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,**,
,3,**,
Departments of * Paediatrics and Nutritional Sciences, University of Toronto, and ** Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada, M5G 1X8; and Department of Agricultural Food and Nutritional Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2P5
3 To whom correspondence should be addressed. E-mail: paul.pencharz{at}sickkids.ca.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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KEY WORDS: • dietary essential amino acids • indispensable • indicator amino acid oxidation • infants • children
Very few data exist for humans on the amino acid needs of infants and children. This is mainly because until recently, amino acid requirements were determined using nitrogen balance. Nitrogen balance studies require sufficient time to permit equilibration of the body's urea pool to changes in intake of the test amino acid. In adults, this takes a minimum of 5–7 d. The existing data based on nitrogen balance were summarized in the 1985 report of an Expert Committee for the FAO/WHO/UNU (1) and consist of data from neonates by Snyderman et al. (2,3), from toddlers by Pineda et al. (4), and from preadolescent children by Nakagawa et al. (5). The North American Dietary Reference Macronutrient Panel (6) regarded these data as too limited to be used to establish the dietary essential amino acid requirements for infants and children and instead opted for a factorial approach. This approach assumes that the maintenance needs of infants and children are the same as for adults.
The advent of amino acid oxidation studies, and in particular, the indicator amino acid oxidation (IAAO) method, permits studies to be conducted with only a few hours of adaptation to the test amino acid (7). A description may be found in a recent interpretative review that outlines all of the amino acid oxidative methods from direct oxidation to 24-h IAAO and balance (7). IAAO is a functional method that is based on the partitioning of the indicator amino acid between incorporation into protein and oxidation in response to graded intake of the test amino acid (7). The initial application of IAAO to studies of humans was on adults and involved intravenous indicator administration, blood sampling, and up to 9 d of the amino acid–based experimental diet. To enable us to study children, we adapted the IAAO method to be minimally invasive (8) by administering the tracer orally, using urine as a means of sampling arterialized blood, and reducing the adaptation period to the experimental diet to 4 h. These adaptations allow us to study a level of the test amino acid in an 8–9-h day. Earlier we showed that between-subject variance was greater than within-subject variance; we therefore adopted an experimental approach whereby each subject was studied over a range of 6–8 levels of the test amino acid. With this minimally invasive IAAO method, we were able to determine the dietary essential amino acid needs of children as young as 3 y (9).
Amino acid requirements in neonates
Owing to their biochemical immaturity, preterm neonates have several conditionally dietary indispensable amino acids including glycine and arginine (10).
The requirements of the classical dietary indispensable amino acids in this group were studied because preterm neonates spend extended periods in neonatal units. The initial studies were conducted by Snyderman et al. (2,3) using nitrogen balance. However, for any one dietary indispensable amino acid, the data are limited. As part of the Dietary Reference process (6), nonlinear regression was applied to the published data to better define population mean requirements; however, the results were less than satisfactory. This led to the decision to use a factorial approach as outlined above (6).
Conversely, the advent of IAAO has allowed a systematic determination of dietary essential amino acid requirements in neonatal piglets fed either enterally or parenterally. The branched chain amino acids (11), sulfur amino acids (12), and threonine (13) are partially retained in the gut. Conversely, tryptophan is not net retained (14), and arginine is net produced by the neonatal piglet gut (15). The relevance of the neonatal piglet–derived results to human neonatal requirements is confirmed for phenylalanine and tyrosine (16,17). Although it is necessary to verify in human neonates some of the other amino acid requirements such as those for sulfur amino acids (12) and threonine (13), it is worth noting that the piglet studies resulted in the conclusion that current commercial neonatal amino acid mixtures are inadequate (18). Particular issues include excess BCAA content (11), excess total sulfur amino acid content (13), and in some cases, inadequate cysteine intake (19) and deficient tyrosine content (20–23). This body of work together with that from the Children's Nutrition Research Center (24) provide quantitative estimates of the importance of amino acid metabolism in the gut.
