3. Amino Acids - ESPEN

1 3. Amino AcidsMETHODSL iterature SearchTime frame: Publication search is 1992 2004 plus relevantpublications before of publications: Case control and cohort studies,randomised controlled studies and systematic : Words: Adolescents, children, neonates, preterm infants,parenteral nutrition, Amino Acids , requirements, toxicity, children with age range from preterm to adolescent wereconsidered in these were developed from a standpoint ofnutrient adequacy. Depending on age groups, nutrient adequacywas based on intrauterine accretion rate, organ development,factorial estimates of requirements and Amino acid Amino Acids are intakes of specific Amino Acids are those that meetthe specific requirement of children in that age group.

2 Maximalintakes are recommended to prevent excessive and potentiallyharmful intakes of Amino ACIDSI ntroductionProteins are the major structural and functional com-ponents of all cells in the body. They consist of chains ofamino acid subunits joined together by peptide chain length ranges from two Amino Acids to thou-sands, with molecular weights subsequently ranging fromhundreds to hundreds of thousands of Daltons. From anutritional perspective, an important aspect of a proteinis its Amino acid Amino Acids are classified as essential (indis-pensable). Those are Amino Acids that cannot be syn-thesized by humans and hence must be provided in thediet or parenteral solution. Non-essential Amino Acids canbe synthesized from other Amino Acids or from otherprecursors.

3 Some Amino Acids are categorized as semi-essential. These Amino Acids can be synthesized fromother Amino Acids but their synthesis is limited undercertain circumstances (1 6). These Amino Acids may beof particular importance for the preterm infant in whoma developmental delay in specific enzymes involved inamino acid synthesis have been demonstrated (7 11).The essential, non-essential and conditionally essen-tial Amino Acids are listed in table 1. Although everybodyagrees on the concept that some none-essential aminoacids are essential under certain circumstances, somedebate exists about the conditional essentiality of arginineand FOR ESTIMATING TOTAL ANDINDIVIDUAL Amino acid NEEDSA mino acid requirements are mainly determined by the rateof net protein synthesis, which depends on the availability ofrate limiting Amino Acids .

4 There are several physiological andbiochemical ways to determine whether the Amino acid intake issufficient or in excess of the needs of children. Different mea-surements in assessing adequacy of Amino acid intake includeanthropometry (weight and length), nitrogen balance, metabolicindices ( Amino acid concentrations, albumin, pre-albumin,total protein concentrations, blood urea nitrogen, metabolicacidosis), whole-body nitrogen kinetics, specific Amino acidkinetics and the indicator Amino acid method. The intake ofeach essential Amino acid required to maintain nitrogen equi-librium in children and infants has been defined as the amountnecessary to obtain growth and nitrogen veterinary medicine the free Amino acid concentrationswere used to detect deficiencies and excesses of dietary aminoacids.

5 This principle works more rapidly than weight gain orgrowth studies (12 15). Most current Amino acid solutions aredeveloped in a similar fashion (16 18).The Amino acid indicator method seems to be an accurate andfast way to determine specific Amino acid requirements. It hasrecently been developed to measure specific Amino acid re-quirements (19 21) and has been validated in animal models ofinfancy (22 24). The technique is based on the partitioning ofessential Amino acid outflow under steady state conditions be-tween oxidation and protein synthesis. When a single essentialamino acid is deficient in the diet, the amount of protein that canbe synthesized is limited. Since the limiting Amino acid alsolimits the use of all other dietary Amino Acids for proteinsynthesis, the body must oxidize excess amounts of these aminoacids.

6 If one increases the dietary amount of the limiting aminoacid, protein synthesis will increase and so will the utilization ofthe other dietary Amino Acids which in turn reduces theiroxidation. Once the requirement for the limiting Amino acid isreached, further increases in its dietary intake will cause nofurther increase in protein synthesis, nor decrease in the oxida-tion of the other essential Amino indicator Amino acid oxidation method uses thisrelationship (19). Subjects are given a series of diets containingvarying amounts of the Amino acid for which the requirementis to be determined. The amounts vary below and aboverequirement. All other Amino Acids are furnished at constantJournal of Pediatric Gastroenterology and Nutrition41:S12 S18 November 2005 ESPGHAN.

7 Reprinted with At the end of each diet period,a dose of another essential Amino acid with a13 Cor14C label(the indicator Amino acid ) is given, and its oxidation ismeasured. The oxidation of the labelled indicator Amino acidwill decrease as the amount of test Amino acid increases, untilrequirement is reached, and then the oxidation will the oxidation of the labelled indicator Amino acidagainst test Amino acid intake should show a breakpoint at therequirement level for the test Amino acid . A slightly differentapproach is the use of the oxidation rate of the investigatedamino acid or its direct metabolite. Such an approach hasrecently been used in the determination of the requirement oftyrosine in parenterally fed infants (25).

8 Most currently used parenteral Amino acid mixtures containamino acid amounts that result in a plasma Amino acid patternresembling the plasma Amino acid patterns of normallygrowing, breast fed infants and children, or cord blood. Thesepaediatric parenteral Amino acid mixtures provide more essen-tial and less non-essential Amino Acids than normally depositedby the infant or utilisation of the Amino acid supply depends on a suf-ficient energy intake, and often an energy supply of 30 to 40kcal per 1 g Amino Acids is Amino acid NEEDS DURINGPARENTERAL NUTRITIOND ifferences Between Enterally Fed andParenterally Fed ChildrenThe Amino acid requirement is lower in parenterallyfed infants and children than in enterally fed infantsbecause the supply bypasses the intestine.

9 No human dataare available but animal studies suggest that the equiv-alent of approximately 30 50% of the protein intake isused by the intestine in neonates (26 28). However, thereis a wide variation in the intestinal uptake and utilizationof specific Amino Acids that changes with age. First pass(intestinal and liver) leucine utilisation in older childrenis 24% (29), while it accounts for approximately 50% ofthe dietary intake in preterm infants (30). Intestinalutilisation of lysine accounts for approximately 20% ofthe intake (31) whereas 50% of glutamine is used (30) inpreterm infants. Thus, the total needs of Amino Acids inparenterally fed children are lower than in enterally fedchildren, but there are huge differences in intestinalutilization of specific Amino Acids .

10 Besides utilization bythe intestine, a number of Amino Acids are also metab-olized and converted into other Amino Acids within theintestine and/or liver upon first pass. Bypassing theintestine will lower systemic availability of these aminoacids and thus increase the parenteral requirements. Inaddition, while ingested phenylalanine and methionineappear to be converted to tyrosine and cysteine, respec-tively. It seems that parenterally administered phenylal-anine and methionine are converted to a lower active peptides are produced within theintestine ( sIgA) and animal studies show that theintestine uses predominantly dietary Amino Acids (morethan Amino Acids that are offered to the intestine from thesystemic circulation) for specific protein synthesis (32).