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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, Canada; and 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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69% of the enteral requirement and suggest that extraction of SAA by first-pass splanchnic metabolism may be responsible for this difference.
KEY WORDS: • amino acid requirements • methionine • cysteine • total parenteral nutrition • indicator amino acid oxidation • piglets
Due to immaturity and congenital abnormalities, neonates frequently require parenteral feeding (1
). Part of the goal of parenteral feeding is to provide essential and nonessential nutrients at a level sufficient to meet maintenance and growth requirements while avoiding an excessive supply, which may put undue stress on these neonates’ immature organ systems (1
,2
). Presently, the amino acid profile of most parenteral solutions is based upon reference proteins such as egg protein or human breast milk (3
). This may not be the correct profile when the gut is atrophied and has a lower metabolic demand (3
). Wykes et al. (4
) demonstrated significant differences in amino acid kinetics [flux, oxidation, synthesis (nonoxidative disposal), and release from protein] between oral and parenteral feeding in neonatal infants. We demonstrated recently that the threonine requirement is 55% lower (5
) and the branched-chain amino acid requirement is 44% lower (6
) in neonatal piglets fed parenterally than in those fed enterally. In addition, we measured the tryptophan requirement in parenterally and enterally fed neonatal piglets and found that it did not differ between the two groups (unpublished data). This suggests that some amino acids are completely absorbed into the portal vein and not metabolized by the splanchnic tissues, that differences in the requirement cannot be detected or that the source of amino acids for intestinal protein synthesis is different. Although the reduced intestinal protein synthesis that is associated with parenteral feeding may account for a proportion of these reductions in amino acid requirements, this alone is not sufficient to account for the observed differences in parenteral and enteral amino acid requirements. Therefore, both parenteral and enteral amino acid requirements must be determined empirically to ensure that they are supplied in an optimum profile.
No reports were found in the literature in which the methionine requirement was determined directly in piglets fed intravenously. Most studies in the parenterally fed neonate have examined the possibility of the essentiality of cysteine and taurine but not the requirement for methionine (7 –10 ). However, there are studies that have determined the methionine requirement in orally fed growing (>5kg body weight) and mature animals by measuring growth, nitrogen balance, plasma free amino acids, plasma or blood urea nitrogen (11 ,12 ). Other researchers have used the ileal digestibility assay (13 ) and to our knowledge, only one researcher has used an isotopic technique to measure the sulfur amino acid (SAA) requirement in young pigs (14 ). Because there is no evidence that cysteine synthesis is limited in neonatal piglets, in the present study, only methionine was used to supply total sulfur amino acids (TSAA).
Classical methods of determining indispensable amino acid requirements, such as nitrogen balance and the direct tracer approach, require prolonged adaptation and are therefore unsuitable for use in human neonates (3 ,15 ). For this reason, we developed a piglet model for the human neonate (16 ). We also validated the novel and minimally invasive technique of indicator amino acid oxidation (IAAO) as a means of determining indispensable amino acid requirements in piglets fed enterally or parenterally (5 ,3 ). In subsequent studies in human neonates, we showed that findings in the piglet model are valid in parenterally fed human infants (17 ). In the present study, the objective was to determine the methionine requirement in a neonatal piglet model receiving total parenteral (TPN) (n = 18) or enteral nutrition (n = 14) by employing the IAAO technique using L-[1-14C]phenylalanine. We hypothesized that the parenteral methionine requirement would be significantly lower than the enteral requirement.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The Animal Care Committee at the University of Alberta approved all procedures in this study. A total of 25 male and 7 female Landrace/Large White piglets (Genex Swine Group) were obtained from the University of Alberta Swine Unit (Edmonton, Canada) at a mean age of 1.7 d and a mean weight of 1.66 kg. Piglets were transported to the Metabolic Unit at the University of Alberta. The piglets were weighed and then anesthetized with acepromazine (0.5 mg/kg; Atravet; Ayerst Laboratories, Montreal, Canada) and ketamine hydrochloride (22 mg/kg; Rogarsetic Rogar STB, Montreal, Canada) and maintained during surgery with 1% halothane. All piglets were then fitted with venous catheters (Ed-Art, Don Mills, Canada) using the modified methods of Wykes et al. (16 ); in enterally fed piglets, gastric catheters were inserted using the method of Rombeau et al. (18 ). In all pigs, an infusion catheter was inserted into the left jugular vein and advanced to the superior vena cava just cranial to the heart and a sampling catheter was inserted into the left femoral vein and advanced to the inferior vena cava just caudal to the heart. After surgery, incision sites were treated with a topical antibiotic (Hibitane Veterinary Ointment: Ayerst Laboratories), and an analgesic (0.1 mg/kg Buprenex, Buprenorphrine HCl, Reckitt and Colman Pharmaceutical, Richmond, VA) was given intramuscularly. Analgesic was given again 8 and 16 h postsurgery. Piglets were then put into cotton jackets, which secure the tether to the piglets. The tether is part of the swivel- tether system (Alice King Chatham Medical Arts, Los Angeles, CA), which enables the pigs to move freely while receiving a continuous dietary infusion and ensuring that the catheters to do not become tangled and occluded.
