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© 2008 American Society for Nutrition J. Nutr. 138:243-246, February 2008

Recent Advances in Nutritional Sciences

Indicator Amino Acid Oxidation: Concept and Application1–3,

Rajavel Elango4,5, Ronald O. Ball4,5,7 and Paul B. Pencharz4,5,6,7,*

4 Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8; 5 Department of Nutritional Sciences and 6 Department of Paediatrics, University of Toronto, Ontario, Canada M5S 3E2; and 7 Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

* To whom correspondence should be addressed. E-mail: paul.pencharz{at}sickkids.ca.


   ABSTRACT
TOP
 ABSTRACT
Introduction
 LITERATURE CITED
 
The indicator amino acid oxidation (IAAO) method is based on the concept that when 1 indispensable amino acid (IDAA) is deficient for protein synthesis, then all other IDAA, including the indicator amino acid, will be oxidized. With increasing intakes of the limiting amino acid, IAAO will decrease, reflecting increasing incorporation into protein. Once the requirement for the limiting amino acid is met, there will be no further change in the indicator oxidation. Originally, the IAAO method was designed to determine amino acid requirements in growing pigs. The minimally invasive IAAO method developed in humans has been systematically applied to determine IDAA requirements in adults. Due to its noninvasive nature, the IAAO method has also been used to determine requirements for amino acids in neonates and children, and in disease. The IAAO model has recently been applied to determine the metabolic availability (MA) of amino acids from dietary proteins and to determine total protein requirements. The IAAO method is robust, rapid, and reliable; it has been used to determine amino acid requirements in different species, across the life cycle, and in diseased populations. The recent application of IAAO to determine MA of amino acids and protein requirements is also very novel.


   Introduction
TOP
 ABSTRACT
 Introduction
LITERATURE CITED
 
The indicator amino acid oxidation (IAAO)8 technique is based on the concept that when 1 indispensable amino acid (IDAA) is deficient for protein synthesis, then all other amino acids, including the indicator amino acid (another IDAA, usually L-[1-13C]phenylalanine), will be oxidized (1). Fundamentally, this is because amino acids cannot be stored and therefore must be partitioned between incorporation into protein or oxidation. With increasing intake of the limiting amino acid, oxidation of the indicator amino acid will decrease, reflecting increasing incorporation into protein. Once the requirement is met for the limiting amino acid, there will be no further change in the oxidation of the indicator amino acid (Fig. 1).


Figure 1
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  		FIGURE 1  Effect of lysine intake on production of 13CO2 from the oxidation of L-[1-13C] phenylalanine (F13CO2) when tracer was infused either i.v. or orally in healthy adult humans. Values are means ± SD, n = 5. IAAO: With increasing intake of the limiting amino acid, oxidation of the indicator amino acid will decrease, reflecting increasing incorporation into protein. Once the requirement is met for the limiting amino acid, there will be no further change in the oxidation of the indicator amino acid. Route of isotope infusion does not impact the breakpoint or requirement estimate. The minimally invasive IAAO model involves oral isotope infusion, followed by collection of breath for 13CO2 measurement and urine for calculation of isotope kinetics. Reprinted from Kriengsinyos et al., 2002 (13).

 
The IAAO method was initially developed by Bayley et al. (2–5) for the determination of amino acid requirements in young growing pigs in a series of elegant experiments. Ball and Bayley (6) validated the concept that the IAAO is inversely related to protein synthesis by showing that the recovery of 14C-phenylalanine radioactivity in piglet liver protein increased with increasing intake of dietary protein and was greatest with the dietary level of protein that minimized phenylalanine oxidation. The inflection point where the oxidation of the indicator amino acid stops decreasing and reaches a plateau is referred to as the breakpoint (Fig. 1). The breakpoint identified with the use of bi-phase linear regression analysis indicates the mean or estimated average requirement of the limiting (test) amino acid (1). Currently, IAAO and the IAAO-based method, 24-h indicator amino acid balance, are accepted as appropriate for the determination of amino acid requirements (7). For discussions about the various methods to determine amino acid requirements, their advantages and disadvantages, and requirement values, the reader is referred to earlier comprehensive reviews (1,8–10). The current article will briefly review developments in the application of the IAAO method to determine amino acid requirements, and introduce the recent adaptation of the IAAO model to determine total protein requirements and the metabolic availability (MA) of amino acids from foods.

