We recently reported the lysine requirement of school-aged children living in Canada consuming a mixed diet to be 35 mg.kg(-1).d(-1). Because the majority of children in the world live on cereal-based diets in developing countries, we measured the daily lysine requirement in healthy children living in India and consuming a mostly cereal-based diet. Our objective in this study was to determine the lysine requirement in healthy, school-aged children in the developing world by using the indicator amino acid oxidation method with l-[1-(13)C] phenylalanine. Six healthy, school-aged children consumed 7 levels of lysine (5, 15, 25, 35, 50, 65, and 80 mg.kg(-1).d(-1)) each in a random order along with an amino acid mixture providing energy and protein intakes of 1.7 x resting energy expenditure and 1.5 g.kg(-1).d(-1), respectively. The mean lysine requirement was determined by applying a 2-phase linear regression crossover analysis on tracer oxidation (F(13)CO(2)) data, which identified a breakpoint (requirement) at minimal F(13)CO(2) in response to the graded lysine intakes. The mean lysine requirements with the upper 95% CI for children were determined to be 33.5 and 46.6 mg.kg(-1).d(-1), respectively, by breakpoint analysis of the F(13)CO(2) data. The mean lysine requirements of Indian children were almost identical to that of Canadian children (35 mg.kg(-1).d(-1)). There is no evidence for any adaptation in lysine requirements in children from developing countries such as India.

The lysine requirement in undernourished children is similar to that of well-nourished children, and intestinal parasitic infestation increased the lysine requirement by ∼20%.

The factorial approach is used to measure the dietary indispensable amino acid (IAA) requirements in children, although recent measurements based on the indicator amino acid oxidation (IAAO) method have begun to generate more direct evidence. Difficulties with the factorial method are that it depends on accurate estimates of the maintenance protein requirement, as well as of protein deposition during growth. Also, a value for the efficiency of utilizing dietary protein for deposition has to be selected, based on published Nitrogen (N) balance studies. In the recent 2007 WHO/FAO/UNU report, the amino acid requirement pattern for infants was taken to be similar to the amino acid composition of breast milk. For pre-school and older children, the factorial method gave values for the scoring pattern of protein that were fairly close to the earlier 1985 WHO/FAO/UNU report for children, since growth progressively became a smaller component of the factorial calculation as age progressed. However, given that there are several assumptions in the derivation of factorial estimates, direct experimental measurements of the amino acid requirement are desirable. The IAAO method, as it is non-invasive, as made it possible to measure the IAA requirements in children. Over the last decade, some of the IAA requirements have been determined by using the IAAO method in healthy school age children; however, the data on IAA requirements in developing country populations are still being conducted. In the elderly, there are not enough data to make a separate recommendation for IAA requirements from that of adults.

There are now several direct experimental methods that are available to measure indispensable amino acid (IAA) requirements. In all these, increasing intakes of amino acids are given until a change is observed in the response curve. The response could be nitrogen (N) balance or growth, although current methods measure amino acid oxidation or its surrogate by the use of stable isotope tracers. N balance or growth progressively increase until the requirement level of intake of the IAA is reached, after which it plateaus. Oxidation of the test IAA is measured as the primary route of loss of that amino acid, such that the IAA balance (intake-oxidation) can be measured. For oxidation, there is no change in response as graded levels of the IAA are fed below the requirement level. However, once the requirement level is reached there is a linear increase in oxidation. The IAA balance follows the same pattern as the N balance. However, both these methods have several requirements and assumptions, and are technically demanding. A third method has also been developed, related to measuring the oxidation of a selected amino acid whose kinetics are well described, in response to graded intakes of the test IAA. The selected amino acid (other than the test IAA) is called an indicator amino acid, and its oxidation (or balance) response is measured. The oxidation of the indicator, particularly in the fed state, is an index of protein synthesis, and will fall as increasing levels of the test IAA are fed until the requirement level is reached, after which there is no further change. The advantage of this method is that it is relatively non-invasive, and effectively is a breath test, since the oxidation of the tracer-labelled amino acid itself acts as a surrogate for whole-body amino acid oxidation. A modification of the indicator amino acid oxidation method relates to using the measured IAA balance as a surrogate for N balance, in 24-h indicator amino acid balance (IAAB) studies. Here, the balance response follows the same pattern as that of N balance and growth. The indicator amino acid oxidation (and balance) technique is now used primarily in both developed and developing countries, to derive estimates of amino acid requirements in humans. Values of IAA requirements based on the direct and indicator amino acid oxidation and balance (IAAO and IAAB) techniques are considerably higher than those derived from N balance studies, and these tracer-based estimates are now the basis of the recent WHO et al. (2007) recommendations for amino acid requirements.