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The analysis carried out by Millward has helped us towards developing a new model for considering the internal dynamics of nitrogen metabolism. In the development and exploration of this model of amino acid metabolism, it is important to differentiate between the fate of carbon and the fate of nitrogen during the oxidative process.
External nitrogen balance only represents a fraction of the intensity of the movement of nitrogen within the body, and there are two major internal cycles for nitrogen. The first, characterised as protein turnover, represents the movement of nitrogen from amino acids into urea, and the return of the urea-N to amino acid synthesis (Figure 7).
The return of the urea-N requires the salvaging of nitrogen through the metabolic activity of the colonic microflora. Within the range of adequate protein intakes, the production of urea is unrelated to the intake. The achievement of nitrogen balance appears to be dependent upon the salvage of urea-N, implying that the activity of the colonic microflora is an integral part of the mechanism through which nitrogen balance is maintained under normal circumstances.
substantial salvaging of nitrogen by the lower bowel forces us to
a fundamental reconsideration of the relations between the
metabolism of nitrogen, amino acids and proteins, and cautions
against the use of the terms interchangeably as synonyms. The
movement of nitrogen through the body can only be measured
directly with the use of nitrogen labels, and not imputed
indirectly from the use of carbon-labelled amino acids.
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