The Role of Protein and Amino Acids in Sustaining and Enhancing Performance (1999) / Chapter Skim
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9 Inherent Difficulties in Defining Amino Acid Requirements
9 Inherent Difficulties in Defining Amino Acid Requirements
Pages 169-216
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From page 169... ...
National Academy Press 9 Inherent Difficulties in Defining Amino Acict Requirements D Joe Millward INTRODUCTION To address the role of protein and amino acids in performance, this chapter is based on the premise that it is an inherently difficult problem to define the dietary requirements of human adults for indispensable amino acids and to assess the nutritional value (protein quality)
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From page 170... ...
Thus, the infant values were largely patterned on the composition of breast milk, while the adult values, measured in balance studies with excess nonessential nitrogen and low levels of indispensable amino acid, would have identified minimum requirement values. They concluded that IAA requirements are complex, include an adaptive component, and can only be defined under specific artificial conditions that would allow definition of a minimum value and that "current estimates of adult requirements may be close to this level." To identify which IAA might be rate limiting for the obligatory N losses (ONL)
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From page 171... ...
Protein requirements are mean values ~ These are the rates of oxidative loss of IAA's predicted to occur based on the assumption that the ONL (54 mg Nlkg/d) derive from the oxidation of amino acids liberated from body protein (amino acid composition as beef)
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From page 172... ...
. This growth would markedly increase the need for indispensable amino acids compared with that of normal preschool children, older children, and especially adults.
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From page 173... ...
Obligatory Metabolic Demand The MD for dietary protein is to provide amino acid precursors for the synthesis of tissue proteins and a range of nonprotein products. Although most proteins are in a dynamic state of constant turnover, little metabolic demand for amino acids is generated by this avenue because of amino acid recycling.
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From page 174... ...
The task, then, is to define the amounts and amino acid pattern of the maintenance requirement. Obligatory Metabolic Demands and Obligatory Oxidative Losses The diverse obligatory maintenance MDs for amino acids represent an important, but small, intrinsic part of MD, the magnitude of which is the main subject of current debate.
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From page 175... ...
All other amino acids with a lower ratio would be present in excess in the OOL but would be nevertheless oxidized because they could not be returned on their own to the tissue protein pool. They argued that if protein turnover is tightly regulated, allowing just enough of the rate-limiting amino acid to be withdrawn from tissue protein to provide for its MD, the OOL of this amino acid should be a reasonable guide to its requirement.
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From page 176... ...
The first is deletion studies, in which individual amino acids are removed from the diet and the extent of the negative balance is monitored. If the maintenance requirement patterns corresponded exactly to the patterns of tissue protein, then there should be a similar negative balance on removal of each IAA.
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From page 177... ...
Leucine and lysine are the two most abundant amino acids in carcass proteins and in the growth requirement patterns for both rat and pig; in the maintenance requirement patterns, the most abundant amino acids are threonine and TSA in the pig; threonine, isoleucine, valine, and TSA in both adult and growing rats. TABLE 9-2 Responses (Negative Balance)
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From page 178... ...
Growing rate Adult rat: Body G M Body G M G M Histidine 0.74 0.65 0.34 0.14 0.41 0.48 Isoleucine 0.92 0.92 0.30 0.81 1.07 0.56 1.08 1.03 Leucine 1.88 1.66 0.43 1.67 1.61 0.38 1.35 0.94 Lysine 1.87 1.45 0.68 1.58 1.23 0.33 1.76 0.74 TSA 0.74 0.76 0.92 0.87 1.40 0.63 0.98 0.97 TAA 1.89 1.80 0.70 1.67 1.25 0.20 1.41 1.15 Threonine 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 TIyptophan nd 0.26 0.21 0.26 0.16 0.06 0.22 0.22 Valine 1.25 1.12 0.38 1.03 1.27 0.66 1.10 1.03 NOTE: G growth; M, maintenance; TSA, total sulfur amino acids; TAA, total aromatic amino acids; nd, not determined.
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From page 179... ...
50 _ o -50 -1 00 -1 50 -200 I wheat +mix -threo -met -ileu -~1 -Iysine FIGURE 9-1 Adult rats were fed limiting amounts of a wheat diet that did not allow balance and were supplemented with a mixture of indispensable amino acids that did allow balance. By removing individual amino acids, the limiting amino acids were identified as threonine and the sulfur amino acids.
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From page 180... ...
Traditionally, this has been defined as "the labile protein reserves," which imply that metabolically it was a pool of protein that varied in size with the dietary protein intake. In the rat, liver and visceral protein content does vary directly with dietary protein intake (see Munro, 1964)
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From page 181... ...
Even for the lowest intake, total losses (assumed to be tissue protein and association amino acids) were in excess of the obligatory MD, which indicates that subjects may not have fully adapted to this low intake in the relatively short (2-week)
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From page 182... ...