Animal housing.
The piglets were kept in rooms between 21 and 27°C, with supplemental heat from heat lamps provided for each individual cage. Lighting was on a 12-h light:dark schedule. Each piglet was placed in an individual circular cage, 75 cm in diameter with toys to enhance their environment.
Diet regimen.
Elemental solutions were designed to meet the requirements of neonatal piglets (16
). Diets were administered using pressure-sensitive infusion pumps. Piglets received 15 g amino acids/(kg · d) and 1.1 MJ metabolizable energy/(kg · d) with glucose and lipid (Intralipid 20%, PharmaciaUpjohn, Stockholm, Sweden), each supplying 50% of nonprotein energy intake. The base amino acid profile of the complete elemental diet fed from d 0 to 5 is described in Table 1. The amino acid profile was based on human milk protein (Vaminolact: Fresenius-Kabi) except phenylalanine and tyrosine, which were provided at their estimated safe levels of intake (19
,20
), and arginine provided at 1.2 g/(kg · d) (21
). Tyrosine was provided as the dipeptide glycyl-tyrosine (16
). Full infusion rates [272 mL/(kg · d)] were adjusted on a weight basis so that energy and nitrogen intake were identical for all piglets. All vitamins were supplied in a commercial solution, MVI Pediatric (Rhone-Poulenc Rorer Canada, Montreal, Canada), which provides a combination of oil- and water-soluble vitamins, formulated especially for incorporation into intravenous solutions. The cofactors involved in the transsulfuration pathway, vitamin B-12, choline, vitamin B-6 and folate, were in the MVI solution at 115% of requirement (22
). Piglets also received a mineral solution including zinc, copper, manganese, chromium, selenium and iodide at 200% of the NRC (22
) recommendation for piglets.
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Tracer infusion, 14CO2 collection and analytical procedures.
Phenylalanine oxidation and flux were determined by a primed [186 kBq (5 µCi/kg)], constant intravenous infusion [130 kBq (3.5 µCi/(kg · h)] of a tracer solution containing 92.8 MBq (2.5 mCi)/L of L-[1-14C]phenylalanine [200 MBq (54 mCi/mmol; American Radiolabeled Chemicals, St. Louis, MO]. To reach a plateau in both blood and breath labeling, the duration of constant infusion was 4 h. Details of infusion protocol, 14CO2 collection and blood collection procedures were described previously (19 ). After the infusion on d 8, piglets were killed by injection of 1000 mg of sodium pentobarbital into a venous catheter.
Reverse-phase HPLC with the use of phenylisothiocyanate derivatives was used to measure plasma amino acids. Collection and liquid scintillation counting of radioactive fractions to determine specific radioactivity (SRA) of plasma phenylalanine and tyrosine were completed as previously described (19 ). The calculations for intake, oxidation, flux, nonoxidative disposal, release from protein breakdown, balance and the percentage of dose oxidized were as reported previously (19 ). SRA for both plasma phenylalanine and tyrosine during the IAAO study were plotted and plateau values were calculated as the mean SRA at plateau. Plasma concentrations are represented as the mean concentration at each test methionine level.
Statistical analyses.
This was a completely randomized design, with methionine intake serving as the main treatment effect. The effects of day of IAAO study (d 6 or 8), gender of pig, initial weight and weight at study were determined not to be significant using an ANOVA (SAS/STAT, version 8.1, SAS Institute, Cary, NC). If P-values were <0.05 for the F-value of the ANOVA model, significant differences between treatments were assessed using the Student-Neuman-Keuls multiple comparison procedure. Determination of methionine requirement was performed using a two-way linear crossover model, as described previously (23 ,24 ). Regression analysis variables were dietary concentration of methionine as the independent variable and percentage of dose oxidized and phenylalanine oxidation as the dependent variable. The 95% confidence intervals (CI), for the estimation of a safe level of intake, were also determined.
