The Journal of Nutrition Vol. 127 No. 9 September 1997, pp. 1842-1846
Copyright ©1997 by the American Society for Nutritional Sciences

Human Amino Acid Requirements

D. Joe Millward

Center for Nutrition and Food Safety, School of Biological Sciences, University of Surrey, Guildford, Surrey GU2 5XH, United Kingdom

LITERATURE CITED

Few issues in nutritional science have aroused such long-standing and deep-seated controversies as protein and amino acid requirements. Those fortunate to read Graham Lusk's description of "A Normal Diet" in his Elements of the Science of Nutrition (Lusk 1928) will be only too aware of the debate that raged around the beginning of the 20th century (and before that) on the issues of the benefits or otherwise of large or small quantities of animal or vegetable protein in the human diet. Lusk records an account of a conversation between Lusk and Chittenden as to whether Lusk's delight and satisfaction with a large slice of cold roast beef consumed on board ship after an austere stay in wartime Britain reflected the replenishment of the "improvement quota" of his protein stores (Lusk's view) or the appetite-creating stimulus of the sea air (Chittenden's view). Lusk wrote that both opinions were proper themes for psychoanalysis. Students of the history of science looking at the current debate about amino acid requirements might have the same response today.

At the center of the debate are three issues: 1) the validity of the amino acid requirement values first assembled for the 1973 FAO/WHO protein requirements report and used subsequently in the 1985 FAO/WHO/UNU report, 2) the validity of the MIT scoring pattern proposed as a replacement for the FAO adult pattern (Young et al. 1989) and 3) the extent to which the amino acid scoring of proteins is a feasible or even valid way of evaluating protein quality in human nutrition (Millward 1994). It is appropriate to consider these issues here, given the most recent article published in The Journal of Nutrition in support of the MIT amino acid scoring pattern (McLarney et al. 1996). In this report, Young and colleagues present an interspecies comparison of the 1985 FAO/WHO/UNU human amino acid requirement recommendations with recommendations for other mammalian and avian species (McLarney et al. 1996). They show that when the values are compared at various stages of development, the human values are higher for infants and lower for adults compared with mean values for non-human species. On the basis of this comparison they conclude that it is difficult to escape the conclusion that the current human amino acid requirement values seem to be anomalous when judged against data from other animal species, especially in the case of the adult values. At the end of their article, they recommend the MIT pattern as an alternative to the FAO 1985 adult values, having argued that the MIT pattern "brings the human data more closely in line with amino acid requirement patterns and their rates of change with development as in other species."

As in previous eras, this is an important issue, the resolution of which has implications for international food and nutritional policies. Young and Pellett (1990) used the MIT scoring pattern to identify a lysine deficiency of cereal-based diets that they say requires animal protein supplementation to rectify it. To many readers, the arguments of Young will be persuasive, coming as they do from someone who has probably contributed the greatest amount of high quality published work in history about human protein and amino acid requirements. But so were the arguments of Liebig about the role of protein as a fuel for muscle, and Liebig was mistaken. So, in my view, are Young and colleagues.

From my reading of the scientific literature, I would agree with Reeds (1988) that with the obvious exceptions (e.g., arginine requirements for growing cats and growing and adult dogs, a taurine requirement of kittens and a high maintenance amino acid requirement in avian species for feather growth), there are few major differences among mammalian species in the fundamentals of amino acid and protein metabolism. On this basis an interspecies comparison of mammalian species is of scientific interest. However, such an exercise only has value if the comparison compares like with like and uses data that are reliable.

