased compared with that used previously (1) to obtain a larger difference in oxidation between the dietary treatments, based on preliminary data. All piglets were administered the complete diet intravenously at 50% of the full rate for 6 h after surgery, then at 75% overnight. For Expt. 1, the diet was then infused at full rate intravenously, whereas for Expt. 2, the piglets were switched to 50% of full rate intragastric feeding on d 1 and increased to the full intragastric feeding by d 2.
Test diets. On d 5, all piglets received diets that provided total BCAA at 75% of parenteral [1.1g/(kg · d)] and enteral [2.0 g/(kg · d)] requirements, as determined previously by IAAO studies (1). The ratio among the three BCAA was 1:1.8:1.2, (isoleucine/leucine/valine) based on NRC recommendations (2). This ratio is also very similar to that found in milk protein (1). An oxidation study was conducted on d 6 and the piglets were returned to the complete diet for 24 h. At 2100 h on d 7, the piglets were randomly assigned to one of the three test diets [+ isoleucine, +leucine, +valine] (Table 1), which provided isoleucine, leucine or valine at 100% of requirement. The 100% requirement value was 1.53 g/(kg · d) for Expt. 1 and 2.64 g/(kg · d) for Expt. 2, as previously determined (1). L-Aspartate, L-serine and L-glycine were used to make the test diets isonitrogenous.
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16–18 h after the start of the test diet infusion. A 30-min period was allowed for the piglets to acclimate and the CO2 to equilibrate in the chamber; then, phenylalanine flux and oxidation were determined by a primed [186 kBq (5 µCi)/kg], constant infusion [130 kBq (3.5 µCi)/(kg · h)] of a tracer solution containing 92.5 MBq (2.5 mCi)/L of L-[1-14C]phenylalanine (American Radiolabeled Chemicals, St. Louis, MO). Air was drawn from the boxes by a pump and the total amount of 14CO2 expired was trapped in a series of gas washing bottles containing CO2 absorber (ethanolamine and ethylene glycol monomethylether, 1:2, v/v). Blood samples (1.5 mL) were drawn at time 0 and every 0.5 h during the 4-h study. The blood samples were centrifuged (3000 x g for 5 min), and plasma collected and stored at -80°C until analysis of phenylalanine specific radioactivity (SRA) and amino acid concentrations. On d 8, a 5-h study was conducted; h 1 was used for breath collection to determine baseline radioactivity before the primed, constant infusion. Immediately upon completion of the oxidation study on d 8, the piglets were injected with a lethal dose (750 mg) of sodium pentobarbital through the venous sampling line. The rate of expiration of 14CO2 was determined by liquid scintillation counting of radioactivity in the CO2 absorber. The plasma concentrations of amino acids and SRA of plasma phenylalanine were analyzed by reversed-phase HPLC; collection and liquid scintillation counting of radioactive fractions were described previously (15). Phenylalanine intake, flux, the percentage of dose oxidized and oxidation were calculated as described previously (15).
Statistical analyses. Each experiment was a fully randomized design. Phenylalanine kinetics and plasma concentrations were analyzed as a one-way ANOVA with diet as the main effect within each route of feeding (n = 12). Differences between d 6 and 8 for oxidation of L-[1-14C]phenylalanine as a percentage of the dose oxidized were analyzed as a 2 x 3 factorial ANOVA with interaction (n = 24), where factor 1 was the route of feeding (parenteral or enteral) and factor 2 was the test diet (+isoleucine, +leucine or +valine). The PROC GLM procedure was employed (SAS/STAT version 8.01, SAS Institute, Cary, NC) and least-square means were considered significantly different at P < 0.05.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiment 1: parenteral BCAA ratio.
Phenylalanine flux [330.4 ± 42.3 µmol/(kg · h)] and phenylalanine intake [121 ± 2.9 µmol/(kg · h)] did not differ (P > 0.05) between 75% BCAA diets and test diets. Supplementation of isoleucine, (+isoleucine), leucine (+leucine) or valine (+valine) to the parenteral 75% BCAA diet decreased phenylalanine oxidation, as a percentage of dose oxidized, calculated within individual pigs (Figs. 1, 2and 3). The percentage decrease (
) in oxidation within pigs fed diets providing 75% of parenteral BCAA requirement compared with +isoleucine and +valine diets was 12.6 and 6.6% respectively (P < 0.05) (Fig. 4). Supplementation of leucine (+leucine), did not affect (P > 0.05) phenylalanine oxidation (2%) (Fig. 4).
