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Nutritional quality of wheat protein in adults

The revised estimate of the lysine requirement in adults, 30 mg/kg/day, has implications for the nutritional quality of wheat proteins. Thus, it is worth considering this important plant protein source as a basis for evaluating the validity of the foregoing analysis of the lysine needs of healthy adults [55]. The lysine content of wheat products is summarized in table 6, together with the lysine content of a number of FAO/WHO amino acid scoring patterns. In addition, the usual concentration of lysine in most animal proteins and legumes [57] and that for the MIT requirement pattern are given for comparison in table 6. Therefore, if an amino acid score ([amino acid content in the food protein/amino acid content in the reference amino acid requirement pattern! x 100) is calculated for wheat flour, it would be greater than 100 when the 1985 FAO/WHO/UNU [9] amino acid requirement pattern for the adult is used as the reference pattern (table 7). This means that the nutritional value of wheat would be equal to that of high-quality animal protein foods, such as milk, egg, or meat [9]. On the other hand, for scoring purposes, the FAO/WHO/UNU [9] pre-school amino acid pattern (or the FAD/WHO pattern [24]) predicts a relative nutritional quality of 41%, and with the MIT pattern the score predicts a slightly higher value of 48%. In each case, lysine is determined to be the most limiting amino acid. These latter and lower estimates of the nutritional quality of wheat proteins in adults are consistent with the results of nitrogen balance experiments in healthy adults carried out at MIT approximately twenty years ago [58].

TABLE 6. Lysine content of wheat flour in comparison with other foods or amino acid requirement patterns

TABLE: 7. Lysine content of whole wheat flour in relation to an estimate of protein quality

a. L = lysine first limiting amino acid, not corrected for digestibility.

The nitrogen balance response to graded intakes of test dietary protein in healthy adults, expressed as relative protein value (RPV= [N balance slope with wheat/N balance slope with reference protein] x 100), was 54 for whole wheat protein, using beef protein as a reference. Expressed as relative nitrogen requirement (RNR = 1/[amount of wheat protein to achieve nitrogen balance in 97.5% of population amount of beef protein] x 100), the response was about 56 (table 8). The MIT amino acid requirement pattern predicted a value of 48. Hence, it is clear that there is very good agreement between these experimentally derived (nitrogen balance) and predicted (from amino acid score) estimates of the nutritional quality of whole wheat proteins. 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 that are faced in attempts to aggregate nitrogen balance data across separate studies carried out in different laboratories or within the same laboratory on different occasions [59], our observations support the conclusion that the 1985 FAO/WHO/UNU lysine requirement value of 12 mg/kg/day for the adult should be discarded. Further, they provide additional justification for the tentative working value of 30 mg/kg/day proposed above (or 50 mg lysine per gram of protein), and they strengthen our recommendation that this figure be used until additional data become available that may make any further change in the recommendation both necessary and desirable.

TABLE 8. Biological assessment of the nutritional quality of whole wheat proteins in young adults^a

a. Expressed in comparison with beef protein as reference protein [58].

Worldwide applicability of estimates of indispensable amino acid requirements

If it can be accepted that the tentative new amino acid requirement values given in table 2 represent a better approximation of the minimal physiological needs for well-nourished, healthy subjects studied largely in North America, it is legitimate to ask whether the amino acid requirements of individuals in developing regions, particularly where protein and/or dietary lysine are likely to be more limiting, are similar to or different from those given above. Although the current international FAD/WHO/UNU [9] amino acid requirement values, based largely on studies conducted in young adult American subjects, are recommended for application worldwide, our reassessment of the requirements for indispensable amino acids emphasizes the need to consider this nutritional-metabolic issue more critically than hitherto.

Unfortunately, there have not been any relevant 13C-tracer studies in subjects outside North America that directly explore this important practical issue. Hence, the Global Cereal Fortification Initiative (GCFI) of Ajinomoto Co., Inc., and Kyowa Hakko Kogyo Co., Ltd., Japan, is now sponsoring a multicentre study designed to confirm our new estimate of the lysine requirements of healthy adults (table 2) and its applicability in other populations. The results to be obtained from these studies within one or two years are expected to give a reasonable indication of the approximate minimum lysine needs of healthy Indian and Thai young adults, whose usual lysine intake levels are likely to be below those of the US subjects that we studied at MIT. This is an exciting and important development with profound implications for international nutritional and metabolic investigation and the value of studies on nutritional requirements in humans.

