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.