. "10 Amino Acid Flux and Requirements: Counterpoint Tentative Estimates are Feasible and Necessary." The Role of Protein and Amino Acids in Sustaining and Enhancing Performance. Washington, DC: The National Academies Press, 1999.
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The Role of Protein and Amino Acids in Sustaining and Enhancing Performance
proteins amounts to about 30 percent, which is consistent with the lysine oxidation data mentioned above. This matter will be raised again below, in some detail, and with respect to the diurnal cycling of protein retention and loss.
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. Further, the data for requirements for adult maintenance in different animal species are not only limited but also contradictory, as we both recognize (McLarney et al., 1996; Millward, 1998, see Chapter 9).
Millward's three positions are not supported adequately. A reasonable initial and tentative definition of the pattern and level of requirements for the IAA in adult humans can be made from estimates of OAALs (Young and El-Khoury, 1995).
Adaptive Aspects of Amino Acid Metabolism
Millward appropriately emphasizes that the adaptive component of amino acid metabolism brings complexity to the assessment of maintenance amino acid needs. The important practical question, however, is how long it takes the integrative aspects of whole body amino acid metabolism to adjust to a new level of amino acid, or total protein, intake so that a reasonable estimate of the steady state amino acid needs for that intake level and status of nutriture can be made by either N balance or tracer techniques. In our studies, we have used relatively short experimental diet periods of 6 to 7 days prior to the conduct of 13C-amino acid oxidation studies (Young, 1994). Millward (Chapter 9) concludes that the adaptation of oxidative catabolism to a change in protein intake is relatively slow and that the extent of postprandial oxidative catabolism reflects mainly the habitual rather than the actual or present protein intake supplied in meals. The validity of this conclusion depends on whether the metabolic responses and adaptations to altered protein or amino acid intakes are indeed as slow as Millward concludes. He bases his argument on his own studies of N balance and leucine oxidation following the transition from a high- to a low-protein diet in healthy adults.
The following six points are relevant, therefore, to the question of whether a 6-day period of "adaptation" is long enough to permit a suitable derivation of nutritionally meaningful amino acid requirement values:
Based on the earlier N excretion and balance studies carried out in our laboratories (Rand et al., 1976, 1979, 1985) and on our evaluation of the data published by others (Rand et al, 1981) it is entirely reasonable to propose that a 6 day adjustment period is suitable for purposes of establishing the amino acid needs of initially well-nourished subjects. Further, relatively short-term