Furthermore, there are few relevant data that can be used to predict whether the indispensable amino acid needs, and the lysine requirements in particular, are similar or different among these various population groups. Studies of obligatory nitrogen losses in US [60-62], Chinese (Taiwan Province) [63], Indian [64], Nigerian [65, 66], and Japanese [67] men reveal that they are remarkably uniform [68]. This implies similar OAALs and similar dietary requirements for indispensable amino acids [5]. This would be so unless there were evidence that the efficiency of specific amino acid retention differed among apparently similar subjects in the population groups. According to FAO/WHO/UNU [9], nitrogen balance studies have not revealed any striking differences in estimates of total protein requirements, in relation to body cell mass, in studies of well-nourished subjects in different countries. Earlier studies suggesting that Nigerian men of low income are adapted to low-protein diets and utilize dietary protein more efficiently [65, 69] than, for example, US students [61] are not appropriate to answer this question. Indeed, they are probably flawed because the nitrogen balance results in the subjects studied indicated that they were depleted and that they were undergoing a body protein repletion response to the good diet given during the course of the experiments. Later studies in young Nigerian adult males [70] indicate that at maintenance nitrogen intakes, the efficiency of dietary protein utilization is essentially the same as that for caucasian and Asian subjects.

In summary, it seems rather unlikely that there would be any major differences in the minimal physiological requirements for lysine among groups of normal healthy adults of different genetic, nutritional, and environmental background. The ongoing GCFI-sponsored multicentre studies should provide evidence to support or refute this view. Thus, the GCFI study is potentially of great practical and international significance, and it is the authors' hope that there soon will be a broader appreciation for this fact by national and multinational authorities concerned with improving the nutritional well-being of underprivileged populations worldwide.

Conclusions and implications for nutrition policies and food programmes

Despite the economic, social, and human dimensions of protein-energy malnutrition in large areas of the developing world, current knowledge about the quantitative needs both for dietary energy and for the indispensable amino acids that are supplied by our food protein remains inadequate. In 1973 a group of senior investigators in the United Kingdom asked the rhetorical question "How much food does a man require?" [71]. Since that time, there has been a considerable amount of research on this topic. This follow-on effort has been facilitated by the development in a number of countries, including the United States, of whole-body calorimeters and by the application of the doubly labelled water technique, which permits a non-invasive, quantitative measure of energy expenditure in free-living individuals [72-75]. Hence, the energy requirements of individuals of all ages, from infants [76, 77] to the elderly [78], are now being refined and the database is being expanded. With reference to the requirements for indispensable amino acids, equivalent advances in technique have been slower to occur, and therefore their application has also been less extensive to date, in comparison with the explosion of studies involving use of the doubly labelled water method. However, it is encouraging to note that the application of amino acid kinetics and, by extension, the 13C-tracer balance method represents one of the most important developments in recent years with respect to the study of human amino acid requirements [28].

Based on these tracer-derived data, it is our view that the new, tentative requirements for indispensable amino acids represent the best available approximations of the needs for these nutrients in adults. Hence, we recommend that they be used as a rational basis for the formulation of amino acid mixtures, or of protein sources, intended for meeting the nutritional support of individuals in institutional settings; the determination of the composition of enteral products is a case in point. Furthermore, these newly proposed values for the amino acid requirements of adults serve, in our view, as a credible benchmark for assessing the quantitative impact of disease and trauma on amino acid requirements. They should aid, therefore, in the design of parenteral nutritional formulations and, hopefully, lead to improvements in their efficacy for supporting the nutritional and metabolic needs of patients in a catabolic state.

Finally, as pointed out above, the revised requirements for adults are similar to those of young children when expressed in relation to the need for dietary protein. On this basis, considerations of dietary protein quality become important in reference to adult human protein as well as in relation to the nutritional well-being of the younger age groups. This challenges the current dogma, as reflected by FAO/WHO/UNU [9], that indigestibility appears to be the most important factor determining the capacity of the protein sources in a usual mixed diet to meet the protein needs of adults. However, as has been stated by Berg and Singer [79] in their assessment of the historical background behind the now dominant use of recombinant DNA technology in biology, changes in human thought and technological developments lead to new issues that challenge traditional ideas. We presume that the ¹³C-tracer techniques used by our group and others represent another, perhaps small, advance in nutritional investigation. We recommended that the adult amino acid requirement values, referred to as the MIT Amino Acid Requirement Pattern, now be used to establish the quantitative profile of the amino acid component of an adequate diet. The new amino acid requirement pattern should be of greater value in identifying the nature and extent of the limiting indispensable amino acid(s) in national and regional diets. This pattern should be of particular assistance to those responsible for developing sound food and nutrition policies and programmes. The pattern should also be useful for evaluating the economic, social, and cultural merits of dietary protein complementation, food protein supplementation, and specific amino acid fortification, such as lysine fortification of wheat flour, as alternative or perhaps simultaneous approaches for improving the nutritional value of diets based predominantly on cereals.

Acknowledgements

The authors' studies were supported by NIH grants DK 15856, DK 42101, and RR 88, and by SHCC grant 15847.

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