    Amino acid requirements in adult humans. The initial application of the IAAO method in adult humans was accomplished by Zello et al. (11) to determine the lysine requirement using intravenous L-[1-13C]phenylalanine as the indicator amino acid. Breath and blood were collected to measure 13CO2 and plasma phenylalanine enrichment, respectively. Biphasic linear regression analysis identified the lysine requirement as 36.9 mg·kg–1·d–1, which was considerably higher than the 1985 FAO/WHO/UNU recommendations of 12 mg·kg–1·d–1 (11). To make the IAAO protocol less invasive, Bross et al. (12) validated the IAAO method with hourly oral isotope doses and sampling of urine to measure isotopic enrichment. To test whether the route of isotope infusion has an impact on the determination of the breakpoint, or requirement estimate, Kriengsinyos et al. (13) determined the lysine requirement in subjects infused i.v. or orally with L-[1-13C]phenylalanine (Fig. 1). Identical requirement estimates of 36.6 mg·kg–1·d–1 for lysine were determined with both routes of isotope infusion. This minimally invasive IAAO method has been systematically applied to determine IDAA requirements in adult humans (10), except histidine (14), for which no requirement could be determined (Table 1). These requirement values obtained using the IAAO method were used to derive amino acid intakes in the recent dietary recommended intakes (DRI) for macronutrients (7).


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  		TABLE 1 Amino acid requirements determined using the IAAO method

 
One of the criticisms of the IAAO model has been that subjects are only adapted to the test amino acid intake on the study day. To examine whether prolonged days of adaptation are necessary to determine requirements using the IAAO method, Moehn et al. (15) tested phenylalanine oxidation following various periods of adaptation to different lysine and protein intakes in growing and adult pigs. Phenylalanine oxidation did not differ following either 2 or 10 d of adaptation to lysine or protein intake. Furthermore, we recently demonstrated that L-[1-13C]phenylalanine oxidation, measured as F13CO2 which is the primary variable used for determination of the breakpoint, did not differ following 8 h, 3 d, or 7 d of adaptation to a wide range of lysine intakes in young men (16). Therefore, the minimally invasive model provides valid estimates of amino acid requirements to be determined in short periods of time and thus is highly suitable for application in vulnerable populations.

    Amino acid requirements in children. Determination of amino acid requirements in children has traditionally been difficult, because it is impractical and unethical to feed deficient amino acid intakes for prolonged periods of time. Therefore, current recommendations for amino acids in children are based on a factorial method. Development of the minimally invasive IAAO model enabled the direct determination of requirements for: total branched chain amino acids (BCAA) (17), total sulfur amino acids (SAA) (18), methionine (with cysteine) (19), and lysine (20) in healthy school-age children (6–11 y) (Table 1). Requirement estimates in children were similar to the estimates in adult humans, which suggests that the experimentally derived values predominantly reflect maintenance requirements and do not take into account all the growth needs (10). To ensure proper growth in children of this age group, we recommend addition of the calculated growth component to the requirement estimate, which has been discussed in detail recently by Elango et al. (10). These recent IAAO studies are the first to our knowledge to be conducted in children using stable isotopes and have clearly established that the factorial method of calculating requirements is indeed valid in healthy children.

    Amino acid requirements in disease. Dietary management of specific diseases requires knowledge of nutrient requirements to have a successful clinical outcome. Metabolic disorders such as phenylketonuria (PKU) require tyrosine supplementation with phenylalanine restriction and maple syrup urine disease requires BCAA restriction. The minimally invasive IAAO model was used to determine tyrosine (21) and phenylalanine (22) requirements in children with classical PKU and the requirements were 19 and 14 mg·kg–1·d–1, respectively. These values suggest that the ratio of aromatic amino acids (AAA) is 60% of tyrosine and 40% of phenylalanine, which is considerably different from the current recommendation of 80 and 20%, respectively, for the management of patients with PKU (9). Similarly, the mean total BCAA requirements in maple syrup urine disease patients was determined to be 45 mg·kg–1·d–1 compared with the requirements of 144 mg·kg–1·d–1in healthy people (23).

