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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.
The
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.
BEARD, J.L., HUEBERS, H.A.,
FINCH, C.A.: Protein depletion and iron deficiency in rats. J.
Nutr., 114,1396-1401(1984).
DANIELSEN, M., JACKSON, A.A.: Limits of adaptation to a diet low in protein in normal man: urea kinetics. Clin. Sci., 83, 103-108 (1992).
DAS, T.K., WATERLOW, J.C.: The rate of adaptation of urea cycle enzymes, aminotransferases and glutamic dehydrogenase to changes in dietary protein intake. Br. J. Nutr., 32, 353-373 (1974).
FAO/WHO/UNU: Energy and Protein Requirements: Report of an Expert Consultation. Technical Report Series No. 724. World Health Organization, Geneva, 1985.
HIBBERT, J.M., JACKSON, A.A.: Variation in measures of urea kinetics over four years in a single adult. Eur. J. Clin. Nutr., 45, 374-351 (1991).
JACKSON, A.A.: Amino acids: essential and non-essential? Lancet, i, 1034-1037 (1983).
JACKSON, A.A.: Dynamics of protein metabolism and their relationship to adaptation. In: Proceedings of the XIII International Congress of Nutrition 1985, pp. 402-409, T.G. TAYLOR, N.K. JENKINS (Eds.). John Libbey, London, 1986.
JACKSON, A.A.: Protein metabolism in man. In: The Contribution of Nutrition to Human and Animal Health, pp. 92-104, E.M. WIDDOWSON, J.C. MATHERS (Eds.). Cambridge University Press, Cambridge, UK, 1992.
JACKSON, A.A., DOHERTY, J., DE BENOIST, M.-H., HIBBBRT, J., PERSAUD, C.: The effect of the level of dietary protein carbohydrate and fat on urea kinetics in young children during rapid catch-up weight gain. Br. J. Nutr., 64, 371-385 (1990).
JACKSON, A.A., WOOTTON, S.A.: The energy requirements of growth and catch-up growth. In: Activity, Energy Expenditure and Energy Requirements of Infants and Children, pp. 185-214, B. Schürch, N.S. Scrimshaw (Eds.). IDECG, Lausarne, Switzerland, 1990.
JAHOOR, F., SHANGRAW, R.E., MIYOSHI, H., WALFISH, H., HERNDON, D.N., WOLFE, R.R.: Role of insulin and glucose oxidation in mediating the protein catabolism of burns and sepsis. Am. J. Clin. Physiol., 257, E323-E331 (1989).
KENNEDY, N., BADALOO, A.V., JACKSON, A.A.: Adaptation to a marginal intake of energy in young children. Br. J. Nutr., 63, 145-154 (1990).
KLEIBER, M.: Dietary deficiencies and energy metabolism. Nutr. Abst. Rev., 15, 207-221 (1945).
LANGRAN, M., MORAN, B.J., MURPHY, J.L., JACKSON, A.A: Adaptation to a diet low in protein: effect of complex carbohydrate upon urea kinetics in normal man. Clin. Sci., 82, 191198 (1992)
MEIJER, A.J., LAMERS, W.H., CHAMULEAU, R.A.F.M.: Nitrogen metabolism and ornithine cycle function. Physiol. Rev., 70, 701-748 (1990).
MILLWARD D.J., RIVERS, J.P.W.: The nutritional role of the indispensable amino acids and the metabolic basis for their requirements. Eur. J. Clin. Nutr., 42, 367-393 (1988).
MORAN, B. J., KARRAN, S.J., JACKSON, A.A.: Enhanced retention of urea-nitrogen in patients on total parenteral nutrition for intestinal failure. Clin. Nutr., 10, 67-70 (1991).
PATHIRANA, C., GRIMBLE, R.F.: Taurine and serine supplementation modulates the metabolic response to tumour necrosis factor-µ in rats fed a low protein diet. J. Nutr., 122,1369-1375 (1992).
RAFOTH, R.J., ONSTAD, G.R.: Urea synthesis after oral protein ingestion in man. J. Clin. Invest., 56, 1170-1174 (1975).
ROSE, W.C.: The nutritive value of the amino acids. Physiol. Rev., 18, 109-136 (1938).
RUDMAN, D., MILLIKAN, W.J., RICHARDSON, T. J., BIXLER, T. J., STACKHOUSE, J., McGARRITY, W.W.: Elemental balances during intravenous hyperalimentation of underweight adults. J. Clin. Invest., 55, 94-104 (1990).
WALSER, M.: Urea metabolism. In: Protein Metabolism in Man, pp. 229-246, J.C. WATERLOW, J.M.L. STEPHEN (Eds.). Applied Science Publishers, London, 1981.
WATERLOW, J.C.: Report on the informal gathering of investigators to review the collaborative research programme on protein requirements and energy intake. In: Protein-Energy-Requirement Studies in Developing Countries: Results of International Research, pp. 351-362, W.M. RAND, R. UAUY, N.S. SCRIMSHAW (Eds.). United Nations University, Tokyo, 1984a.
WATERLOW, J.C.: Protein turnover with special reference to man. Quart. J. Exp. Physiol., 69, 409-438 (1984b).
WATERLOW, J.C., JACKSON, A.A.: Nutrition and protein turnover in man. Br. Med. Bull., 37, 5-10 (1981).
WHEELER, R.A., JACKSON, A.A., GRIFFITHS, D.M.: Urea production and recycling in neonates. J. Pediatr. Surg., 26, 575-577 (1991).
YOUNG, V.R., PELLETT, P.L.: Current concepts concerning indispensable amino acid needs in adults and their implications for international nutrition planning. Food Nutr. Bull., 12, 289-300 (1990).