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) in oxidation from 75% BCAA diets (Fig. 4), the +valine diet tended to decrease (P = 0.08) phenylalanine oxidation (2.4%; Fig. 3). Changes in plasma BCAA concentrations were observed in the enterally fed piglets. The +isoleucine diet increased plasma isoleucine concentration significantly (P < 0.05) (Table 3), but had no effect (P > 0.05) on leucine and valine concentrations. For the +leucine diet, plasma isoleucine concentration decreased (P < 0.05) and plasma leucine concentration increased (P < 0.05). Plasma valine concentrations tended to decrease (P = 0.067). In general, all three BCAA were within normal plasma concentrations for enterally fed piglets (1). In the +valine diet, plasma isoleucine concentration decreased significantly (P < 0.05) (Table 3). Leucine and valine plasma concentrations were not (P > 0.05) affected. No changes were observed in the rest of the plasma amino acids.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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56% of the enteral requirement. The ratio among the BCAA in the previous study was kept constant at 1:1.8:1.2 (isoleucine/leucine/valine), based on the NRC recommendations for swine (2). Although we supplied a constant ratio of BCAA in the diet, we observed marked differences in plasma concentrations of the BCAA due to the route of feeding. This suggested that the appropriate ratio of BCAA might be different and specific for each route of feeding. We decided to use the IAAO method to determine whether the ratio among the BCAA used was appropriate for enteral and parenteral feeding. The IAAO method was used successfully in pigs and humans to determine the amino acid requirements of indispensable amino acids (16,17). The primary assumption of the technique is that the partitioning of any indispensable amino acid between oxidation and protein synthesis is sensitive to the most limiting amino acid provided in the diet (18). Thus, when the most limiting indispensable amino acid is supplied, protein synthesis will increase and oxidation of all amino acids including the indicator amino acid will decrease (Fig. 5). This concept was applied to the present experiment by providing a deficient intake of all of the BCAA in a fixed ratio and then supplementing each BCAA individually. All other indispensable amino acids were provided in excess of their requirement. Therefore, the BCAA that reduces the oxidation of the indicator must be the limiting amino acid.
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In Expt. 1 (parenteral BCAA ratio), significant decreases (P < 0.05) in the percentage of dose oxidized and were observed in phenylalanine oxidation from 75% BCAA diets due to addition of isoleucine (12.6%) and valine (6.6%) (Figs. 1and 3), whereas the supplementation of leucine had no effect (2%) (Fig. 2). Thus, the most limiting BCAA for protein synthesis during parenteral nutrition under the current ratio of 1:1.8:1.2, isoleucine/leucine/valine is isoleucine, followed by the second limiting amino acid, valine. Parenteral nutrition by-passes first-pass metabolism by the splanchnic organs, and thus nutrients including amino acids are provided to the peripheral organs at different concentrations than via the oral route. The current ratio among BCAA is based on oral requirement studies and the pattern of BCAA in reference proteins such as human milk (1). The results from the current study indicate that during parenteral nutrition, the ratio of 1:1.8:1.2 of isoleucine/leucine/valine is inappropriate.
Supplementation of individual BCAA to the parenteral test diet resulted in the expected increase in the plasma concentration of the BCAA that was added, for isoleucine and for valine; however, there was no increase in plasma leucine when the intake of parenteral leucine was increased. The addition of parenteral isoleucine to the 75% BCAA diet increased plasma leucine almost fivefold, presumably due to a reduction in leucine oxidation. Further, the decrease in plasma concentration of lysine and threonine observed when isoleucine was supplemented in the diet supports the interpretation that isoleucine was first limiting.