To compare the breakpoints or requirement estimates between parenterally and enterally fed piglets, we treated the breakpoint as a sample mean and used the pooled two-sample t procedure (25 ). Based on the assumption that the subjects are derived from the same population and identical procedures were used, the true variance was assumed to be the same. The pooled variance was calculated by averaging each sample variance with weights equal to its degrees of freedom. Therefore, pooled variance for the two groups was determined and used to test whether the two breakpoints were different using a pooled two-sample t procedure.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Parenteral and enteral methionine requirement.
Values for 14CO2 recovery and plasma SRA for both parenteral and enteral requirements are summarized in Tables 2and 3, respectively. Plateaus in breath 14CO2, plasma phenylalanine SRA and plasma tyrosine SRA were reached 2 h after the initiation of the primed, constant infusion in all pigs. The ratio of plasma tyrosine SRA/plasma phenylalanine SRA did not differ among diet treatments; thus, the rate of conversion of phenylalanine to tyrosine did not affect the breakpoint estimate.
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Phenylalanine flux, intake, nonoxidative disposal and release from protein did not differ (P > 0.05) among dietary treatments (Table 4). The similarity in flux among dietary treatments indicates that the differences observed in phenylalanine oxidation reflect a shift in partitioning between oxidation and protein synthesis. Furthermore, the lack of difference in Tyr:Phe SRA demonstrates that when dietary tyrosine is in excess, phenylalanine is channeled directly to oxidation within the hepatocyte and is partitioned between protein synthesis and oxidation (19 ,26 ). Phenylalanine oxidation, expressed as a percentage of the dose oxidized and as the absolute rate, was significantly influenced by methionine intake (P = 0.005 and P < 0.0001, respectively (Fig. 1 ). As methionine intake increased from 0.05 to 0.29 g/(kg · d), phenylalanine oxidation (% of dose) decreased (P < 0.05). Further increases in methionine intake did not affect phenylalanine oxidation (P > 0.05, slope not different from zero) (Fig. 1) .
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50 µmol/L) (27
), these were very low.
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Phenylalanine flux, intake, nonoxidative disposal and release from protein did not differ (P > 0.05) among dietary treatments (Table 6). Phenylalanine oxidation, expressed as a percentage of the dose oxidized and as an absolute rate was significantly influenced by methionine intake (P < 0.005). As methionine intake increased from 0.1 to 0.42 g/(kg · d), phenylalanine oxidation (% of dose) decreased (P < 0.05) (Fig. 3 ). Further increases in methionine intake did not affect phenylalanine oxidation (P > 0.05, slope not different from zero) (Fig. 3) .
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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-ketobutyrate to propionyl CoA. To the authors’ knowledge, the present report is the first direct measurement of parenteral requirements for the SAA in a neonatal model under well-controlled conditions. Comparison of the parenteral and enteral requirements also provides an estimation of the extent of first-pass metabolism of the SAA by the splanchnic tissues. The mean parenteral methionine requirement, as determined by a two-phase linear crossover model, was 0.29 g/(kg · d) when based on phenylalanine oxidation as a percentage of the dose oxidized, and 0.26 g/(kg · d) when based on phenylalanine oxidation. The mean enteral requirement was 0.42 g/(kg · d) when based on phenylalanine as a percentage of the dose oxidized, and 0.44 g/(kg · d) when based on phenylalanine oxidation. The upper 95% CI was estimated and is assumed to meet the methionine requirement of 95% of the population. This safe level of intake in parenteral feeding of methionine was 0.42 g/(kg · d) and for enteral feeding, it was 0.48 g/(kg · d).
There have been numerous studies that have measured methionine requirements in pigs fed a grain-based diet, most commonly a corn-soybean diet. However, it should be noted that few data presently exist on pigs < 5 kg (22 ); in fact, the amino acid requirements for these pigs is generally extrapolated from studies done on larger pigs. The present estimate of 0.42 g/(kg · d) is 84% of the NRC (22 ) recommendation of 0.50 g/(kg · d). However, the NRC (22 ) recommendation assumes a grain-based diet in which the amino acids are less available than in the synthetic diet used in the present experiment. Kim and colleagues (14 ) used the IAAO method with an oral isotope to measure the methionine and TSAA requirements in piglets of approximately the same age (10–14 d old) and weight (3 kg) consuming a diet based on a mixture of amino acids and dried skim milk. These researchers determined both the methionine requirement, when excess cysteine and choline was provided, and the TSAA requirement when only methionine was provided, using a broken line regression model. This resulted in a methionine and TSAA requirement of 2.7 and 4.4 g/kg diet, respectively. This yields a requirement of 0.26 g/(kg · d) for methionine and 0.43 g/(kg · d) for the TSAA. This compares closely with our estimate of 0.42 g/(kg · d) for TSAA, suggesting that methionine can meet the entire TSAA requirement. The TSAA requirement has been estimated to be 0.41–0.47 g/(kg · d) (29 ), 0.51 g/(kg · d) (30 ), and 0.41 g/(kg · d) (31 ) in piglets consuming a semipurified (29 ,31 ) or skim milk-soybean oil diet, respectively (30 ). These estimates were performed on piglets weighing between 1 and 5 kg, and used weight gain, feed efficiency and nitrogen balance to calculate the methionine and TSAA requirement. The agreement between these results and the present estimate suggests that there is not a separate requirement for cysteine in piglets of this age.
