The problem

The current, authoritative, internationally stated estimates of the amino acid requirements in humans at various ages are those in the 1985 report of the Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements [9]. These estimates, for preschool and school-age children and for adults, are summarized in table 1. Except for the requirements for pre-school children, these estimates are taken from a 1973 FAO/WHO report [11] or are similar to those summarized by others [10].

TABLE 1. FAO/WHO/UNU (1985) estimates of amino acid requirements in pre-school and school-aged children and in adults^a

a. From tables 4 and 39 in FAO/WHO/UNU [9].
b. Also based on a summary by Williams et al. [10].
c. Sulphur amino acids.

We will not discuss dietary amino acid requirements for infants, because the 1985 FAO/WHO/UNU report [9] recommended that these requirements be met by breastmilk. However, it is worth noting that breastmilk provides higher amounts of all the indispensable amino acids than the estimated requirements for infants in the 1973 FAO/UNU report [11], except perhaps for threonine. Moreover, in a paper presented at the 1994 meeting of the International Dietary Energy Consultative Group (IDECG), Dewey et al. [12] suggested, based on a factorial approach, that the needs for indispensable amino acids in infants three to six months of age were much lower than those in the 1985 FAO/WHO/UNU report [9]. For example, the requirements for lysine were estimated at 63 mg/kg/day by Dewey et al. and at 103 mg/kg/day by FAO/WHO/UNU. For the sulphur amino acids (methionine and cystine), the values were 27 mg/kg/day for Dewey et al. and 58 mg/kg/day for FAO/WHO/UNU. These are very large differences and they demand more careful scrutiny and experimental verification. It is apparent, therefore, that the quantitative needs for indispensable amino acids in the first year of life are still a matter of considerable uncertainty, as has been pointed out earlier [13].

With respect to the requirement figures given in table 1, some points should be made. First, the values for pre-school children are based on studies carried out by investigators at the Institute of Nutrition of Central America and Panama (INCAP), and only a summary of the findings from these studies is currently available in the literature [14, 15]. Thus, a detailed analysis of these important experiments cannot be made at this time. Second, the children who were studied in this series of investigations may not have fully recovered from earlier protein-energy malnutrition, as suggested by the magnitude of the positive nitrogen balances that were obtained in the experimental subjects. For example, at the estimated requirement value for lysine of 64 mg/kg/day, the children were found to be in a positive nitrogen balance of approximately 80 mg N/kg/day. However, the nitrogen increment in normal 18-month-old boys is only about 18 mg N/kg/day [16]; according to the data of Pineda et al. [14], a dietary intake of between 39 and 53 mg lysine/kg/day would have been sufficient to induce this lower rate of daily nitrogen retention. Of course, we do not know whether the children were using dietary lysine more efficiently than might be the case for a normal child. If this were the case, then the total daily amino acid requirements may not have been very different from those of normal and fully repleted children. However, the requirement figures given in table I for pre-school children may turn out to be somewhat higher than the true minimal physiological needs for fully repleted, normal, pre-school children. Additional studies in this age group would be highly desirable. The requirement figures for school-age children are based on the metabolic balance studies carried out at the Institute of Public Health in Tokyo [17] and are designed to help establish the minimal needs for the specific indispensable amino acids in this age group. Thus, in the absence of replication and confirmation of these earlier studies, the reliability of the values given in table 1 for the school-age child is still a question that future research will need to resolve.

The amino acid estimates for adults in the 1985 FAO/WHO/UNU report [9] were taken directly from the 1973 FAO/WHO report [11] and are based largely on the studies by Rose and co-workers in men [18] and on similar studies by various investigators in women [19]. There are major problems in the design and interpretation of these studies [2 4, 8]. These problems will not be discussed here, but it is our strong recommendation that the results of these earlier nitrogen balance experiments not be used to establish the minimum quantitative dietary requirements for specific indispensable amino acids that are needed to support long-term maintenance of protein nutrition status in adults.

From the foregoing, a case can be made that current knowledge about human amino acid requirements is highly unsatisfactory. There is an urgent need to close our wide gap of ignorance about the amino acid requirements of humans, if rational and safe nutrition policies and programmes, especially in reference to the nutritional well-being and development of populations in developing regions, are to be designed and implemented successfully. Therefore, we have sought new experimental data to support our arguments and to strengthen our previous conclusions [2, 8], and to make tentative new estimates of the amino acid requirements of humans from pre-school to adult [2, 4].