To the authors’ knowledge, the current experiment is the only study examining the optimum ratio of BCAA during parenteral nutrition in pigs. Iwasawa et al. (19) examined the optimal ratio of individual BCAA in injured rats being fed parenterally. They maintained isoleucine/valine at a ratio of 1:1 and supplemented leucine at 0.5, 1, 2 and 4 on a molar ratio basis and measured nitrogen balance, urinary 3-methyl-histidine concentrations and plasma-free amino acids over a period of 7 d. They observed no changes in mean cumulative 7-d nitrogen balance and 3-methyl-histidine excretion. Infusion of BCAA solutions of 1:0.5:1 and 1:4:1 (isoleucine/leucine/valine) significantly changed plasma BCAA concentrations from preinfusion values and BCAA solutions of 1:1:1 and 1:2:1 (isoleucine/leucine/valine) tended to allow BCAA concentrations to approach preinfusion values. The researchers concluded that the optimal ratio of BCAA during parenteral nutrition in the injured rat model lies between 1:1:1 and 1:2:1. Bonau et al. (20) examined the relationship between composition and efficacy of enriched BCAA amino acid solutions in postoperative adult surgical patients receiving parenteral nutrition. Three BCAA solutions, one with a ratio of 1:1.28:1.2, but 25% BCAA enriched, and the other two at ratios of 1:0.24:1.6 and 1:2:1.1 (isoleucine/leucine/valine) at 45% BCAA enrichment of total amino acids were tested over a period of 7 d. Whole-body protein kinetic studies conducted using infusion of 15N-glycine on d 3 and 4 postoperative indicated no significant changes in whole-body protein flux, but protein catabolism was significantly higher in the group infused 1:0.24:1.6, followed by the 1:2:1.1 group and lowest in the 1:1.28:1.2 group. In addition, nitrogen balance was highest in the group infused with 1:1.28:1.2 (isoleucine/leucine/valine). As observed in the current study, both the previously mentioned studies indicated that the appropriate ratio among BCAA during parenteral nutrition is closer to 1:1:1 of isoleucine/leucine/valine.
In Expt. 2 (enteral BCAA ratio), phenylalanine oxidation (the percentage of the dose oxidized and ) did not change significantly due to individual addition of any of the BCAA (Figs. 1234). This demonstrates that during enteral feeding, all three BCAA were co-limiting and thus no change in IAAO was observed. The ratio of 1:1.8:1.2 of isoleucine/leucine/valine used in the initial total BCAA requirement study (1) is appropriate during enteral feeding in neonatal piglets. The NRC (2) recommended BCAA ratio was derived predominantly from oral feeding studies and is thus optimal.
We recently reported a similar study in orally fed adult men in which we demonstrated that valine was the most limiting BCAA amino acid in egg protein (21). In the current study, although the addition of valine was not significant (P = 0.08), nonetheless there was a numeric reduction in phenylalanine oxidation, which is consistent with our human studies. The pattern of BCAA used in the current piglet study is similar to human milk, rather than egg, and the proportional content of valine is higher in milk than in egg. Based on a comparison of the present study with that of Riazi et al. (21), it appears that the BCAA balance in milk protein is more ideal for enteral feeding than the pattern of BCAA in egg protein.
In the enterally fed piglets, supplementation of leucine to the 75% BCAA diet resulted in plasma isoleucine concentrations that were 10 times lower, but there was no effect on valine (Table 3). Similarly, supplementation of valine yielded plasma isoleucine concentrations that were 3 times lower. The addition of leucine and valine to the 75% enteral BCAA diet appeared to antagonize isoleucine metabolism. In orally fed adult humans, Pelletier et al. (22,23) examined BCAA interactions and the relative effect of each on amino acid requirement. In the first set of experiments (22), the authors examined valine metabolism at different leucine intakes, keeping isoleucine constant. In the second set of experiments (23), leucine metabolism was examined at different valine and isoleucine intakes. Their results indicated that within the range of intakes studied, the interactive effect on the remaining BCAA was minimal, and it did not interfere with the requirement values for leucine or valine. In the current study we examined the relative change in phenylalanine oxidation and plasma concentrations using diets formulated to supply 75%, a deficient intake of the mean requirement for total BCAA. Although there was no difference in IAAO, meaning that the three BCAA were co-limiting for protein synthesis, the plasma concentrations of isoleucine suggested that some form of antagonism was occurring.
Phenylalanine oxidation as a percentage of dose was higher in the parenterally fed piglets compared with enterally fed pigs, although the route of isotope infusion was parenteral in both groups. This suggests that during enteral feeding, there is a greater partitioning of phenylalanine to protein synthesis during first pass by the splanchnic tissues. Phenylalanine utilization by the gut and liver was also reported by Stoll and colleagues (24) during enteral infusion of 13C algal protein. In our previous studies of threonine (14) and methionine (25) requirements, we observed similar differences in phenylalanine oxidation due to route of feeding. Further, to determine whether the route of isotope infusion has an effect on the IAAO requirement estimates, we determined tryptophan requirements during enteral and parenteral routes of 1-14C-phenylalanine infusion (26) and found no influence on the requirement estimation due to the route of isotope infusion.
