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* Laboratory of Human Nutrition, School of Science, Massachusetts Institute of Technology, Cambridge, MA 02142-1308, U.S.A.
With the aid of a descriptive model (MILLWARD and RIVERS, 1988), Millward considers the major metabolic processes that are involved in the utilization of amino acids and, thus, in determining the dietary intakes required to support growth in the young and maintenance in the adult. Whether this qualitative model, including the concept of an 'anabolic drive', will help to define, in quantitative terms, the needs for protein and individual indispensable amino acids remains unclear. However, it would be generally agreed that a more complete understanding of the metabolic basis of the protein (amino acid) requirement will be necessary if major advances are to be made in refining the definition of, and approaches taken to determine, requirement values in humans at various stages of life and under different circumstances.
Millward raises a number of intriguing issues and significant points; some of them are highly speculative, and some must be challenged. It is not my intent here to consider all of these issues and points in this brief commentary. Rather, I will discuss and/or argue against just some of them, in the hope that the reader will then become somewhat better informed about the many complex problems raised in the Millward paper, particularly with reference to the issue of amino acid requirements.
It is pointed out, appropriately, in the introduction to Millward's paper that: "the measurement of nitrogen balance alone does not give us sufficient information to judge the adequacy of the dietary intakes...". Indeed, as previously discussed (YOUNG, 1986), the N-balance technique could well give rise to erroneous conclusions. Hence, it is important to recognize where Millward's points and arguments are based on interpretation of N-balance data and to consider whether these interpretations are reasonable and consistent. For example, Millward makes the point that there are differences between the amino acid requirement patterns for maintenance and growth in the growing pig, based on data from N-balance studies, and he then uses this information to question use of obligatory oxidative losses (OOL) of amino acids for predicting human amino acid requirements. I have discussed this issue previously and, in disagreement with Millward, drawn the tentative conclusion that a useful, initial prediction of the amino acid needs of the adult, as well as the young child, can be obtained from a consideration of OOL (YOUNG, 1991). Furthermore, we have pointed out, that the growing pig is not a suitable model for purposes of evaluating the amino acid requirements of human subjects, and we have given the reasons for this view (YOUNG, 1991). Also, it is interesting that, in the abstract to Millward's paper, the statement "no relationship should be expected between protein quality for growing animals and human needs" is made. This is consistent with the foregoing concern that I raised about the pig model, and yet it seems to contradict a number of the arguments Millward himself makes later in his paper.
Support for use of the amino acid requirement pattern that we have proposed (YOUNG et al., 1989), based on a consideration of OOL, is forthcoming from results of a series of tracer studies (YOUNG et al., 1989; YOUNG and MARCHINI, 1990) and further strengthened by data emerging from the Toronto group (ZELLO et al., 1990; 1992). Millward raises, quite rightly, a number of technical problems that ought to be discussed, and in a recent exchange of letters (MILLWARD, 1992; YOUNG et al., 1992) we have addressed a number of these issues. Again, in contrast to Millward, we remain convinced that the lysine requirement (Rmin) of healthy adults is significantly higher than the figure that Millward appears to accept. We accept Millward's concern that estimates of amino acid balance, as derived from our original stable isotope tracer protocols, are not sufficiently precise and so could be criticized as either overestimating or underestimating actual balance. However, we have stated repeatedly that our earlier calculations tend to overestimate balance, and it is unfortunate that Millward has chosen lysine for his specific example of this problem in his paper. Thus, at the time of our initial publication (MEREDITH et al., 1986) we indicated that our figures for lysine oxidation were underestimates due to the fact that we were obliged to use, for our calculations, the plasma 13C-lysine enrichment data, rather than the unknown but lower enrichment of the intracellular lysine pool from which tracer and tracee oxidation was taking place. We did not have the benefit of using a plasma intracellular marker, analogous to that of KIC for leucine (MATTHEWS et al., 1982), to circumvent this problem. The estimated large positive daily balance of lysine at the generous lysine intake of 100 mg/kg/d, for example, supports this interpretation. Recent measurement of the labeling of aminoadipic acid (ARENDS and BIER, 1991) as an index of the enrichment of intracellular lysine suggests a correction of approximately 1.5 or somewhat higher than for our 13C-leucine oxidation studies (MATTHEWS et al., 1982). Thus, if this correction is applied to the original lysine balance data reported by MEREDITH et al. (1986) for the 