Thus, after a meal, these amino acids are rapidly removed by oxidative catabolism if not deposited in protein by high 250 200 150 100 50 Obligatory and adaptive metabolic demand from postabsorptiYe leucine oxidation mgN,kg per day 1~ ~ O ~V~:341li days on diet 0 intake: 1.86 gpratein~kg 84 38 0.76 0.76 0.76 adaptive metabolic demand obligatory metabolic demand FIGURE 9-3 Rate of change in the adaptive metabolic demand during transition from a high- to lower-protein diet. Because of the slow change in the adaptive metabolic demand after the reduction in protein intake, there was a negative nitrogen balance throughout the 14 days after the diet change.
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From page 183... ...
The overall metabolic scheme describing these various aspects of the MD is shown in Figure 9-4. The metabolic fate of dietary protein is shown as providing amino acids into Me tissue tree amino acid pool, which is in a state of dynamic protein intake _ 7 free mino-~cid protein pool turnover irreversible metabolism and oxidathre losses / memory demoted Of ~ F meal-dependent obligatory metabolism variable losses ileal losses and surface losses adaptive, habitual di et-dependerit losses epletion of fasting loss and growth body protein pool FIGURE 9-4 Scheme to describe the metabolic fate of amino acids in relation to the metabolic demand.
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From page 184... ...
Does it mean that regardless of the amino acid composition of the intake that induces the adaptive MD, the adaptive MD will have a tissue protein pattern of amino acids to enable postprandial protein deposition? Young and El-Khoury (1995)
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From page 185... ...
By 7 hours, the concentration of all the BCAAs, methionine, and the aromatic amino acids had fallen below the baseline, but for threonine and lysine, there was still an excess of amino acid over the baseline value. Some indication of how much of these free pools of lysine and threonine might be available to supplement dietary protein that was inadequate in lysine and threonine was indicated by biopsy studies after feeding subjects a proteinfree meal.
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From page 186... ...
Summary of the Metabolic Demand for Indispensable Amino Acids In normal adults, the MD for IAAs includes the following: · an obligatory component, which in most cases is less than that contained in 0.33 g tissue protein and has a pattern that cannot be predicted from first principles. However, on the basis of animal data, it is likely to contain considerably less lysine and leucine than in the tissue protein pattern.
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From page 187... ...
For example, Young and colleagues have argued that because of "conceptual limitations" and poor reproducibility, N balance studies in adults in relation to assessment of amino acid requirements are of questionable value (Marchini et al., 1993; Young, 19861. Some of the difficulties reflect the practical problems that arise in performing N balance studies.
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From page 188... ...
. In fact, as is well documented in protein requirement balance studies, prediction of a zero balance intake intercept from a few balance points by linear regression will result in requirement values that vary according to where the intake values lie on the balance curve (see Millward and Roberts, 1996~.
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From page 189... ...
(1971) tested in one group the pattern that Rose fed or in another group the pattern with a 30 percent addition of all essential amino acids together with nonessential N to give a total level of 7 g N
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From page 190... ...
When rapid growth is occurring, differences in dietary protein quality are observed. Graham and colleagues studied the nutritive value of various unsupplemented plant protein sources (wheat, maize, potato, rice, beans, and sorghum)
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From page 191... ...
A large number of adult trials of protein quality have been conducted with a variable range of findings. Although differences among protein sources have been reported in N balance studies in young adults (e.g., relative biological values of 0.66 for wheat compared with beef [Young et al., 1975~)
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From page 192... ...
The impact of this adjustment is shown in Table 9-5; the total requirement values were doubled for most amino acids. Nevertheless, when these values are compared with the pattern obtained using OOL, the former (values recalculated for sedentary individuals on moderate N intakes)
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From page 193... ...
Recalculatedt mg/d mg/lcg (70~§ mg/d mg/lcg(70) OOL: Histidine - - 11.5 Ileucine 550 7.91,083 15.5 16.2 Leucine 730 10.41,580 22.6 27.4 Lysine 545 7.81,118 16.0 30.1 TSA 350 5.0960 13.7 13.4 TAA 350 5.01,184 16.9 27.0 Threonine 375 5.4942 13.5 15.5 T'yptophan 168 2.4222 3.2 4.0 Valine 622 8.9811 11.6 16.9 Total 3,690 7,898 162 % protein 10.5 22 48 NOTE: FAO, Food and Agricultural Organization; OOL, Obligatory oxidative loss; TAA, total aromatic amino acid; TSA, total sulfur amino acid.
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From page 194... ...
Day-to-day variation is large for N balance studies, some of which may reflect the operation of the regulator of body composition. The extent of day-to-day variation in leucine balance is currently unknown.
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From page 195... ...
195 Problems 2 to 4 are solvable by appropriate methodologies. Problem 5 can probably be ignored for leucine and most carboxyl-labeled amino acids, but possibly not for threonine, which exhibits unrealistic positive balances (Zhao et al., 1986)
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From page 196... ...
being supplied during the postabsorptive 14-hour period. The likely effect of this would be to reduce the capacity for net postprandial protein synthesis through leucine limitation and to increase oxidation and negative balance in the postabsorptive state, with an overall leucine balance that is more negative than otherwise.