As far as comparing like with like, given the basic concepts of amino acid requirements for growth and maintenance that have been understood since the earliest experiments of Osborne and Mendel (1916), interspecies comparisons are of value only when comparisons take into account the markedly different growth rates among species. It is well known that newborn pigs grow more than 10 times faster than newborn infants before weaning, (i.e., 286 g/d to gain 4 kg in 14 d compared with 22 g/d to gain 4 kg in 180 d) and 60 times more faster after weaning (i.e., 571 g/d to reach 90 kg in 148 d compared with 10 g/d to reach 70 kg in 17.5 y). This means that, as commented by Said and Hegsted (1970) in their evaluation of amino acid requirements of rats, in human infants after the first year of life requirements of indispensable amino acids for growth approach insignificance compared with maintenance needs. Because of this, it is by no means clear to me what value is to be gained by comparing human values for preschool or older children (early growth and growth) with other species that grow faster than human infants. For the fast-growing species such as pigs or rats amino acid needs are almost entirely for tissue growth whereas for humans after the first year amino acid needs are almost entirely for maintenance. Although elsewhere (Young and El-Khoury 1995) Young argues that growth patterns need not bear any relationship to tissue amino acid content, it is a fact (accepting the essentially of some amino acids) that the growth requirement pattern must provide at least the content of tissue growth. The only way that it can differ is through any additional dietary need due to any inefficiency of utilization or any non-growth metabolic demand. In the rapidly growing rat the pattern of indispensable amino acid requirement reported by Benevenga et al. (1994) is (contrary to the assertions of Young and El-Khoury 1995), with one exception, very similar to that of rat mixed body proteins reported by Davis et al. (1994). Thus, after equalizing each pattern for threonine, relative ratios of all other amino acids ranged from 0.8 to 1.3 except for the sulfur amino acids, for which the rat requirement values are higher than expected (ratio of 2.3), presumably reflecting hair growth. In the case of growing pigs, Fuller's requirement pattern for accretion (Fuller et al. 1989) is even closer to the composition of mixed body proteins, with relative ratios ranging from 0.8 to 1.1. This means that 1) there can be little debate about the amino acid requirement for growth apart from the efficiency of utilization, which will determine any additional need; and 2) because humans grow at rates that begin to approach those of animals only during catch-up growth or in preterm infants, the requirement patterns of most other species are largely irrelevant to human needs after the first 6-12 mo of life. What is needed is a specific focus as far as is possible on maintenance requirements. Again, if like is to be compared with like, then consideration must be given to the protein allowance chosen, because the scoring patterns as examined by McLarney et al. (1996) (milligrams of amino acid per gram of protein allowance) depend on the choice of protein value in the denominator. The FAO/WHO/UNU 1985 adult pattern quoted by Young is based on the 1985 safe allowance (0.75 g protein), a value which in any case was somewhat arbitrarily derived (see Millward et al. 1989). The values in the 1973 FAO/WHO scoring pattern were all 36% higher because the same amino acid requirement values were related to a lower safe protein requirement (0.55 g/kg). The MIT pattern is based on 0.6 g protein, the 1985 mean requirement.

As to the reliability of the data, at the outset McLarney et al. (1996) state that they will use sets of values mainly reported by the National Research Council as "the most widely accepted figures and, therefore, useful for the present purpose" even after pointing out that other more recent values (e.g., values for rats; Benevenga et al. 1994) may represent better estimates. In fact, for the most important data in the present context, i.e., the mature values, the data quoted (turkey, dog, rat and swine) are generally poor apart from the data for rats. Thus as indicated by the authors, the turkey data are predicted. The NRC data for adult dogs is recognized to be based on very limited data (Schaeffer et al. 1989) and in fact derives from a single Ph.D. thesis, and, as the authors report elsewhere (Young and El Khoury 1995), the data set for the boar is generally considered to be based on very weak data.

As to the reliability of the human data, there are several criticisms that can be made of these data as a description of human requirements. The infant scoring pattern derives from the composition of breast milk that was recommended by FAO after pointing out that breast milk contains higher levels of tryptophan and the S-amino acids than experimental estimates of infant amino acid requirements values. The preschool values are very difficult to assess because they have never been published in full. Furthermore, examination of what data are available (i.e., for lysine) raises the question of whether the results derive from measurements in children exhibiting abnormal growth for their age due to prior malnutrition (see Millward 1994). The data on older children were rejected by FAO/WHO as unreliable in their 1991 report (FAO/WHO 1991).

