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14-18 y, Girls

Histidine

11

< 0.5

12

14

Isoleucine

15

< 0.5

16

19

Leucine

34

1

35

44

Lysine

31

1

32

40

Methionine + cysteine

15

< 0.5

16

19

Phenylalanine + tyrosine

27

1

28

35

Threonine

16

< 0.5

17

21

Tryptophan

4

< 0.5

4

5

Valine

19

1

20

24

a Derived from die adult EAR for specified amino acids (from Table 10-20).

* Derived using the following equation: Amino acid deposition = mean protein deposition (from Table 10-9) X amino acid composition of whole body protein (from Table 10-19).

c EAR for ages 7 mo-13 y = maintenance + amino acid deposition X 1.72. EAR for ages 14—18 y = maintenance + amino acid deposition x 2.13.

d RDA for ages 7 mo-13 y = EAR + 2 X V [(0.12 X maintenance) 2 + (0.43 X 1.72 X mean protein deposition) 2]. RDA for ages 14—18y = EAR + 2 X V[(0.12 X maintenance)2 + (0.43 X 2.13 X mean protein deposition)2].

676 DIETARY REFERENCE INTAKES

-25 ; _1___r______. _____,_____, __________________

-25 ; _1___r______. _____,_____, __________________

Lysine Intake (mg-kg ^cf1}

FIGURE 10-7 Relationship between nitrogen balance and test amino acid intake using four different one-fit regression equations: linear (A), square root (B), log (C), and exponential asymptotic (D), superimposed on the original data. Reprinted, with permission, from Rand and Young (1999). Copyright 1999 by the American Society for Nutritional Sciences.

support to the lysine DAAO estimate (Kurpad et al., 2001a; Meredith et al., 1986).

The 24-hour balance model is regarded as being the best from a theoretical point of view, especially when performed with the indicator approach. However, from a practical point of view, the 24-hour amino acid balance studies are very labor intensive with the result that only three or four levels of intake of the test amino acid have been studied for each of leucine, lysine, phenylalanine + tyrosine, and threonine.

Direct Amino Acid Oxidation. The DAAO method has been utilized to investigate six or seven amino acid levels, so it was possible to reanalyze these data using two-phase linear crossover regression analysis and define a breakpoint (which is regarded as the EAR). DAAO can only be used if the carboxyl group of the test amino acid is released to the body bicarbonate pool when the amino acid is committed to degradation. As shown in

PROTEIN AND AMINO ACIDS 677

TABLE 10-22 Indispensable Amino Acid Studies in Adults

Estimated

Method Useda

Average

(Number of Levels/

Requirement

Reference

Amino Acid

Number of Data Points)

(mg/kg/d)

Meguid et al., 1986a

Leucine

DAAO reanalyzed (8/52)

24.5

El-Khoury et al., 1994a

Leucine

24-h AAB (3/10)

38.3

Kurpad et al., 2001b

Leucine

24-h AAB (4/40)

40

Meredith et al., 1986

Lysine

DAAO reanalyzed (8/28)

26.6

Zello et al., 1993

Lysine

IAAO (7/42)

36.9

Rand and Young, 1999

Lysine

N-Balance reanalyzed

30

(8/53)

Kurpad et al., 2001a

Lysine

24-h IAAB (4/32)

29

Kriengsinyos et al.,

Lysine

IAAO (5/60)

35

2002

Kurpad et al., 2002a

Lysine

24-h IAAB (4/36)

29

Reynolds et al., 1958

Methionine

N-balance reanalyzed

20

+ cysteine

(6/42)

Young et al., 1991

Methionine

Methionine balance (1/5)

13

+ cysteine

Di Buono et al., 2001

Methionine

IAAO (6/36)

12.6

+ cysteine

Zello et al., 1990

Phenylalanine

DAAO (7/41)

9.1

Roberts et al., 2001

Tyrosine

IAAO (7/42)

