|Energy and Protein Requirements, Proceedings of an IDECG workshop, November 1994, London, UK, Supplement of the European Journal of Clinical Nutrition (International Dietary Energy Consultative Group - IDECG, 1994, 198 pages)|
|The requirements of adult man for indispensable amino acids|
For the sake of completeness we must look briefly at a completely different method of assessing amino acid requirements. There were attempts in the 1970s to use plasma amino acid concentrations as an indicator, the idea being that if concentration is plotted against intake, there will be a sharp inflexion at an intake corresponding to the requirement. Although these expectations were not fulfilled, it seems worthwhile to look at the results of the early tracer balance experiments, since they involved measurements over a wide range of intakes. The results (Figure 6) show a more or less continuous fall of amino acid concentration with decreasing intakes, but nothing that could be interpreted as a breakpoint.
Brooks et al (1972) extended the method to oxidation. They plotted Lysine oxidation against intake in the rat and found a clear-cut breakpoint at an intake of about 100 mg/d (although it is not clear how they calculated the Lysine oxidized from measurement of the output of labelled CO2, since there was no analysis of precursor activity). They considered that the method was more sensitive and specific than similar breakpoint measurements on plasma.
The fed-state oxidation rates plotted in Figure 7 show so much variation that it is impossible to predict the requirement from such data. Bayley and coworkers extended the idea. Instead of measuring the oxidation of the test amino acid at different levels of intake, they used oxidation of a labelled indicator amino acid at different levels of the test amino acid. This method gives curves which are the inverse of the original method; at the point where the requirement is fulfilled, oxidation of the indicator amino acid is at its lowest. Some of the breakpoints obtained on pigs are very clear; for example with 14C-phenylalanine as indicator, there were sharp breakpoints for tryptophan and histidine, but not for Lysine (Kim et al, 1983; Ball & Bailey, 1984, 1986).
This approach has been used by Zello et al (1993) (section 8) to measure the Lysine requirement with phenylalanine as indicator amino acid, but I know of no other studies in man.