The explanation for the large differences in plasma valine and isoleucine, but not leucine concentration, between the enteral and parenteral groups probably involves the BCAA-catabolizing enzymes. In addition to the general branched-chain amino transferase (BCAT) and branched-chain dehydrogenase (BCDH) complex, leucine-specific BCAT and BCDH were reported in various tissues and species (11); this is probably one of the reasons why leucine utilization rate is greater compared with valine and isoleucine. Leucine was also reported to activate BCDH activity in rat tissues (27). We observed previously that with graded supplementation of total BCAA, plasma leucine concentrations were consistently lower (1) compared with plasma isoleucine and valine concentrations. The BCAT isozyme for valine has a higher Km than the isozymes for leucine and isoleucine, resulting in a lower rate of clearance from the plasma pool (28). Further, Staten et al. (28) also reported that valine concentrations were high in plasma, but ketoisovalerate concentrations were half of ketoisocaproate concentrations in plasma. Thus it appears that although BCDH may be the enzyme regulating the BCAA decarboxylation step, the transamination step regulated by BCAT may regulate individual plasma BCAA concentrations. Information on BCAA-catabolizing enzymes is extensive for liver and muscle tissues, but scarce for the intestinal cells. The previous study (1) and the present study clearly indicate a significant influence by the gut on BCAA metabolism; further studies are warranted to examine the activity and regulation of BCAA enzymes in the small intestine.
In conclusion, the current study combined with the previous study (1) clearly demonstrates that not only does the total BCAA requirement differ between the two routes of feeding, parenteral and enteral, but the optimal ratio among the BCAA also differs. The currently used ratio of 1:1.8:1.2 (isoleucine/leucine/valine) is inappropriate during parenteral nutrition, with isoleucine being first limiting and valine second limiting for protein synthesis. We suggest that an improved ratio for parenteral feeding would be 1:1:1 (isoleucine/leucine/valine). During enteral nutrition, the ratio found in milk protein of 1:1.8:1.2 (isoleucine/leucine/valine) is close to optimal. Thus future studies in BCAA supplementation must be conducted with the appropriate ratio of BCAA suited for each route of feeding.
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FOOTNOTES |
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2 Supported by Alberta Pork, Alberta Agricultural Research Institute (AARI), Canadian Institutes of Health Research (CIHR) and Degussa AG.
4 Abbreviations used: BCAA, branched-chain amino acids; BCAT, branched-chain amino transferase; BCDH, branched-chain dehydrogenase; IAAO, indicator amino acid oxidation; SRA, specific radioactivity; TPN, total parenteral nutrition.
Manuscript received 1 August 2003. Initial review completed 11 September 2003. Revision accepted 7 October 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Elango, R., Pencharz, P. B. & Ball, R. O. (2002) The branched-chain amino acid requirement of parenterally fed neonatal piglets is less than the enteral requirement. J. Nutr. 132:3123-3129.
2. National Research Council (1998) Nutrient Requirements for Swine 10th ed. 1998 National Academy Press Washington, DC.
3. Duffy, B. & Pencharz, P. B. (1986) The effect of feeding route (IV or oral) on the protein metabolism of the neonate. Am. J. Clin. Nutr. 43:108-111.
4. Bertolo, R.F.P., Pencharz, P. B. & Ball, R. O. (2000) Organ and plasma amino acid concentrations are profoundly different in piglets fed identical diets via gastric, central venous or portal venous routes. J. Nutr. 130:1261-1266.
5. Brunton, J. A., Ball, R. O. & Pencharz, P. B. (2000) Current total parenteral nutrition solutions for the neonate are inadequate. Curr. Opin. Clin. Nutr. Metab. Care 3:299-304.[Medline]
6. Lal, H. & Chugh, K. (1995) Metabolic and regulatory effects of branched-chain amino acid supplementation. Nutr. Res. 15:1717-1733.
7. Blazer, S., Reinersman, G. T., Askanazi, J., Furst, P., Katz, D. P. & Fleischman, A. R. (1994) Branched-chain amino acids and respiratory pattern and function in the neonate. J. Perinatol. 4:290-295.
8. Teasley, K. M. & Buss, R. L. (1989) Do parenteral nutrition solutions with high concentrations of branched-chain amino acids offer significant benefits to stressed patients?. Ann. Pharmacother. 23:411-416.[Abstract]
9. Vente, J. P., Soeters, P. B., von Meyenfeldt, M. F., Rouflart, M.M.J., van der Linden, C. J. & Gouma, D. J. (1991) Prospective randomized double blind trial of branched-chain amino acid versus standard parenteral nutrition solutions in traumatized and septic patients. World J. Surg. 15:128-133.[Medline]
10. Block, K. P. (1989) Interactions among leucine, isoleucine and valine with special reference to the branched-chain amino acid antagonism. Friedman, M. eds. Absorption and Utilization of Amino Acids 1:229-244 CRC Press Boca Raton, FL. .
