New approaches and tentative new amino acid requirement values

Our earlier studies [20--22] have already been reviewed, and the following discussion will focus on our work done during the 1990s. Our experimental approach has involved the application of C-labelled amino acid tracer techniques, the estimation of body amino acid carbon balance at varying test levels of amino acid intake [3, 4, 23], and the prediction of the obligatory amino acid losses and intakes necessary to just balance these, especially for the amino acids that we have not yet studied directly using C tracers [5].

Our working definition of the requirement for an indispensable amino acid in healthy individuals is "that minimal intake level which represents a single point on a dose-response curve and that is sufficient to maintain a specific criterion of nutritional adequacy (such as growth performance, body composition, body amino acid balance, or measure of organ [liver, muscle] or system [immune/defence, nervous] function)." For practical reasons we have chosen to use body amino acid balance, as determined by the difference between the intake of the test amino acid (e.g., leucine or lysine) and the whole-body oxidation of that amino acid, as measured by the appearance of the C label of the amino acid in expired carbon dioxide. Measures of organ or system function have not yet been sufficiently explored to permit them to be used for estimating the requirements for specific indispensable amino acids.

From our initial series of C-tracer studies, we estimated the requirements in adults for leucine, lysine threonine, and methionine (without dietary cystine) to be 30-40, 30, 15, and 13 mg/kg/day, respectively [4]. Except for the sulphur amino acid requirement, these estimates are far higher than the 1985 FAO/WHO/UNU values for adults shown in table 1. In addition, we attempted to predict the requirements for those indispensable amino acids that we had not yet studied with the aid of tracer techniques, based on considerations of obligatory amino acid loss (OAAL). This approach has been described in detail elsewhere [2, 5]. We appreciate that for some amino acids, and perhaps lysine in particular, the daily OAAL might be lower than the predicted value. This difference could be due to the possibility that some of the amino acid is retained in the free amino acid pool (possibly in muscle) during the fasting period and then reutilized for protein synthesis when amino acids are again supplied from the ingestion of protein-containing meals. Our preliminary C-lysine oxidation data suggest that possibly one-third of the lysine released via protein breakdown during the fasting period may be retained without undergoing terminal oxidation. However, the revised requirement for lysine discussed below, is based on the lower figures derived from metabolic data rather than on the somewhat higher predicted obligatory lysine loss.

Based on these earlier C-tracer studies and estimates of OAAL, revised values for the amino acid requirements of adults were proposed. These are presented in table 2 expressed in terms of milligrams of amino acid required per kilogram body weight per day. Table 2 also expresses these estimates in the form of an amino acid scoring pattern relating these estimates of mean requirements for specific amino acids to the mean requirement for total dietary protein [9]. The proposed new requirement pattern is quite different from that proposed by FAO/WHO/UNU [9]. Indeed, our proposed pattern for adults is similar to the FAO/WHO/UNU pattern for the pre-school child. The latter is also the amino acid requirement pattern that FAO/WHO [24] has recommended as an interim measure for use in the assessment of dietary protein quality for all humans over two years of age.