Children with liver disease are hypothesized to have increased BCAA requirements based on measurements of plasma amino acid concentrations. We therefore applied the IAAO method using L-[1-13C]phenylalanine to determine total BCAA needs in children with cholestatic liver disease (24). The mean requirement was 209 mg·kg–1·d–1, which is 30% higher than the mean requirement estimate of 147 mg·kg–1·d–1 determined earlier in healthy children (17). Using a similar protocol, the mean total BCAA requirements in children after liver transplantation was determined to be 172 mg·kg–1·d–1 (25). Therefore, post liver transplantation BCAA requirements are lower compared with children with liver disease but remain higher compared with the requirements for healthy children. These IAAO-derived requirement values are the first direct estimates, to our knowledge, in various diseases and disorders in children.

    Amino acid requirements in neonates. Amino acid requirement studies in preterm and term neonates are extremely difficult to conduct due to ethical and practical concerns. During the adaptation of the IAAO method from growing pigs to adult humans, simultaneous work was conducted to adapt the IAAO protocol in parenterally (26,27) and enterally fed piglets (28) as a surrogate model for the human neonate. The results from the series of IAAO studies in piglets (28–31) revealed important and clinically relevant information, including: 1) the profile of amino acids in current total parenteral nutrition (TPN) solutions are inappropriate and are present in considerably higher concentrations than the requirements (Table 1); and 2) in enterally fed piglets, there is a considerable involvement of the splanchnic tissues, especially the small intestine, in amino acid utilization, which leads to higher requirements compared with parenterally fed piglets. The requirements for threonine (28), total BCAA (29), and total SAA (30) were 55, 44, and 31% higher, respectively, in enterally fed piglets, whereas tryptophan (31) requirements were not affected by route of feeding.

Recently, the IAAO model was applied to parenterally fed piglets to identify the limiting amino acids in a commercially used TPN solution (Vaminolact) and a new parenteral profile (32). Using 14C-lysine as the indicator amino acid, it was observed that Vaminolact was deficient in AAA (phenylalanine+tyrosine) and supplementation with AAA reduced lysine oxidation and hence increased protein synthesis. Similarly, the new parenteral profile was deficient in SAA (methionine+cysteine). Evidence that results from animal studies are readily applicable in human neonates is provided by comparing earlier tyrosine requirements determined using the IAAO method in piglets (27) and human neonates (33) (Table 2). At a constant phenylalanine intake, the mean tyrosine requirements in piglets and neonates were determined to be 2.7 and 3.1% of total amino acids, respectively. Currently available TPN solutions provide 0.9% of the total amino acids as tyrosine and clearly are not sufficient to promote normal growth and protein accretion in neonates.


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  		TABLE 2 Comparison of IAAO-derived requirements in piglets vs. human neonates fed parenterally1

 
    Application of IAAO to determine MA of amino acids. The nutritional value of amino acids is directly related to the availability of amino acids metabolically at the site of protein synthesis. MA is dependent on digestibility, subsequent absorption, and the fraction of absorbed amino acids that is utilized for protein synthesis. Currently, the nutritional value of proteins is determined by cumbersome long-term growth/balance studies, or by using the protein digestibility-corrected amino acid score method. The IAAO method, which is based on the concept that with increasing intakes of the limiting amino acid, oxidation of the indicator amino acid will decrease and therefore reflect increasing whole body protein synthesis, can be applied to determine the bioavailability or MA of amino acids. At a given amino acid intake, differences in the IAAO rate between test and reference proteins will be proportional to the whole body MA of the test amino acid and thus account for all losses of dietary amino acids during digestion, absorption, and cellular metabolism. Moehn et al. (34) applied the IAAO model using 14C-phenylalanine to determine the MA of free lysine compared with protein-bound lysine from peas and heated peas in 15-kg pigs. The MA of lysine from peas was determined to be 88%, compared with 55% from heated peas. These values are comparable to earlier published estimates of 85% for peas and 48% for heated peas, determined using slope-ratio growth assays.

