Salvage of urea-nitrogen and protein requirements.

The metabolism of N in the colon is dominated in quantitative terms by the metabolic activity of the microflora. The microflora are not found exclusively in the colon, as there are increasing numbers in the terminal ileum towards the ileocaecal valve; however, the working approximation is that functionally significant activity is colonic. There is a complex, and as yet inadequately defined, interaction of the luminal, mucosal and intramucosal populations of bacteria which is almost certainly of functional relevance. As for all organisms, the metabolic profile of the microflora is determined by the availability of substrate, which gives rise to an interdependence of protein, amino acid and N metabolism with the availability of energy; but for the practical purposes of the present analysis the reasonable assumption has been made that for all the work quoted, energy and other nutrients are not limiting. In general bacteria utilize NH3 as their preferred source of N, and other forms of protein or amino acids are reduced to NH3 before being used metabolically. The microflora represent a complex ecosystem, with aspects of a continuous-flow culture. Our knowledge of the details of this system in vivo are limited and, in particular, we are not clear of the extent to which the dwell time in the caecum and ascending colon justifies consideration of this section of the bowel as a fermentative vat, compared with the role for water and electrolyte extraction in the more distal regions (Stevens, 1988). The objective here is to identify the extent to which the hydrolysis of urea by the microflora provides N to the host in a form which enables the host’s requirements for amino acids to be satisfied under any circumstance. Of necessity this leads to consideration of the concept of requirements itself.