Factorial approach to determine dietary essential amino acid requirements in infants and children
In animals, dietary essential amino acid requirements consist of two components: maintenance and growth. As mentioned above, in light of the very limited available data on human infants, toddlers, and children, it was proposed (6) that a factorial approach should be taken to estimate their essential amino acid requirements. Using amino acid oxidation techniques, the dietary essential amino acid requirements for adults have been nearly completed (6,7). Furthermore, in 2000, new data were published of changes in total body potassium, which were prospective in a birth-to–2-y-old cohort (25) and were cross-sectional above that age (26). From these data, it is possible to calculate protein deposition during growth. Finally, from a knowledge of the average amino acid composition of tissue, it is possible to calculate the amino acid deposition associated with growth (6). Hence, for the first time there are enough data upon which to base reasonable factorial estimates of amino acid requirements in children. However, there was no independent verification of the validity of this approach in humans. Very recently, we determined total BCAA requirements for young adults (27) to be 144 mg·kg–1·d–1 and for 6- to 10-y-old children (9) to be 147 mg·kg–1·d–1, thereby providing data to support the validity of the factorial model.
In addition, the IAAO approach was used to determine tyrosine (19 mg·kg–1·d–1) (28) and phenylalanine (14 mg·kg–1·d–1) (29) requirements for children with phenylketonuria. This is the first time it has been possible to determine requirements for these patients. Finally, we are able to show that in children as young as 3 y, end-stage liver disease increases BCAA requirements by 
40% (30). Using the minimally invasive IAAO model, the way is clear to the determination of dietary essential amino acids for infants and children in health and in disease.
Definition of the upper limits of dietary essential amino acids in infants and children
Apart from observations in patients with inborn errors of amino acid metabolism such as phenylketonuria, we were unable to find any data on infants and children relating to the upper limits of amino acid requirements. However, for preterm neonates, there is some information available about the ability of neonates and young infants to handle a load of amino acids that is delivered as the protein source in a preterm- or term-infant formula. The amino acids of particular note are phenylalanine, threonine, and tyrosine (10). The protein sources primarily used for infant formulas are derived from cow's milk. Cow's milk is casein dominant, and casein is high in phenylalanine and tyrosine compared to human milk. Human milk is higher in whey protein (70%) than cow's milk (18%). Based on this observation, the protein source for preterm- and term-infant formulas was altered to be whey predominant (60%). With this higher whey content, more threonine is delivered, and infants fed these formulas have high plasma threonine levels compared with infants fed human milk or casein-dominant formulas. Similarly, infants fed casein-dominant formulas have increased plasma levels of phenylalanine and tyrosine. Interestingly, there are no differences in nitrogen retention between infants fed casein- and whey-dominant formulas despite the marked differences in plasma amino acid patterns (31). The concern arises in the finding that preterm infants fed high protein levels (6 g·kg–1·d–1) from casein-dominant formulas have worse school performance at age 6 y (32). We studied the ability of infants to oxidize dietary threonine (33) and phenylalanine (P. Darling, R. O. Ball, and P. B. Pencharz, 2004; unpublished data) in response to ingestion of infant formulas compared with their mother's own milk. Human milk–fed infants were able to increase oxidation of threonine and phenylalanine as their intake increased; conversely, formula-fed infants were unable to increase oxidation.
In our piglet studies of total parenteral nutrition, we were able to study phenylalanine oxidation in response to graded levels of phenylalanine intake in the presence of excess tyrosine. With an excess of tyrosine, excess dietary phenylalanine is channeled within hepatocytes directly to oxidation. At high levels of intake, it was possible to detect a maximal rate of phenylalanine oxidation at which point plasma levels rose rapidly (22). We suggest that this is a metabolic example in which the upper limit of a particular amino acid (in this case, phenylalanine) can be defined. It is true that neurotoxicity might occur at lower levels of phenylalanine intake. However, once the intake of phenylalanine exceeds the maximum oxidative capacity of the piglet and phenylalanine starts to accumulate (as it does in phenylketonuria), then clearly the upper limit of phenylalanine intake has been exceeded. The piglets we study are quite homogenous genetically, so variance is less than in human infants. Conversely with humans, maximal phenylalanine oxidative capacity is more variable.
