The Role of Protein and Amino Acids in Sustaining and Enhancing Performance (1999) / Chapter Skim
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10 Amino Acid Flux and Requirements: Counterpoint Tentative Estimates are Feasible and Necessary
10 Amino Acid Flux and Requirements: Counterpoint Tentative Estimates are Feasible and Necessary
Pages 217-242
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From page 217... ...
His points are multiple and well articulated, although he presents little direct experimental data from his own laboratories that are directly relevant to the quantitative determination of adult human amino acid requirements or that are in contrast to our own findings. Nevertheless, I agree with a number of important elements in his argument; indeed, I accept some of his criticisms of our work, which is overviewed briefly below.
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From page 218... ...
should be used for evaluating the amino acid adequacy of diets and/or for planning diets intended to meet adequately the physiological needs of consumers in healthy populations. With respect to the issue of indispensable amino acid requirements and dietary protein quality, the most recent UK expert panel (Department of Health, 1991)
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From page 219... ...
with the same amino acid composition of the mixed proteins in the body. He is correct except that the values for lysine, sulfur amino acids, and threonine, in particular, were derived more from the results of our tracer studies, which were limited in scope.
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From page 220... ...
, but this does not necessarily mean that the amino acid requirement pattern would be that different. Additionally, body protein maintenance in the adult involves depletion of body proteins during the fasting period of the day and their repletion during the fed period of the day.
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From page 221... ...
Third, Millward concludes, from the animal data, that the maintenance pattern differs from that of growth, and criticizes our estimates for adult human amino acid requirements. I have said before that maintenance in a growing animal is a very different metabolic condition than that of nutritional maintenance in an adult and probably also that of a pre-school child.
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From page 222... ...
This question is raised despite the fact that the current international FAO/WHO/UNU (1985) amino acid requirement values, based largely on studies conducted mainly in young adult American subjects (Irwin and Hegsted, 1971)
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From page 223... ...
to assess the lysine requirement of healthy Indians whose long-term lysine intake appears to be about 60 percent of the level characteristic of the North American subjects. Results from this laboratory do not suggest that adequately nourished Indian subjects require a different lysine intake for maintenance as compared to MIT subjects.
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From page 224... ...
This hypothesis should now be further validated through metabolic studies of varying duration and, preferably, in population groups in different geographic regions of the world. Diurnal Cycling and Amino Acid Requirements Millward and coworkers (1996)
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From page 225... ...
Clearly, it would be worthwhile to obtain data on changes in the free amino acid concentrations in muscle tissue under the relevant dietary conditions of our \3Camino acid tracer studies. However, technical difficulties and ethical constraints make this an unlikely research activity in the near fixture.
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From page 226... ...
This design would be expected to enhance the retention of lysine during the "requirement" phase of the study. For this reason, as well as the fact that only at TABLE 10-3 An Illustration of the Limitation of Nitrogen Balance as a Basis for Estimation of Lysine Requirements in Five College Women Time on Diet (days)
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From page 227... ...
(1955a) was appropriate, it is difficult to accept, on metabolic grounds, a lysine requirement of 6.25 mg/l~g/d.
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From page 228... ...
of the magnitude of the prandial retention of protein necessary to balance subsequent post-absorptive losses appears to be at odds with estimates of the minimum requirement level for lysine; this observation is based on the following line of reasoning. First, for purposes of this argument, the minimum needs for total protein in adults can be estimated Tom the response of body nitrogen balances measured over relatively short experimental diet periods (e.g., 10 to 14 days)
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From page 229... ...
For these reasons, we have not used N balance data to enhance interpretation of the results of our own tracer experiment that was concerned with testing the 1985 FAO/WHO/UNU amino acid requirement pattern (Marchini et al., 1993, 1994~. We anticipated that the balance data would be difficult to assess for their nutritional significance and chose, instead, to evaluate the amino acid kinetic and plasma amino acid data.
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From page 230... ...
Hence, if an amino acid score [(amino acid content in the food protein/amino acid content in the reference amino acid requirement pattern)
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From page 231... ...
In contrast, use of the 1985 FAO/WHO/UNU adult amino acid pattern gives an invalid estimate of the nutritional value of wheat protein, in that this pattern makes wheat proteins nutritionally equivalent to beef proteins. Notwithstanding the problems faced when attempting to aggregate N balance data across studies carried out in different laboratories or within the same laboratory on different occasions (e.g., Millward et al., 1989)
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From page 232... ...
Again, as discussed earlier, the tissue free amino acid pools, particularly muscle, can serve as a reservoir for IAAs when intakes exceed immediate needs for protein synthesis (Bergstrom et al., 1990~. Thus, these "stored" amino acids can later be used when there is an intake of the other IAAs.
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From page 233... ...
criticizes the Toronto studies in human subjects, even though they were designed in relation to the various indicator amino acid studies conducted in experimental animals, which have given amino acid requirement estimates that are consistent with those derived from growth studies (Zello et al., 1995~. In a recent paper, Millward (1997)
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From page 234... ...
This is because there could well be a "replete" free lysine pool that would serve as an unaccounted source of utilizable lysine, in addition to the actual intake supplied by the six small hourly meals given beginning 2 hours before and during the 4-hour isotope tracer study. It is also of possible interest that in estimating protein requirements in healthy elderly and young adults Tom ~3C-leucine balance studies, Millward and colleagues (Fereday et al., 1997)
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From page 235... ...
. ., in my view, definition of adult indispensable amino acid requirements for protein quality scoring is not currently possible or likely to be useful in the future." We (Metges et al., 1997)
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From page 236... ...
the practical nutritional relevance of the amino acid requirement values proposed initially by Rose and used together with comparable data from other investigators to arrive at the 1985 FAO/WHO/UNU adult indispensable amino acid requirement pattern and (b) determination of amino acid requirement values that are likely to be minimally sufficient to maintain body amino acid homeostasis, and by implication adequate body function, over the long term.
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From page 237... ...
in order to weigh the merits of our respective views. It is my judgment that the minimum physiological requirements for amino acids to maintain health and body function should be defined and that it would be prudent to apply the tentative requirement values proposed herein for the protein and amino acid nutrition of healthy adults.
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From page 238... ...
1996. Report of the working group on protein and amino acid requirements.
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From page 239... ...
1993. The requirements for indispensable amino acids in adult humans: A longer-term amino acid kinetic study, with support for the adequacy of the Massachusetts Institute of Technology amino acid requirement pattern.
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From page 240... ...
1981. Protein quality in relation to estimates of essential amino acid requirements.
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From page 241... ...
1989. A theoretical basis for increasing current amino acid requirements in adult man, with experimental support.
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From page 242... ...
Pent, and R.O.BaB.1993. Dieted lysine ~quiremenl of young adult males detennined by oxidation of L-[1-13Clpbenylalanine.
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