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close this bookEnergy 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)
close this folderThe requirements of adult man for indispensable amino acids
View the document(introductory text...)
View the document1. Introduction
View the document2. The problem in defining requirements
View the document3. Protein quality
View the document4. The maintenance requirement (MR)
View the document5.Diurnal cycling: the Millward-Rivers model
View the document6. Theoretical basis of the MIT tracer balance studies
View the document7. Technical problems of tracer balance studies
View the document8. Results of the MIT tracer balance studies
View the document9. Relation between leucine oxidation and nitrogen excretion
View the document10. Factors relating to the design of tracer balance experiments
View the document11. Breakpoint analysis
View the document12. Effect of protein/amino acid intake on protein synthesis and breakdown
View the document13. The colon: losses or gains?
View the document14. Conclusion
View the documentReferences
View the documentDiscussion
View the documentReferences

4. The maintenance requirement (MR)

This is a subject on which there is still a difference of opinion. Clearly when growth is rapid the IAA requirement must to a large extent be determined by the pattern of IAAs in the tissue that is being laid down. In the non-growing adult the MR derives from the need to replace IAAs that are consumed in a variety of irreversible pathways (Table 3). There is no a priori reason why this consumption pattern should bear any relation to that of deposition; a view that has been widely held by nutritionists in the past, e.g. Osborne & Mendel (1916, quoted by Millward, 1992) and Said & Hegsted (1970), and which is accepted in principle by Young (personal communication). Thus Fuller et al (1989), in experiments on young pigs, have assessed separately the pattern of requirements for growth and the pattern when there is no growth (maintenance), and found the two patterns to be quite different.

Table 3 Some non-protein pathways of amino acid utilization

Amino acid

Pathway

Methionine

Methylation reactions


Creatine


Choline

Cysteine

Glutathione


Taurine

Tyrosine

Neurotransmitters

Glutamate

Neurotransmitters

Lysine

Carnitine

Glycine

Nucleic acid bases


Haem


Creatine

From Reeds (1990).

Nevertheless, Young and El Khoury (1995) maintain that in practice in man the maintenance IAA pattern resembles that of tissue protein, on the grounds that there is a close correspondence between the tissue pattern and that of the IAA requirements of pre-school children, as observed by the workers at INCAP (Pineda et al, 1981) (Table 1B). Since in the pre-school child growth represents only some 10% of the requirement, it is reasonable to extrapolate these results to adults.

Is it possible to reconcile these two opposing points of view? On a protein-free diet or fasting the obligatory N loss (ONL) must reflect the pattern of tissue proteins, since the only source of nitrogen is from protein breakdown. In this situation one may suppose that the IAA with the largest consumption pathway 'drives' the obligatory loss. If protein breakdown provides more of a particular IAA than is needed to make good its loss through irreversible pathways, this extra amount must nevertheless be oxidized. When protein is fed, the pattern changes and the IAAs are only needed in a pattern that balances their losses in the consumption pathways. Thus if fasting and feeding occur in a 12 h cycle, 50% of the 24 h requirement will reflect the composition of body protein and 50% that of the consumption pathways.

The picture, however, is complicated by the diurnal cycling of deposition and loss of body protein (see below). Fed state deposition can be regarded as a form of temporary growth, and therefore requires that amino acids should be provided in the concentrations in which they occur in body protein, although Fuller (personal communication) has suggested that temporary protein storage could have a composition quite different from that of body protein as a whole. To a large extent these amino acids must be derived from protein breakdown; if the food intake is, say, one fifth of the flux, four fifths of the amino acids deposited will be derived from breakdown. At maintenance levels of intake, deposition is relatively small, but at higher intakes, when deposition is increased, the rate of protein breakdown is greatly reduced (section 11), so that intake from the diet becomes more important. Millward et al (1991) have discussed whether amino acids liberated from body protein during the fasting period could be held over, as it were, and be available for meeting needs in the fed state. However, the free amino acid pools, particularly those of the branched chain amino acids (BCAs), are too small for this to be likely. Therefore, in the fed state, particularly with generous intakes, the intake from food plays an essential role in topping up the amino acid supply for protein deposition, and to the extent that it is used in this way, this intake probably must have the pattern of tissue protein.

It follows that conceptually the IAA requirement pattern will be some kind of halfway house between the pattern of irreversible losses and the pattern of body protein, and the relative proportions of the two patterns will be influenced by the level of protein intake. It seems to me that the next step must be to measure the individual IAA losses through irreversible pathways. In the meantime, because the two processes of fasting loss and diurnal cycling tend to shift the maintenance pattern towards that of body protein, it may be considered that Young's proposal represents a reasonable working compromise until more direct observations become available. It should be noted that on Young's hypothesis, if the requirement of one IAA is established, those of all the others follow, whereas if an amino acid, e.g. Lysine, is differentially conserved, the requirement of each IAA must be determined separately.