100 and 58 mg/kg/d lysine intake levels, then the markedly positive balances are reduced to small positive balances, which would be expected to occur under these conditions. Further, in their studies, mean lysine balance would become negative at about the 30 mg/kg/d intake level rather than being a small positive balance as reported originally. This suggests that the isotopically-derived lysine balance calculations made originally were appropriate, with the caveat that they were overestimates to the extent that we were not able to determine the intracellular free lysine enrichment. This strengthens our earlier conclusion (MEREDITH et al., 1986; YOUNG et al., 1989) that the approximate mean lysine requirement in healthy adults exceeded 20-30 mg kg/d. In his paper, Millward has recalculated our original balance by including in his determination an amount greater than the actual quantity of 13C-lysine tracer infused. Thus, his Figure 13 is misleading, since we had already included, in the estimate of the 12-hour balance, the labeled-tracer intakes provided by the priming dose and the 3-hour constant infusion. Further, despite their limitations, the kinetic studies and the lysine balances so derived, together with the nutritional interpretations drawn from them, are consistent with the simultaneously obtained plasma amino acid data, also suggesting inadequacy of lysine intake below about 33 mg/kg/d (MEREDITH et al., 1986). Hence, these studies of lysine kinetics, together with those involving valine, threonine, methionine and also our more extensive series with leucine, consistently support the view for a substantial revision of the requirement values for indispensable amino acids as reported by FAO/WHO/UNU (1985). Furthermore, the higher lysine content of our proposed tentative scoring pattern (YOUNG et al., 1989), compared with that in the FAO/WHO/UNU (1985) pattern, is also supported by: (a) the results of human studies by ZELLO et al. (1992), obtained with the indicator amino acid oxidation method, which concluded that the mean dietary lysine requirement in healthy young adults is met at an intake of about 35 mg/kg/d, (b) our earlier N-balance studies on the nutritional quality of opaque-2 corn (YOUNG et al., 1971), whole wheat proteins (YOUNG et al., 1975) and isolated soy proteins (YOUNG et al., 1984). For example, our proposed amino acid scoring pattern (YOUNG et al., 1989) predicts that whole wheat protein would have a net protein utilization of about 0.4-0.5, and this was found by direct N-balance measurements to be the case (YOUNG et al., 1975). In contrast, the FAO/WHO/UNU (1985) adult amino acid requirement pattern, with its much lower lysine content, predicts a relative nutritional value of ³1 which does not agree with the experimentally determined value. As Millward points out, there is considerable variability among N-balance studies, but it can sometimes be a useful technique for comparison within a given set of study conditions. In this context, a valuable research effort would be to help establish and understand the quantitative effect of various metabolic factors on N balance and how this information could be integrated to predict the N balance of an individual subject for a given set of experimental conditions. It would be interesting to learn whether Millward's model might assist in this direction.
In the meanwhile, and without direct evidence by either Millward or others to the contrary, it would be prudent to accept the revised, tentative amino acid requirement values proposed by YOUNG et al. (1989) for purposes of designing and evaluating the capacity of diets to meet physiological requirements. Clearly, additional investigations on this topic would be highly desirable and if possible, as Millward recommends, with an emphasis on inclusion of functional indices. This, however, remains a difficult challenge, and I consider that there is still a great deal more that we can learn about the 'physiological requirement' from further studies of amino acid kinetics in vivo.
areas of metabolic focus are raised by Millward; these concern
urea salvage in the lower gut and are discussed by Alan Jackson
in more detail elsewhere (this publication), and the possible de
novo synthesis of indispensable amino acids (IAA). Should
there be a significant synthesis of IAA which, in turn, could be
made available to meet physiological needs, then, as Millward
says, the implications would be profound. The statement in his
abstract, namely: "In any case, the emerging evidence that
changes in the dietary amino acid composition can be achieved
during urea salvage and bacterial amino acid synthesis in the
lower gut... " would, however, seem to be premature and
certainly makes too strong an inference, in my opinion. There is
as yet no direct or satisfactory evidence to support the
hypothesis that there is a significant intestinal synthesis of
IAA, which benefits the host, and alternative explanations of the
data presented by Millward need to be ruled out first. The lack
of methodological detail and the preliminary and sketchy nature
of the reports that he refers to in his discussion makes it
difficult to evaluate the merits, or otherwise, of these
ARENDS, J., BIER, D.M.: Labeled amino acid infusion studies of in vivo protein synthesis with stable isotope tracers and gas chromatography - mass spectrometry. Anal. Chim. Acta, 247, 255-263 (1991).