Humayun et al. (35) recently adapted the method in humans to determine the MA of SAA from casein vs. soy protein isolate (SPI) using L-[1-13C]phenylalanine as the indicator amino acid. All other amino acids except the SAA were present in excess and identical in content between the test proteins. Therefore, changes in the IAAO between free methionine vs. SAA from casein or SPI will reflect MA. The MA of SAA in casein and SPI were 87 and 72%, respectively, and are comparable to earlier published net protein utilization values of 80–85% for milk proteins and 71–78% for soy proteins. The IAAO method to determine MA has the potential to revolutionize the field of determination of protein quality of foods and is preferable to existing methods, because it can be conducted in a relatively short period of time in a minimally invasive way.

    Application of IAAO to determine protein requirements. Traditionally, total protein requirements for humans have been determined using nitrogen balance. The recent DRI recommendations for mean and population safe intakes of 0.66 and 0.8 g·kg–1·d–1, respectively, of high quality protein in adult humans are based on meta-analysis of nitrogen balance studies using linear regression analysis (7). Considering the inherent problems associated with the nitrogen balance method (1), we hypothesized that the protein requirements are underestimated. Therefore, we examined the total protein requirement in adult humans using the IAAO method (36), as previously applied in young pigs (6). Graded intakes of a mixture of amino acids in the pattern present in egg protein, except phenylalanine, ranging from 0.1 to 1.8 g·kg–1·d–1, were fed and indicator amino acid (L-[1-13C]phenylalanine) oxidation was measured. The mean protein requirement was 0.93 g·kg–1·d–1 and is 41% higher than the current DRI recommendation. This value is also in agreement with our reanalysis of previous nitrogen balance studies using bi-phase linear regression analysis, which identified a mean protein requirement of 0.91 g·kg–1·d–1 (36). The IAAO-derived protein requirements for adult humans are significantly higher than current recommendations and suggest an urgent need to reassess protein intake recommendations.

In conclusion, the IAAO method is a robust, rapid, and reliable method to determine amino acid requirements in different species, across the life cycle, and in diseased populations. The novel application of the IAAO to determine MA is a major step forward in the determination of protein quality of various foods. The recent adaptation of the IAAO method to determine protein requirements in humans suggest that reassessment of protein intake recommendations for adult humans is necessary. Due to the minimally invasive nature of the IAAO method, it is now possible to determine amino acid and protein requirements in other vulnerable populations, including pregnant and lactating women and the elderly.


   FOOTNOTES
 
1 Supported by the Canadian Institutes for Health Research (grant nos. MOP 10321 and FRN 12928). Back

2 Author disclosures: R. Elango, R. O. Ball, and P. B. Pencharz, no conflicts of interest. Back

3 Manuscript received 24 October 2007. Back

8 Abbreviations used: AAA, aromatic amino acid; BCAA, branched chain amino acid; DRI, dietary recommended intake; IAAO, indicator amino acid oxidation; IDAA, indispensable amino acid; MA, metabolic availability; PKU, phenylketonuria; SAA, sulfur amino acid; SPI, soy protein isolate; TPN, total parenteral nutrition. Back


   LITERATURE CITED
TOP
 ABSTRACT
 Introduction
 LITERATURE CITED
 

1. Pencharz PB, Ball RO. Different approaches to define individual amino acid requirements. Annu Rev Nutr. 2003;23:101–16.[Medline]

2. Kim KI, McMillan I, Bayley HS. Determination of amino acid requirements of young pigs using an indicator amino acid. Br J Nutr. 1983;50:369–82.[Medline]

3. Kim KI, Bayley HS. Amino acid oxidation by young pigs receiving diets with varying levels of sulphur amino acids. Br J Nutr. 1983;50:383–90.[Medline]

