13. Conclusions

My aim at the outset was to explore the extent of our understanding of the metabolic basis of the requirements for amino acids and protein, and to argue for a more rational model of the organism's needs. Having elaborated such a model, I am conscious of the likely response: "Where does this get us, and how does this help in interpreting the data in Figure 1?" If the model is of any value at all, it should help in allowing better definition of protein requirements, amount and quality, not least in young children.

In my view, the crucial point is recognition of the two levels of requirement - minimum and optimum. Most of our efforts to date, namely the N-balance data reviewed in the 1985 report (FAO/WHO/UNU, 1985), have focused on R_min. The stable isotope metabolic balance studies (e.g., MEGUID et al., 1986) are in effect attempting to establish an R_operative which needs bear little or no relationship to either R_min or R_opt. What they do establish, discounting any technical problems, is the leucine, valine, threonine and lysine intakes required to balance losses of these amino acids generated by a diet containing all other amino acids at a level equivalent to that of 0.8 g egg protein. The information from these and subsequent studies is important in advancing our understanding of amino acid interactions as influences on oxidation rates, a long-standing important metabolic question (HARPER and ELVEHJEM, 1955) but does not, in my view, help us better define nutritional requirements for protein.

I believe our task for the future is the determination of R_opt. The first step has in fact been taken in the recent publications of dietary reference values in the UK (UK Department of Health, 1991), by defining an upper limit for adults beyond which it may not be safe. Our task now is to agree on functional indicators of adequacy, i.e., targets of the anabolic drive, which enable us to define L_r opt, and hence R_opt. Such indicators are unlikely to be simple to measure. Height growth, immunocompetence, and the extent of urea recyling are potential indicators which all require considerable expertise and resources for their study, assuming they prove to be appropriate. However, in my view, without an investment in such studies, we are unlikely to be able to generate any new data of sufficient worth to warrant adjustment of the existing data. Thus, resolution of the dilemma posed by the data in Figure 1 requires measurements of the functional responses of these children to their protein intakes, measurements which may well take many years, but for which, in my view, there is no alternative solution.