Statistical comparison of the requirements for methionine by the two different routes of feeding demonstrated that these are significantly different, suggesting that the gut is utilizing 30% methionine and/or possibly cysteine. This is intriguing considering that it has been reported that the metabolism of the SAA is accomplished, in its entirety, by only the liver and kidney (32
,33
). Met has not been considered to be catabolized to a large extent by intestinal mucosa because of the low activity of S-adenosyl-L-methionine synthase (32
) and the transsulfuration enzyme, cystathionase (34
) when their activity is compared with that of the liver. In addition, methionine adenosyltransferase (MAT) has at least three isozymic forms, of which the "high Km" form (MAT-III) exists only in the liver, suggesting that only the liver (and not the gut) may be unique in its ability to adapt immediately to high levels of methionine (32
). However, these previously reported enzymatic data require reexamination (35
).
Previous research has measured a release of methionine (108% of dietary intake) and cysteine (109% of dietary intake) using arterial-venous (A-V) differences (36 ). This suggests that the gut utilizes negligible amounts of the SAA and that the splanchnic release is consistent with the dietary concentration of these amino acids. In addition, these researchers examined A-V differences in adult rats consuming a purified diet. The apparent negligible release of these amino acids from the gastrointestinal tract may be a result of recycling of mucins and glutathione (cysteine). Within the same study, these researchers found a very large uptake of sulfate by the gut; this may indicate that methionine and cysteine are catabolized by the gut to sulfate for conjugation of potentially toxic compounds or for synthesis of sulfated compounds (37 ).
Stoll and colleagues (38 ) reported results that are consistent with the present data. Using the mass balance technique to measure the appearance of amino acids in the portal blood in 28-d-old piglets administered sow’s milk replacer, these researchers reported a 48% appearance of methionine in the portal blood, suggesting a splanchnic methionine uptake of 52%. In the study by Garcia and Stipanuk (36 ), rats were fed a casein-based semipurified diet containing 8 g/kg Met, 0.6 g/kg cysteine and 1 g/kg sulfate. In contrast, Stoll et al. (38 ) fed a milk-based diet and did not report any addition of sulfate. Perhaps, in the absence of sulfate, the SAA are catabolized to produce adequate amounts of sulfate for its various functions in the splanchnic organs. However, both our data and those of Stoll et al. (38 ) support the idea that there is selective utilization of the SAA by the splanchnic tissues.
The differences between previous estimates of amino acid uptake by the gut and our estimate may be due to the differences in dietary components and the age of the piglets. Semisynthetic and milk replacer diets present both free amino acids and peptides to the enterocyte. In addition, suckling milk-fed piglets will also receive immune factors, growth factors, cysteine and glutamine in their diet, all of which may alter intestinal metabolism or affect the estimate of the requirement. The presence of peptides in the enterocyte stimulates transport of individual amino acids (39 ) and this may account for a larger amount of amino acids passing into the systemic circulation. In addition, previous data demonstrated that the efficiency of methionine utilization was much lower when an enzymatic hydrolysate of milk vs. an elemental diet was fed enterally when measured using A-V balance (40 ). Chung and Baker (13 ) determined a 1:2, TSAA:lysine ratio for young, 5- to 10-kg pigs, which increases to 1:1.7 in 10- to 20-kg pigs. Our estimate as a ratio to the lysine requirement as determined by Kim et al. (41 ) is 1:2.7, demonstrating that as the pigs age, the requirement of the TSAA increases in proportion to lysine. Thus, differences in diet and age can affect estimates of the requirement and may alter the extent of amino acid utilization by the gut.