4. Kim KI, Elliott JI, Bayley HS. Oxidation of an indicator amino acid by young pigs receiving diets with varying levels of lysine or threonine, and an assessment of amino acid requirements. Br J Nutr. 1983;50:391–9.[Medline]

5. Ball RO, Bayley HS. Tryptophan requirement of the 2.5-kg piglet determined by the oxidation of an indicator amino acid. J Nutr. 1984;114:1741–6.[Abstract/Free Full Text]

6. Ball RO, Bayley HS. Influence of dietary protein concentration on the oxidation of phenylalanine by the young pig. Br J Nutr. 1986;55:651–8.[Medline]

7. Institute of Medicine Food and Nutrition Board. Dietary reference intakes: energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. Washington, DC: National Academy Press; 2002/2005.

8. Zello GA, Wykes LJ, Ball RO, Pencharz PB. Recent advances in methods of assessing dietary amino acid requirements for adult humans. J Nutr. 1995;125:2907–15.[Abstract/Free Full Text]

9. Brunton JA, Ball RO, Pencharz PB. Determination of amino acid requirements by indicator amino acid oxidation: applications in health and disease. Curr Opin Clin Nutr Metab Care. 1998;1:449–53.[Medline]

10. Elango R, Ball RO, Pencharz PB. Individual amino acid requirements in humans: an update. Curr Opin Clin Nutr Metab Care. 2008;11:34–39.[Medline]

11. Zello GA, Pencharz PB, Ball RO. Dietary lysine requirement of young adult males determined by oxidation of L-[1-13C]phenylalanine. Am J Physiol. 1993;264:E677–85.[Medline]

12. Bross R, Ball RO, Pencharz PB. Development of a minimally invasive protocol for the determination of phenylalanine and lysine kinetics in humans during the fed state. J Nutr. 1998;128:1913–9.[Abstract/Free Full Text]

13. Kriengsinyos W, Wykes LJ, Ball RO, Pencharz PB. Oral and intravenous tracer protocols of the indicator amino acid oxidation method provide the same estimate of the lysine requirement in healthy men. J Nutr. 2002;132:2251–7.[Abstract/Free Full Text]

14. Kriengsinyos W, Rafii M, Wykes LJ, Ball RO, Pencharz PB. Long-term effects of histidine depletion on whole-body protein metabolism in healthy adults. J Nutr. 2002;132:3340–8.[Abstract/Free Full Text]

15. Moehn S, Bertolo RF, Pencharz PB, Ball RO. Indicator amino acid oxidation responds rapidly to changes in lysine or protein intake in growing and adult pigs. J Nutr. 2004;134:836–41.[Abstract/Free Full Text]

16. Elango R, Humayun MAH, Ball RO, Pencharz PB. Indicator amino acid oxidation (1–13C-phenylalanine) is not affected by day of adaptation (1, 3 or 7d) to a wide range of lysine intake in young men [abstract]. Faseb J. 2006;20:29.4.

17. Mager DR, Wykes LJ, Ball RO, Pencharz PB. Branched-chain amino acid requirements in school-aged children determined by indicator amino acid oxidation (IAAO). J Nutr. 2003;133:3540–5.[Abstract/Free Full Text]

18. Turner JM, Humayun MA, Elango R, Rafii M, Langos V, Ball RO, Pencharz PB. Total sulfur amino acid requirement of healthy school-aged children as determined by indicator amino acid oxidation technique. Am J Clin Nutr. 2006;83:619–23.[Abstract/Free Full Text]

19. Humayun MA, Turner JM, Elango R, Rafii M, Langos V, Ball RO, Pencharz PB. Minimum methionine requirement and cysteine sparing of methionine in healthy school-age children. Am J Clin Nutr. 2006;84:1080–5.[Abstract/Free Full Text]

20. Elango R, Humayun MA, Ball RO, Pencharz PB. Lysine requirement of healthy school-age children determined by the indicator amino acid oxidation method. Am J Clin Nutr. 2007;86:360–6.[Abstract/Free Full Text]