In contrast, the data on adults, the main focus of attention in the current debate, represent the results of large body of specific work by several investigators on adult men and women aimed at establishing minimum requirements with specific protocols designed to do that (as discussed by Millward and Rivers 1988). Furthermore, the final values selected by FAO/WHO/UNU (1985), deriving from an earlier report (FAO/WHO 1973) that compared values for men reported by Rose (1957) and a set of values published by Hegsted (1963) after careful consideration of all pre-1963 published data, were mainly the highest values "to emphasize the upper range of individual requirements." These were mainly the values reported by Rose. Young has rejected the entire body of work (focusing in his analysis exclusively on Rose's studies), on the grounds that 1) excessive energy was used, 2) no account was taken of miscellaneous losses and 3) nitrogen balance cannot be validated. In fact the first point is true only for the Rose studies, which were recognized by Hegsted (1963) as inferior in quantitative terms to later studies but which in any case generally resulted in higher values than most other reports that used weight maintenance energy intakes. Point 2 will make a difference as indicated by Hegsted (1963). In fact, Fuller and Garlick (1994) recalculated the values from Hegsted's regression values assuming a need for 0.3 g N for unmeasured losses, and this resulted in a doubling of most values. On the other hand, in the case of lysine several studies have taken a positive retention as the criterion of adequacy and generally confirmed the FAO values (e.g., Clark et al. 1963). As for point 3 above, notwithstanding the well-known difficulties associated with all balance studies, their complete rejection (in favor of ¹³C carbon balance studies) is a counsel of despair and logically means the scrapping of all protein requirement values. Thus the human data at the outset are unlikely to reflect accurately the true developmental change in metabolic demands for amino acids because these have not been systematically studied.

It follows from the above that there is, in fact, a great difficulty in making a sensible interspecies comparison, given both the difficulties of identifying comparable physiological states across species that correspond to human development and a lack of unambiguous data. Most work has been done in rats, pigs and human adults, with specific studies in adult rats and only limited studies in adult pigs. Hegsted (1973) compared his own rat data with the FAO human values, and these are shown in Table 1.

Table 1. Adult rat and human amino acid requirements1 [View Table]

Two comments can be made about this comparison. Firstly, both sets of values differ from those quoted by Young and colleagues. The rat values differ from the NRC (1978) values both in terms of the pattern of amino acids and in the total indispensable amino acids, yet it is difficult to know what data the NRC could have worked with apart from Hegsted's data. Secondly, in contrast to the conclusions reached by Young and colleagues, on the basis of this comparison the conclusion reached by Hegsted (1973) seems more appropriate: "It is perfectly clear that there are very apparent similarities between the pattern of amino acid requirements of rats and man. The relative requirements of the various amino acids are also similar."

In addition, maintenance values have been obtained for growing rats and pigs and can be compared, knowing that these values represent amino acid consumption rates under physiologically unusual circumstances. Given that in practice the current debate is centered mainly on lysine requirements, I have compared maintenance lysine requirements for rats, pigs and humans in Table 2 as mg/kg^0.75. Clearly, the basis for an exponent of 0.75 for interspecies comparisons of lysine requirements has not been rigorously established, but assuming such a comparison is valid the human values are entirely unremarkable and indeed higher than the "unphysiological values" derived from growing rats or pigs. Table 2 also shows lysine requirements as a fraction of total indispensable amino acid requirements.

Table 2. Interspecies comparison of lysine requirements [View Table]

What the values show is that in comparison with average tissue protein, the maintenance lysine requirement is a smaller fraction of total indispensable amino acids in all cases with the lowest values for the rat. Again this means that there is nothing remarkable about the human values.

