|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|
So far we have been concerned more with ideas than with numerical estimates of requirements. We come now to the tracer balance studies of Young and coworkers (Young et al, 1989). The new MIT pattern was supported by two general propositions. The first is that the amount of the IAA requirement is determined by the obligatory N loss (ONL) and that the pattern reflects the composition of body protein. 'The oxidation rates of individual amino acids occur in proportion to the pattern or concentration of amino acids in mixed body protein' (Young et al, 1989). Thus, if the ONL is 54 mg/kg/d (FAO/WHO/UNU, 1985), equivalent to 0.34 g protein, and if the leucine content of body protein is 80 mg/g, then the loss of leucine will be 27 mg/kg/d. Of course, a factor also has to be applied for the efficiency with which dietary amino acids are able to replace this loss. I have already argued (section 4) that this reasoning is doubtful and that the adult requirement pattern of the IAAs cannot be determined simply by the composition of body protein.
The second general proposition is that the ONL represents that proportion of total protein turnover that is not recycled back into protein synthesis. We know from tracer studies that, at intakes in the region of maintenance, recycling is about 90% (Waterlow, 1968). If the ONL of 54 mg/kg/d represents the 10% of protein turnover that is not recycled, then total protein turnover must be 540 mg N or about 3.4 g protein/kg/d. This estimate fits well with actual values obtained by various methods (Waterlow, 1984). However, the argument tends to be circular, because although it is consistent with what we know about protein turnover, it is still based, as before, on the ONL and the composition of body protein. It is therefore not surprising that the two approaches give identical results, give or take a milligram or two for rounding off, as shown in Tables 7 and 8 of Young et al (1989).
These criticisms, like the arguments themselves, are really irrelevant. The MIT pattern stands or falls by the extensive body of well planned and well executed studies that have been carried out by Young and his colleagues over more than a decade. The original tracer balance studies of 1986 were followed by others designed to eliminate possible sources of error and to extend the data base.