21. Bross R, Ball RO, Clarke JT, Pencharz PB. Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation. Am J Physiol Endocrinol Metab. 2000;278:E195–201.[Abstract/Free Full Text]

22. Courtney-Martin G, Bross R, Raffi M, Clarke JT, Ball RO, Pencharz PB. Phenylalanine requirement in children with classical PKU determined by indicator amino acid oxidation. Am J Physiol Endocrinol Metab. 2002;283:E1249–56.[Abstract/Free Full Text]

23. Riazi R, Rafii M, Clarke JT, Wykes LJ, Ball RO, Pencharz PB. Total branched-chain amino acids requirement in patients with maple syrup urine disease by use of indicator amino acid oxidation with L-[1–13C]phenylalanine. Am J Physiol Endocrinol Metab. 2004;287:E142–9.[Abstract/Free Full Text]

24. Mager DR, Wykes LJ, Roberts EA, Ball RO, Pencharz PB. Branched-chain amino acid needs in children with mild-to-moderate chronic cholestatic liver disease. J Nutr. 2006;136:133–9.[Abstract/Free Full Text]

25. Mager DR, Wykes LJ, Roberts EA, Ball RO, Pencharz PB. Effect of orthotopic liver transplantation (OLT) on branched-chain amino acid requirement. Pediatr Res. 2006;59:829–34.[Medline]

26. Wykes LJ, Ball RO, Pencharz PB. Development and validation of a total parenteral nutrition model in the neonatal piglet. J Nutr. 1993;123:1248–59.[Abstract/Free Full Text]

27. House JD, Pencharz PB, Ball RO. Tyrosine kinetics and requirements during total parenteral nutrition in the neonatal piglet: the effect of glycyl-L-tyrosine supplementation. Pediatr Res. 1997;41:575–83.[Medline]

28. 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]

29. Elango R, Pencharz PB, Ball RO. The branched-chain amino acid requirement of parenterally fed neonatal piglets is less than the enteral requirement. J Nutr. 2002;132:3123–9.[Abstract/Free Full Text]

30. Shoveller AK, Brunton JA, Pencharz PB, Ball RO. The methionine requirement is lower in neonatal piglets fed parenterally than in those fed enterally. J Nutr. 2003;133:1390–7.[Abstract/Free Full Text]

31. Cvitkovic S, Bertolo RF, Brunton JA, Pencharz PB, Ball RO. Enteral tryptophan requirement determined by oxidation of gastrically or intravenously infused phenylalanine is not different from the parenteral requirement in neonatal piglets. Pediatr Res. 2004;55:630–6.[Medline]

32. Brunton JA, Shoveller AK, Pencharz PB, Ball RO. The indicator amino acid oxidation method identified limiting amino acids in two parenteral nutrition solutions in neonatal piglets. J Nutr. 2007;137:1253–9.[Abstract/Free Full Text]

33. Roberts SA, Ball RO, Moore AM, Filler RM, Pencharz PB. The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirement. Pediatr Res. 2001;49:111–9.[Medline]

34. Moehn S, Bertolo RF, Pencharz PB, Ball RO. Development of the indicator amino acid oxidation technique to determine the availability of amino acids from dietary protein in pigs. J Nutr. 2005;135:2866–70.[Abstract/Free Full Text]

35. Humayun MA, Elango R, Moehn S, Ball RO, Pencharz PB. Application of the indicator amino acid oxidation technique for the determination of metabolic availability of sulfur amino acids from casein versus soy protein isolate in adult men. J Nutr. 2007;137:1874–9.[Abstract/Free Full Text]

36. Humayun MA, Elango R, Ball RO, Pencharz PB. Reevaluation of the protein requirement in young men with the indicator amino acid oxidation technique. Am J Clin Nutr. 2007;86:995–1002.[Abstract/Free Full Text]




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R. Elango, M. A. Humayun, R. O. Ball, and P. B. Pencharz
Indicator Amino Acid Oxidation Is Not Affected by Period of Adaptation to a Wide Range of Lysine Intake in Healthy Young Men
J. Nutr., June 1, 2009; 139(6): 1082 - 1087.
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