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From page 197... ...
Furthermore, the lysine study involved a method that does not depend on balance, that is, the indicator amino acid oxidation method. As shown in studies of animals fed amino acid mixtures, when the test amino acid intake falls below the requirement for adequate postprandial net protein synthesis, oxidation of a labeled indicator amino acid increases.
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From page 198... ...
In the context of the diurnal cycling model of balance, the increasing amplitude of gains and losses with increasing protein intake, does involve changes in protein synthesis (although changes in proteolysis are the main mediator of cycling)
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From page 199... ...
On this basis, it appears that the data suggesting that protein synthesis falls at inadequate intakes of individual amino acids are artifactual. Thus, with balanced amino acid mixtures and whole proteins, whole body protein synthesis changes only minimally at intakes below those capable of supporting overall balance.
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From page 200... ...
In growing animals, dietary protein intakes in excess of those associated with maximum efficiency of protein utilization have been shown to increase rates of bone growth through hormonal mechanisms involving insulin, thyroid hormones, and insulin-like growth factor-I (IGF-I)
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From page 201... ...
As for physical activity, it is often assumed that increasing protein intakes is of benefit (e.g., Lemon, 1996) , but much of the data indicating apparent increased protein requirements of athletes are based on arguably misleading N balance studies.
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From page 202... ...
....... ~ -~ .,.;, I ',,.,, , ~ FAO preschool Adjusted FAO valued adult values' rapid growth met~olic demand adaptive oxidative losses adaptive postprandial needs adaptive Iysine recycling _um obligatory ~ metabolic demand FIGURE 9-7 Amino acid requirements can be defined for rapid growth, which may be close to the FAO preschool values, and for a minimum obligatory metabolic demand, possibly represented by the adjusted values in Table 9-5.
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From page 203... ...
A specific question that this workshop addressed related to amino acid requirements is the following: Is there evidence that a different mix of amino acids would optimize military performance (cognitive function, long-tenn muscle mass preservation) during high workload, psychological stress, and/or energy deficit?
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From page 204... ...
1990. Effect of a test meal with and without protein on muscle and plasma free amino acids.
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From page 205... ...
1963. Variation in requirements of nutrients: Amino acids.
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From page 206... ...
1981. Protein quality and digestibility of sorghum in pre school children: Balance studies and plasma free amino acids.
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From page 207... ...
1981. Protein quality in relation to estimates of essential amino acids requirements.
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From page 208... ...
1990. Mechanisms and nutritional significance of metabolic responses to altered intakes of protein and amino acids with reference to nutritional adaptation in humans Am.
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From page 209... ...
Now, Mike Rennie will present data on physical activity and intakes and deal with this problem to a certain extent. But, in my view, the real point about the human response to protein intake is that we adapt our metabolic demand according to how much we eat.
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From page 210... ...
That argument is made basically in the Young and Marchini paper, which he quotes in his abstract, where they show that whole body protein synthesis rates plummet dramatically as the intake of essential amino acids plummet. I think that data are wrong, to be quite honest.
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From page 211... ...
I think we are trying to address a problem about intake and requirements in the face of a body that is filled with amino acids, as you pointed out in your remark about the utilization and recycling. It becomes very difficult, I think, to solve the problems in that context when really what we are looking for is some change from the baseline.
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From page 212... ...
I have always taken the view that, if you cannot actually measure the total amount of body protein that is there and whether it is going up or whether it is going down, any other information is basically irrelevant. You have got to start with good data about what is the state of the overall system, i.e.
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From page 213... ...
Because we performed a study a number of years ago, albeit not in normal subjects, but in severely burned patients in a crossover study in which they were given different levels of protein, and three different essential amino acids were used, and all three tracers showed parallel increases in whole body protein turnover when the protein intake was increased, although none indicated an improvement in balance. So I think the fact that the same amino acid tracer as the test compound has been used with different levels of protein intake may not be valid, and that it is a jump to go from that observation to the conclusion that there is no effect of protein intake on protein turnover.
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From page 214... ...
I think that Bill Evans' group (when he was still at Tufts) , recently published data that suggested that exercise training in older individuals, whether they had a dietary protein intake of .8 grams per kg or 1.6 grams per kg, essentially achieved the same level of muscle strength and I think those individuals who were on the lower protein intake were still in positive nitrogen balance, but not as great as those on the higher protein intake.
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From page 215... ...
There are good data on muscle biopsy studies, which I referred to in my talk, of the rate at which amino acids disappeared from the intracellular muscle pool after a meal. It is quite clear that, whereas leucine, all three BCAAs, the aromatic amino acids, and the sulfur amino acids disappear quite quickly, lysine and threonine stick around for a long time.
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From page 216... ...
We would say that the majority of our protein intake is coming from meat. But that is switching as we look at newer concepts in our rations, where we have more vegetarian items.
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