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FOOTNOTES |
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2 Work from the authors' laboratories was supported by grants from the Canadian Institutes for Health Research, the National Sciences Engineering Research Council, the Alberta Agricultural Research Council, and the Alberta Pork Board.
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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1. Food and Agriculture Organization of the United Nations, World Health Organization, and United Nations University (1985) Energy and Protein Requirements. Report of a Joint FAO/WHO/UNU Expert Consultation. Technical Report Series no. 724. World Health Organization, Geneva, Switzerland.
2. Snyderman, S. E., Norton, P. M., Fowler, D. I. & Holt, L. E., Jr. (1959) The essential amino acid requirements of infants: lysine. Am. J. Dis. Child. 97: 175–185.
3. Snyderman, S. E., Roitman, E. L. & Holt, L. E., Jr. (1961) Essential amino acid requirements of infants: leucine. Am. J. Dis. Child. 102: 157–162.
4. Pineda, O., Torun, B., Viteri, F. E. & Arroyave, G. (1981) Protein quality in relation to estimates of essential amino acid requirements. In: Protein Quality In Humans: Assessment and In Vitro Estimation (Bodwell, C. E., Adkins, J. S. & Hopkins, D. T., eds.), pp. 29–42. AVI Publishing, Westport, CT.
5. Nakagawa, I., Takahashi, T., Suzuki, T. & Kobayashi, K. (1964) Amino acid requirements in children: nitrogen balance at the minimal level of essential amino acids. J. Nutr. 83: 115–118.
6. Panel on Macronutrients (2002) Dietary Reference Intakes: Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Institute of Medicine, National Academy Press, Washington, DC.
7. Pencharz, P. B. & Ball, R. O. (2003) Different approaches to define individual amino acid requirements. Annu. Rev. Nutr. 23: 101–116.[Medline]
8. Bross, R., Ball, R. O. & Pencharz, P. B. (1998) Development of a minimally invasive protocol for determination of phenylalanine and lysine kinetics in humans during the fed state. J. Nutr. 128: 1913–1919.
9. Mager, D. R., Wykes, L. J., Ball, R. O. & Pencharz, P. B. (2003) Branched chain amino acid requirements in school aged children determined by indicator amino acid oxidation (IAAO). J. Nutr. 133: 3540–3545.
10. 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? In: Recent Developments in Infant Nutrition: 10th Nutricia Symposium (Bindels, J. G., Goedhart, A. C. & Visser, H.K.A., eds.), vol. 21, pp. 278–296. Kluwer Academic, Dordrecht, The Netherlands.
11. Elango, R., Pencharz, P. B. & Ball, R. O. (2002) Branched chain amino acid requirement of parenterally fed neonatal piglets is less than enteral requirement. J. Nutr. 132: 3123–3129.
12. Shoveller, A. K., Brunton, J. A., Pencharz, P. B. & Ball, R. O. (2003) The methionine requirement is lower in the parenterally fed neonatal piglet than in the enterally fed. J. Nutr. 133: 1390–1397.
13. Bertolo, R.F.P., Chen, 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.
14. Cvitkovic, S., Bertolo, R. F., Brunton, J. A., Pencharz, P. B. & Ball, R. O. (2004) Enteral tryptophan requirement determined by oxidation of gastrically or intravenously infused phenylalanine is not different than parenteral requirements in neonatal piglets. Pediatr. Res. 55: 630–636..[Medline]
15. Brunton, J. A., Bertolo, R.F.P., Pencharz, P. B. & Ball, R. O. (1999) Proline ameliorates arginine deficiency during enteral feeding but not during parenteral feeding in neonatal piglets. Am. J. Physiol. Endocrinol. Metab. 277: E223–E231.