However, what is most instructive is an interspecies comparison of metabolic behavior in terms of adaptive changes in amino acid oxidation and consequent conservation of amino acids in response to low intakes, because this will determine the nutritional value of various food sources. This is because it is now quite clear that for many indispensable amino acids and for lysine in particular their requirement is primarily to satisfy oxidative losses. Indeed, on the basis of a metabolic model (Millward and Pacy 1995, Millward and Rivers 1988) in which the metabolic demand for indispensable amino acids is a function of 1) needs for growth, 2) needs for obligatory metabolic demands and 3) needs for regulatory oxidative losses at rates that reflect habitual protein intakes, definition of a simple unqualified maintenance requirement, irrespective of the background dietary state, is not possible. Within this model all that can be defined is the minimum intake to which adaptive reductions of oxidation rates can accommodate without compromise of bodily functions or composition. This is true even accepting the needs for postprandial protein deposition within the diurnal cycle of postabsorptive losses and postprandial gains. Because the amplitude of this diurnal cycle is variable and because as discussed elsewhere (Millward 1992, Millward and Pacy 1995) amino acid recycling can occur (i.e., amino acids such as threonine and lysine released by postabsorptive net proteolysis can be recycled for postprandial protein gain), this allows wheat protein to be utilized for postprandial protein deposition with an efficiency close to that of milk (Fereday et al. 1994 and 1997). With an adaptive reduction in the oxidation rates of lysine and other limiting amino acids, balance can be maintained on low intakes, as Young et al. (1975) have shown (see Millward 1994).

In this context, the interspecies data from rats and pigs are entirely consonant with very low obligatory needs for lysine compared with other amino acids such as threonine and the sulfur amino acids. Studies on young rats (Benevenga et al. 1994), adult rats (Said and Hegsted 1970, Yokogoshi and Yoshida 1981, Yoshida and Moritoki 1974) and growing pigs (Fuller et al. 1989) have all very clearly shown that, at maintenance, lysine needs are the lowest of all individual indispensable amino acids in that removal of lysine from the diet has a very much smaller influence on nitrogen balance compared with removal of the sulfur amino acids and threonine in pigs and compared with removal of threonine, the sulfur amino acids and isoleucine in rats. Indeed, as Hegsted (1973) has pointed out, because of this the balance-intake curve is extremely shallow for lysine and this means that measurement of a requirement value is very difficult, being dependent on the exact criterion for adequacy. Although several early reports of rats maintaining body weight for 6-mo periods when consuming very low lysine diets [e.g., zein, Osborne and Mendel (1916) or even lysine-free diets (Bender 1961)] are probably explained by coprophagy, given the clear evidence of a metabolic need for lysine in terms of the rapid onset of symptoms in humans consuming a lysine-free diet (Rose 1957), no evidence exists for anything other than a very low metabolic need for this amino acid. Yoshida has done most to explore the concept that rate-limiting amino acids at maintenance differ from those that rate-limit growth. He has shown that in adult rats fed limiting amounts of rice or wheat diets, the limiting amino acids were threonine and the sulfur amino acids, which, when added to the cereal diets, restored nitrogen balance and transformed body weight loss to growth (Yoshida 1983). This may explain why attempts to show in supplementation trials that lysine is the limiting amino acid in wheat in human adults were so disappointing (Scrimshaw et al. 1973).

Although the nature of the relative metabolic need for individual amino acids is by no means clear, Fuller's work in pigs has pointed to ileal amino acid losses as a partial explanation, accounting for some 60% of pig amino acid maintenance requirements (Wang and Fuller 1989). Table 3 compares ileal losses of pigs and humans.

Table 3. Ileal indispensable amino acid losses of pigs and humans in comparison to the FAO adult requirement values [View Table]

These data show that in each case, threonine is the largest component, and although the patterns differ to a certain extent, most importantly, the absolute values are much lower in humans than in the pigs. On this basis then, the human requirement values shown in Table 3 are unremarkable, as are the many human nitrogen balance studies on men and women reporting lysine requirements ranging from 17 down to as little as 1 mg/kg per day (see Irwin and Hegsted 1971).

