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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

13. The colon: losses or gains?

Nitrogen is, of course, lost in the faeces, the obligatory loss being 10 - 20 mg/kg/d. Reeds & Harris (1981) and Fuller & Garlick (1994) have published data on the IAA content of ileostomy fluid, showing that small but not negligible amounts are delivered to the colon. It is immaterial, from the point of view of balance, whether these amino acids are endogenous or derived from the food, but if these amounts pass into the faeces, then they should be added to the losses given by the tracer balances. The fact is, we do not know the fate of these IAAs.


Figure 12
Influence of protein intake on protein synthesis and breakdown, measured with 1- 13C leucine in the fed and fasted states. Data of Pacy et al (1994), reproduced by courtesy of the authors and the publishers of Clinical Science.

The possibility has been raised that amino acids synthesized by the microbial flora in the colon may be available to the body. One of the sources from which this synthesis could occur is urea. It is well recognized since the pioneer work of Walser & Bodenlos (1959) that in the normal adult about 20% of the urea produced passes into the colon and is hydrolysed to ammonia by bacterial urease. Jackson and coworkers have shown that the proportion of urea production that is 'salvaged' in this way (T/Pu) tends to increase in conditions where N balance is under stress, as in subjects on low protein diets or children recovering from malnutrition (Jackson et al, 1990; Langran et al, 1992). If all the ammonia produced by hydrolysis of urea was delivered to the liver, our studies (unpublished) with 15N ammonium chloride suggest that about 70% of it would be immediately converted to urea. In fact, the fraction of urea transferred to the colon that is recycled to urea is only about 25% or less (Jackson et al, 1990; Langran et al, 1992). It has usually been assumed that the ammonia which has not been recycled is taken up into the amino-N pool by transamination. However, there is also the possibility that the ammonia is utilized by bacteria for the synthesis of IAA as well as non-IAA.

Table 20 Fed-state protein turnover rates with 15N-glycine. Data of Pacy et al (1994)

Protein intake (g/kg/12h)

0.34

0.72

1.50

mg N/kg 12 h




N loss

49

74

151

Qu

426

494

651

Qa

165

328

453

Qa/Qu

0.39

0.66

0.70

g protein/kg/12 h




Qav

1.85

2.57

3.45

Sav

1.54

2.11

2.51

Bav

1.51

1.85

1.95

Qu = flux calculated from enrichment of urea.
Qa = flux calculated from enrichment of ammonia.
Qav = arithmetic mean of Qu and Qa
Sav = synthesis rate calculated as Qav -N loss.
Bav = breakdown rate calcuated as Qav-N intake.

This possibility must be seen against the background of a very large nitrogen flux through the colon. Jackson's calculations (Jackson et al, 1984) estimate this flux at about 15 g N/d in a normal adult, or about 1/3 of the whole body flux (this, incidentally, is why we speculate that the substantial pool turning over by lifetime kinetics (section 12) might be related to the gastrointestinal tract). This evidence for a large flux through the colon is supported by an experiment of Wrong et al (1985), which showed that the enrichment of faecal ammonia was many-fold less than that of the plasma urea from which the ammonia must have been derived. Thus we have the picture of a large intra-colonic flux, of which only about 10%/d is disposed of into the faeces, the remainder being recycled back into the body.

The validity of this picture depends on the possibility of reabsorption of amino acids from the colon. Moran & Jackson (1990a,b) found that in adults 60-80% of 15N-urea instilled into the colon was retained in the body. Heine et al (1987) instilled 15N-yeast protein into the colon of infants with Hirschsprung's disease and found that about 90% of the 15N was retained, with significant labelling of plasma proteins. This could have been ammonia that had been fixed by transamination; even so, on this preliminary evidence it looks as if amino acids as well as urea can pass out of the colon. Read et al (1974) suggested this possibility as early as 1974 but could not pursue it because of the civil war in Lebanon. On the basis of an experiment with 15N yeast in infants they wrote: 'Although the colon is not capable of digesting the intestinal flora it can absorb amino acids and there is a high probability that a percentage of the amino acids synthesized by the bacteria will be released into the surrounding medium'.

The final stage in the hypothesis that the body can utilize IAAs synthesized by colonic bacteria must be the direct demonstration of their appearance in body protein. So far the evidence is preliminary. Japanese workers in 1980 claimed to have isolated 15N-labelled Lysine from plasma proteins in subjects in Papua New Guinea to whom they had given 15N-urea (Tanaka et al, 1980), but this claim was greeted by most workers with some scepticism. In a recent study (Torrallardona et al, 1993) a pig was given 15N-ammonium chloride and 14C polyglucose for 10 d. Significant labelling of both N and C was found in the carcass protein. It was estimated that synthesis by the bacterial flora provided Lysine to the amount of 43 mg/kg/d if calculated from 15N and 33 mg/kg/d if calculated from 14C (see also Fuller & Garlick, 1994).

Young has commented that if bacterial synthesis of IAAs occurred to any significant degree, El Khoury et al (1994a) would not have found an almost exact correspondence between leucine intake from the food and leucine oxidized (Table 16). It is difficult to suppose any mechanism by which the colonic bacteria could 'recognize' that their host's dietary intake of an amino acid was high, and therefore reduce their own production. If, then, we visualize a more or less constant and significant addition of IAA from the colon, in striking a balance this addition will be taken into account on the output side (oxidation) but not on the input side (food). The result will be that when oxidation is substracted from the amino acid intake in the food, there will be a negative tracer balance, and this balance will always be negative, however high the intake. In fact this does not happen: the work of both Young and Millward shows that at higher intakes, e.g. 80 mg/kg/d and above, tracer balances become progressively more positive - a well known fact that has never been explained.

I conclude that, in people living on Western diets, colonic synthesis and release of IAAs is unlikely to be significant. However, the microbial flora of the large gut is variable, depending, among other things, on the nature of the carbohydrate and the fibre content of the diet. An effect which does not occur in habitually well nourished subjects might well become significant in those whose diets are less privileged. Research on this subject is now active and the results will be awaited with interest.