16. Roberts, S. A., Ball, R. O., Filler, R., Moore, A. & Pencharz, P. B. (1998) Phenylalanine and tyrosine metabolism in neonates receiving parenteral nutrition differing in pattern of amino acids. Pediatr. Res. 44: 907–914.[Medline]
17. Roberts, S. A., Ball, R. O., Moore, A. M., Filler, R. M. & Pencharz, P. B. (2001) The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirements. Pediatr. Res. 49: 1–9.[Medline]
18. 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]
19. Shoveller, A. K., Brunton, J. A., Pencharz, P. B. & Ball, R. O. (2003) Dietary cysteine reduces methionine requirement by equal proportion in both parenterally and enterally fed piglets. J. Nutr. 133: 4215–4224.
20. Wykes, L. J., House, J. D., Ball, R. O. & Pencharz, P. B. (1994) Amino acid profile and aromatic amino acid concentration in TPN: effect on growth, protein metabolism, and aromatic amino acid metabolism in the neonatal piglet. Clin. Sci. (Lond.) 87: 75–84.[Medline]
21. Wykes, L. J., House, J. D., Ball, R. O. & Pencharz, P. B. (1994) Aromatic amino acid metabolism of neonatal piglets receiving total parenteral nutrition: effect of tyrosine precursors. Am. J. Physiol. Endocrinol. Metab. 267: E672–E679.
22. House, J. D., Pencharz, P. B. & Ball, R. O. (1997) Phenylalanine kinetics and requirements determined using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine. Am. J. Clin. Nutr. 65: 984–993.
23. House, J. D., Pencharz, P. B. & Ball, R. O. (1997) Tyrosine kinetics and requirements during total parenteral nutrition in the neonatal piglet: the effect of glycyl-L tyrosine supplementation. Pediatr. Res. 41: 575–583.[Medline]
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. Butte, N. F., Hopkinson, J. M., Wong, W. W., Smith, E. O. & Ellis, K. J. (2000) Body composition during the first 2 years of life: an updated reference. Pediatr. Res. 47: 578–585.[Medline]
26. Ellis, K. J., Shypailo, R. J., Abrams, S. A. & Wong, W. W. (2000) The reference child and adolescent models of body composition. A contemporary comparison. Ann. N. Y. Acad. Sci. 904: 374–382.
27. Riazi, R., Wykes, L. J., Ball, R. O. & Pencharz, P. B. (2003) Requirement of total branched chain amino acids determined by indicator amino acid oxidation using L-[1-13C]phenylalanine. J. Nutr. 133: 1383–1389.
28. Bross, R., Ball, R. O., Clarke, J. T. R. & Pencharz, P. B. (2000) Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation. Am. J. Physiol. Endocrinol. Metab. 278: E195–E201.
29. Courtney-Martin, G., Bross, R., Rafii, M., Clarke, J.T.R., Ball, R. O. & Pencharz, P. B. (2002) Phenylalanine requirement in children with classical phenylketonuria determined by indicator amino acid oxidation. Am. J. Physiol. Endocrinol. Metab. 283: E1249–E1256.
30. Mager, D., Wykes, L. J., Ball, R. O. & Pencharz, P. B. (2003) Chronic liver disease increases total branched chain amino acid requirements in children. FASEB J. 17: A701 (abs.).
31. Pencharz, P. B., Farri, L. & Papageorgiou, A. (l983) The effects of human milk and low protein formulae on the rates of total body protein turnover and urinary 3-methylhistidine excretion of preterm infants. Clin. Sci. (Lond.) 64: 611–6l6.
32. Goldman, H. I., Goldman, J. S., Kaufman. I. & Liebman, O. B. (1974) Late effects of early dietary protein intake on low-birth-weight infants. J. Pediatr. 83: 764–769.
33. Darling, P. B., Dunn, M., Sarwar, G. G., Brookes, S., Ball, R. O. & Pencharz, P. B. (1999) Threonine kinetics in preterm infants fed their mother's milk or formula with varying ratios of whey to casein. Am. J. Clin. Nutr. 69: 105–114.
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