The differences seen in methionine requirement between enterally and parenterally fed piglets may be attributed in part to a decreased use of the intravenously infused amino acids for maintenance of intestinal mass. Parenteral feeding causes gut atrophy (42
–44
), decreased lower jejunum weight and a reduced fractional and absolute protein synthetic rate compared with enterally fed animals (45
). However, in the study by Stoll et al. (38
) in which they observed a 52% uptake of methionine, there was no gut atrophy. Furthermore, they estimated that < 20% of amino acid utilization could be accounted for by intestinal protein synthesis. However, differences in the parenteral and enteral tryptophan requirement were not significant (Cvitkovic et al., unpublished data), whereas the parenteral threonine requirement (5
) was 45% of the mean enteral requirement and the parenteral branched-chain amino acid requirement was 56% of the mean requirement in enterally fed piglets (6
). The differences in these results are most likely due to the additional metabolic roles of these amino acids in the gut.
There are several potential uses for the SAA by the gastrointestinal tissue. Reduced intestinal mass and function resulting from intravenous feeding may account for a large proportion of the observed difference in the dietary methionine requirement. A net appearance of taurine in portal blood (40 ), possibly from its synthesis from cysteine, could account for a proportion of the net utilization of the SAA by the splanchnic tissue. In addition, there is glutathione synthesis in the intestinal tissue (46 ), which may account for a proportion of the SAA utilization (47 ). Finally, methionine is a major methyl donor in vivo and may be used substantially during gut protein synthesis (32 ), resulting in the end product of the demethylation of methionine, homocysteine, being exported from the splanchnic bed and being catabolized through the transsulfuration pathway by the liver.
In conclusion, a mean methionine requirement when no cysteine was provided was determined to be 0.42 and 0.29 g/(kg · d) for enterally and parenterally fed piglets, respectively, using the IAAO technique. This estimate of the TSAA requirement is 84% of the current recommended intake of 0.5 g/(kg · d) (22 ) and agrees with previously published data (14 ,29 ,31 ). The differences in parenteral and enteral requirement suggest a 30% utilization of methionine, cysteine, glutathione or sulfate by the gut, which is consistent with some previous estimates of splanchnic use of the SAA.
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FOOTNOTES |
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2 Supported by grants from the Alberta Pork, Alberta Agricultural Research Institute and by the CIHR Fund # 12928.
4 Abbreviations used: A-V, arterial-venous; IAAO, indicator amino acid oxidation; MAT, methionine adenosyltransferase; Met, methionine; SAA, sulfur amino acids; SRA, specific radioactivity, TPN, total parenteral nutrition; TSAA, total sulfur amino acids.
Manuscript received 11 October 2002. Initial review completed 20 November 2002. Revision accepted 3 February 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Ball, R. O., House, J. D., Wykes, L. J. & Pencharz, P. B. (1996) A piglet model for neonatal amino acid metabolism during total parenteral nutrition. Tumbleson, M. School, L. eds. Advances in Swine Biomedical Research: International Symposium 1996:713-731 Plenum Press New York, NY. .
2. Pencharz, P. B., House, J. D., Wykes, L. J. & Ball, R. O. (1996) What Are the Essential Amino Acids for the Preterm and Term Infant? 10th Nutricia Symposium Vol. 21:278-296 Kluwer Academic Publishers Dordrecht, The Netherlands. .
3. 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]
4. Wykes, L. J., Ball, R. O., Menendez, C. E., Ginther, D. M. & Pencharz, P. B. (1992) Glycine, leucine, and phenylalanine flux in low-birth-weight infants during parenteral and enteral feeding. Am. J. Clin. Nutr. 55:971-975.
5. 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.
6. Elango, R., Pencharz, P. B. & Ball, R. O. (2002) Branched-chain amino acid requirement of parenterally fed neonatal piglets in less than the enteral requirement. J. Nutr. 132:3123-3129.
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19. House, J. D., Pencharz, P. B. & Ball, R. O. (1997a) 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.
20. House, J. D., Pencharz, P. B. & Ball, R. O. (1997b) Tyrosine kinetics and requirements during total parenteral nutrition in the neonatal piglet: the effect of glycyl-tyrosine supplementation. Pediatr. Res. 41:575-583.[Medline]
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28. House, J. D., Pencharz, P. B. & Ball, R. O. (1998) Lysine requirement of neonatal piglets receiving total parenteral nutrition as determined by oxidation of the indicator amino acid L-[1-14C]phenylalanine. Am. J. Clin. Nutr. 67:67-73.[Abstract]
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38. Stoll, B., Henry, J., Reeds, P. J., Yu, H., Jahoor, F. & Burrin, D. G. (1998) Catabolism dominates first pass intestinal metabolism of dietary essential amino acids in milk protein-fed piglets. J. Nutr. 128:606-614.
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