11. Harper, A. E., Miller, R. H. & Block, K. P. (1984) Branched chain amino acid metabolism. Annu. Rev. Nutr. 4:409-454.[Medline]
12. Wykes, L. J., Ball, R. O. & Pencharz, P. B. (1993) Development and validation of a total parenteral nutrition model in the neonatal piglet. J. Nutr. 123:1248-1259.
13. Rombeau, J. L., Barot, L. R., Low, D. W. & Twomey, P. L. (1984) Feeding by tube enterostomy. Rombeau, J. L. Caldwell, M. D. eds. Clinical Nutrition, Vol. 1: Enteral and Tube Feeding 1984:275-285 W. B. Saunders Philadelphia, PA. .
14. Bertolo, R.F.P., Chen, C.Z.L., Law, G., Pencharz, P. B. & Ball, R. O. (1998) Threonine requirement of neonatal piglets receiving total parenteral nutrition is considerably lower than that of piglets receiving an identical diet intragastrically. J. Nutr. 128:1752-1759.
15. House, J. D., Pencharz, P. B. & Ball, R. O. (1997) Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine. Am. J. Clin. Nutr. 65:984-993.
16. Brunton, J. A., Ball, R. O. & Pencharz, P. B. (1998) Determination of amino acid requirements by indicator amino acid oxidation method: applications in health and disease. Curr. Opin. Clin. Nutr. Metab. Care 1:449-453.[Medline]
17. Pencharz, P. B. & Ball, R. O. (2003) Different approaches to define individual amino acid requirements. Annu. Rev. Nutr. 23:101-116.[Medline]
18. Zello, G. A., Wykes, L. J., Ball, R. O. & Pencharz, P. B. (1995) Recent advances in methods of assessing dietary amino acid requirements for adult humans. J. Nutr. 125:2907-2915.
19. Iwasawa, Y., Kishi, T., Morita, M., Ikeda, K., Shima, H. & Sato, T. (1991) Optimal ratio of individual branched-chain amino acids in total parenteral nutrition of injured rats. J. Parenter. Enteral Nutr. 15:612-618.[Abstract]
20. Bonau, R. A., Ang, S. D., Jeevanandam, M & Daly, J. M. (1984) High branched-chain amino acid solutions: relationship of composition to efficacy. J. Parenter. Enteral Nutr. 8:622-627.[Abstract]
21. Riazi, R., Rafii, M., Wykes, L. J., Ball, R. O. & Pencharz, P. B. (2003) Valine may be the first limiting branched-chain amino acid in egg protein, in men. J. Nutr. 133:3533-3539.
22. Pelletier, V., Marks, L., Wagner, D. A., Hoerr, R. A & Young, V. R. (1991) Branched-chain amino acid interactions with reference to amino acid requirements in adult men: valine metabolism at different leucine intakes. Am. J. Clin. Nutr. 54:402-407.
23. Pelletier, V., Marks, L., Wagner, D. A., Hoerr, R. A & Young, V. R. (1991) Branched-chain amino acid interactions with reference to amino acid requirements in adult men: leucine metabolism at different valine and isoleucine intakes. Am. J. Clin. Nutr. 54:395-401.
24. Stoll, B., Henry, J., Reeds, P. J., Yu, H., Jahoor, F. & Burrin, D. G. (1998) Catabolism dominates the first pass intestinal metabolism of dietary essential amino acids in milk protein fed piglets. J. Nutr. 128:606-614.
25. Shoveller, A. K., Brunton, J. A., Pencharz, P. B. & Ball, R. O. (2003) The methionine requirement is lower in neonatal piglets fed parenterally than in those fed enterally. J. Nutr. 133:1390-1397.
26. Cvitkovic, S., Pencharz, P. B. & Ball, R. O. (2000) Comparison of oral and intravenous isotopic tracers determining tryptophan requirement of piglets. FASEB J. 14:A745 (abs).
27. Frick, G. P., Tai, L., Blinder, L. & Goodman, H. M. (1980) L-Leucine activates branched-chain ketoacid dehydrogenase in rat adipose tissue. J. Biol. Chem. 256:2618-2620.
28. Staten, M. A., Bier, D. M. & Matthews, D. E. (1984) Regulation of valine catabolism in man: a stable isotope study. Am. J. Clin. Nutr. 40:1224-1234.
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