Taken together, this necessarily limited interspecies comparison leads me to conclude that 1) minimal metabolic needs for lysine are low; 2) adaptive reduction in lysine oxidation and recycling in response to reduced intakes is extensive; and 3) the FAO amino acid requirement values and especially the lysine requirement derived from the human N balance trials are entirely unremarkable. Furthermore, on the basis of well-conducted studies showing that the lysine requirements for growth are much higher than those for maintenance [e.g., lysine-tryptophan ratios in the growth and maintenance patterns of 8.2 and 3.4 in rats (Said and Hegsted 1970) and 5.7 and 1.8 in pigs (Fuller et al. 1989)], the conclusion is unavoidable that minimal maintenance requirements for lysine for humans, as in both rats and pigs, are very much lower than the relative requirements for tissue accretion. Thus, during periods of very rapid normal and catch-up growth in preterm and term human infants, the amino acid requirement pattern will need to include a much higher lysine content than in older children and adults. The fact that this is contrary to the much lower change in lysine requirements from very young to adult mean NRC values for non-human species (58-33 mg/g) assembled by Young and colleagues (McLarney et al. 1996), leads me to question the NRC values rather than the human values.

As argued elsewhere (Millward 1990, 1992 and 1994, Millward et al. 1990), the MIT scoring pattern is based on assumptions that the amino acid requirements for maintenance can be predicted from the amino acid composition of body proteins, an assumption never made before to my knowledge and which is unwarranted, and which has found no support apart from acceptance that the values for leucine requirements may be higher than those in the FAO/WHO/UNU pattern (e.g., Fuller and Garlick 1994, Waterlow 1996). Contrary to what was published, it is not the case that at an international meeting of an expert group "a large majority of the group accepted as an interim operational pattern that proposed by Young et al." (Clugston et al. 1996) because, as subsequently reported (Millward and Waterlow 1996), this statement emerged during post-meeting editing.

Young has yet to publish reliable stable isotope studies for lysine, and in my view no other unequivocal values have been published. The indicator oxidation method studies reported by Zello et al. (1992), which support much higher requirement values for lysine, consistent with Young's position, make no attempt to allow for any adaptation in lysine oxidation in response to a reduced intake. Furthermore, although they are presented as more straightforward than many other studies, they are in fact complicated and in my view by no means unequivocal. Thus the increase in ¹³CO₂ excretion from the indicator amino acid (infused phenylalanine), which identifies a fall in dietary intake of the test amino acid (lysine) below the requirement, accurately predicts the true increase in phenylalanine oxidation only if there are no changes in the true precursor enrichment, which, according to my understanding of biochemistry, is tyrosine rather than phenylalanine as assumed by the authors.

As concluded elsewhere (Millward 1994), given 1) the low minimal obligatory needs for indispensable amino acids, 2) the fact that their metabolic demand reflects the extent to which adaptive changes in oxidation occurs and 3) the growing evidence for the availability of indispensable amino acids, including lysine, deriving from colonic microbial de novo amino acid synthesis from salvaged urea (see Fuller and Garlick 1994, Yeboah et al. 1996, Gibson et al. 1997), 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. However, given that protein scoring, which is based on a simple and attractive concept, represents a deeply embedded foundation stone of nutritional quality evaluation, I recognize that few will easily accept my conclusions today in the same way that similar views of others expressed over a quarter of a century ago (e.g., Said and Hegsted 1970, Yoshida and Moritoki 1974) have been consistently ignored, as have the many published studies showing clear evidence of the capacity to adapt to cereal-based diets involving low lysine intakes. The study of Edwards et al. (1971), showing both body weight and N balance maintenance in North American students fed only 46 g of protein (~0.6 g/kg) of wheat (76%) and potatoes (20% protein) for 60 d, which would supply about 17 mg/kg lysine, is one such report. The pressing scientific question is not, in my view, the identity of a scoring pattern to determine the extent to which plant-based diets are nutritionally adequate in human nutrition, because scoring will not provide an answer to that question. Rather we should be focusing on long-term feeding trials with the dietary protein sources under investigation to enable a better understanding of the complexities of the adaptive responses of human amino acid and protein metabolism to changes in intakes, and the extent of any untoward functional consequences of such adaptation.

FOOTNOTES

Manuscript received 26 November 1996. Initial reviews completed 10 December 1996. Revision accepted 13 May 1997.

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