6.0

Phenylalanine

15.1

+ tyrosine

Basile-Filho et al.,

Phenylalanine

24-h AAB

39.0

1998

+ tyrosine

Zhao et al., 1986

Threonine

DAAO reanalyzed (7/33)

13.5

Wilson et al., 2000

Threonine

IAAO (7/36)

19.0

Borgonha et al., 2002

Threonine

24-h IAAB (3/15)

15.0

Kurpad et al., 2002b

Threonine

24-h IAAB (6/48)

15.0

Lazaris-Brunner et al.,

Tryptophan

IAAO (8/36)

4.0

1998

Meguid et al., 1986b

Valine

DAAO reanalyzed (7/37)

19.2

a AAB = amino acid balance, DAAO = direct amino acid oxidation, IAAB = indicator amino acid balance and oxidation, IAAO = indicator amino acid oxidation.

Table 10-22, DAAO studies of indispensable amino acid requirements are limited to leucine (Meguid et al., 1986a), lysine (Meredith et al., 1986), phenylalanine (Zello et al., 1990), and valine (Meguid et al., 1986b). DAAO was also utilized to determine the threonine requirement (Zhao et al., 1986). However there are theoretical concerns for this amino acid,

678 DIETARY REFERENCE INTAKES

since there are two pathways of degradation for threonine; the second pathway, threonine dehydrogenase (TDG), ends in the label being retained in glycine. In practical terms this may not be a serious error since others have shown that the TDG pathway is a minor pathway in adults (Darling et al., 2000).

Indicator Amino Acid Oxidation. IAAO has the advantage that the requirement of any amino acid can be determined, since either phenylalanine (in the presence of an excess of tyrosine to ensure that there is no label retention in the body tyrosine pools) or lysine can and have been used as indicator amino acids in humans and in animals (Bross et al., 2000; Brunton et al., 1998; Zello et al., 1995). A further strength of the IAAO studies is that each adult was fed at six or seven levels of the test amino acid, which has made it possible to define requirements for individuals by two-phase, linear cross-over regression analysis (Brunton et al., 1998; Zello et al., 1995). As shown in Table 10-22, IAAO estimates have been reported for lysine (Kriengsinyos et al., 2002; Zello et al., 1993), methionine + cysteine (Di Buono et al., 2001), tyrosine (Roberts et al., 2001), threonine (Wilson et al., 2000), and tryptophan (Lazaris-Brunner et al., 1998).

As shown in Table 10-22, currently there are amino acid oxidation estimates in which two-phase linear crossover regression analysis has been performed for leucine (DAAO), lysine (both DAAO and IAAO), methionine + cysteine (IAAO), phenylalanine (DAAO), tyrosine (IAAO), threonine (both DAAO and IAAO), tryptophan (IAAO), and valine (DAAO).

Other Indicators. Nonlinear regression was used on two sets of nitrogen balance data as shown by Rand and Young (1999). The first was for lysine in which the original data were in women, each of whom were studied at two to five levels (Jones et al., 1956). This data set was reanalyzed using nonlinear regression, including the addition of 5 or 8 mg of nitrogen/kg/d as miscellaneous losses (Rand and Young, 1999), and these reanalyzed data are included in Table 10-22. Using a similar approach, the data of Reynolds and coworkers (1958) for methionine + cysteine were reanalyzed, and these data are included in Table 10-22. The result is consistent with the data of Zezulka and Calloway (1976a, 1976b), who studied the effect on nitrogen balance of three levels of methionine added to soy protein at a constant and adequate level of total nitrogen. Since there are no direct estimates of the isoleucine requirement, it is estimated from the leucine and valine estimates. The isoleucine requirement was therefore calculated by multiplying the isoleucine requirement calculated from the protein requirement (Table 10-20) by 1.55, the average of the ratios for leucine and valine. Similarly, the requirement for histidine, for which there have also been no direct determinations, is calculated from the protein require-

PROTEIN AND AMINO ACIDS 679

ment (Table 10-20) multiplied by 1.7, the average ratio for all amino acids in Table 10-19.

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