FAO/WHO/UNU: Energy and protein requirements. Report of a Joint FAO/WHO/UNU Expert Consultation. Technical Report Series No. 724. World Health Organization, Geneva, 1985.
MATTHEWS, D.E., SCHWARZ, H.P., YANG, R.D., MOTIL, K.J., YOUNG, V.R., BIER, D.M.: Relationship of plasma leucine end µ-ketoisocaproate during aL-[13C]leucine infusion in man: A method for measuring human intracellular leucine tracer enrichment. Metabolism, 31, 1105-1112 (1982).
MEREDITH, C.N., WEN, Z.-M., BIER, D.M., MATTHEWS, D.E., YOUNG, V.R.: Lysine kinetics at graded lysine intakes in young men. Am. J. Clin. Nutr., 43, 787-794 (1986).
MILLWARD, D.J., RIVERS, J.P.W.: The nutritional role of indispensable amino acids and the metabolic basis for their requirements. Eur. J. Clin. Nutr., 42, 367-393 (1988).
MILLWARD, D.J.: Stable isotope tracer studies of amino acid balance and human indispensable amino acid requirements (Letter to Editor). Am. J. Clin. Nutr., 56 (submitted, 1992).
YOUNG, V.R.: Nitrogen balance studies: indicators of human requirements or of adaptive mechanisms? J. Nutr., 116, 700-703 (1986).
YOUNG, V.R.: Nutrient interactions with reference to amino acid and protein metabolism in non-ruminants; particular emphasis on protein-energy relations in man. Z. Ernährungswiss., 30, 29-45 (1991).
YOUNG, V.R., OZALP, I., CHALAKOS, B.V., SCRIMSHAW, N.S.: Protein value of Colombian opaque-2 corn for young adult men. J. Nutr., 101, 1475-1481 (1971).
YOUNG, V.R., FAJARDO, L., MURRAY, E., RAND, W.M., SCRIMSHAW, N.S.: Protein requirements of man: Comparative nitrogen balance response with the submaintenance-to-maintenance range of intakes of wheat and beef proteins. J. Nutr., 105, 534-542 (1975).
YOUNG, V.R., PUIG, M., QUEIROZ, E., SCRIMSHAW, N.S., RAND, W.M.: Evaluation of the protein quality of an isolated soy protein in young men: relative nitrogen requirements and effect of methionine supplementation. Am. J. Clin. Nutr., 39, 16-24 (1984).
YOUNG, V.R., BIER, D.M., PELLETT, P.L.: A theoretical basis for increasing current estimates of the amino acid requirements in adult man, with experimental support. Am. J. Clin. Nutr., 50, 80-92 (1989).
YOUNG, V.R., MARCHINI, J.S.: Mechanisms and nutritional significance of metabolic responses to altered intakes of protein and amino acids, with reference to nutritional adaptation in humans. Am. J. Clin. Nutr., 51, 270-289 (1990).
YOUNG, V.R., BIER, D.M., MATTHEWS, D.E., PELLETT, P.L.: Stable isotope tracer studies of amino acid balance and human indispensable amino acid requirements: A reply to D.J. Millward. Am. J. Clin. Nutr., 56 (submitted, 1992).
ZELLO, G.A., PENCHARZ, P.B., BALL, R.O.: Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C] phenylalanine. Am. J. Physiol., 259, E838-E843 (1990).
ZELLO, G.A., PENCHARZ, P.B., BALL, R.O.: Lysine requirement in young adult males determined by indicator amino acid oxidation. FASEB J., 6(5), A1943 (1992).
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