Urea as a Dietary Supplement for Humans

Kies, Constance; Fox, Hazel M.

doi:10.1007/978-1-4684-3366-1_7

Cited by 13 publications

(4 citation statements)

References 21 publications

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“…This is indeed true in the steady state; 15 N from the gut, converted to amino-N, is simply substituting for unlabelled amino-N. However, in growing infants or in subjects on lowprotein diets the N from salvaged urea will contribute to the¯ux of amino acids available for protein synthesis, just like any other source of non-speci®c N (see, for example, Snyderman et al 1962;Tripathy et al 1970;Kies & Fox, 1978). Moreover, the proportion of urea production that is salvaged increases in situations where there is an increased demand for N, so it is reasonable to regard salvage as contributing to the body's N economy when it is needed.…”

Section: The Metabolism Of Ureamentioning

confidence: 99%

The mysteries of nitrogen balance

Waterlow¹

1999

Nutr. Res. Rev.

112

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The ®rst part of this review is concerned with the balance between N input and output as urinary urea. I start with some observations on classical biochemical studies of the operation of the urea cycle. According to Krebs, the cycle is instantaneous and automatic, as a result of the irreversibility of the ®rst enzyme, carbamoyl-phosphate synthetase 1 (EC 6.3.5.5; CPS-I), and it should be able to handle many times the normal input to the cycle. It is now generally agreed that acetyl glutamate is a necessary co-factor for CPS-1, but not a regulator. There is abundant evidence that changes in dietary protein supply induce coordinated changes in the amounts of all ®ve urea-cycle enzymes. How this coordination is achieved, and why it should be necessary in view of the properties of the cycle mentioned above, is unknown. At the physiological level it is not clear how a change in protein intake is translated into a change of urea cycle activity. It is very unlikely that the signal is an alteration in the plasma concentration either of total amino-N or of any single amino acid. The immediate substrates of the urea cycle are NH 3 and aspartate, but there have been no measurements of their concentration in the liver in relation to urea production. Measurements of urea kinetics have shown that in many cases urea production exceeds N intake, and it is only through transfer of some of the urea produced to the colon, where it is hydrolysed to NH 3 , that it is possible to achieve N balance. It is beginning to look as if this process is regulated, possibly through the operation of recently discovered urea transporters in the kidney and colon. The second part of the review deals with the synthesis and breakdown of protein. The evidence on whole-body protein turnover under a variety of conditions strongly suggests that the components of turnover, including amino acid oxidation, are in¯uenced and perhaps regulated by amino acid supply or amino acid concentration, with insulin playing an important but secondary role. Molecular biology has provided a great deal of information about the complex processes of protein synthesis and breakdown, but so far has nothing to say about how they are coordinated so that in the steady state they are equal. A simple hypothesis is proposed to ®ll this gap, based on the self-evident fact that for two processes to be coordinated they must have some factor in common. This common factor is the amino acid pool, which provides the substrates for synthesis and represents the products of breakdown. The review concludes that although the achievement and maintenance of N balance is a fact of life that we tend to take for granted, there are many features of it that are not understood, principally the control of urea production and excretion to match the intake, and the coordination of protein synthesis and breakdown to maintain a relatively constant lean body mass.

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Section: The Metabolism Of Ureamentioning

confidence: 99%

The mysteries of nitrogen balance

Waterlow¹

1999

Nutr. Res. Rev.

112

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“…The data inevitably tend to suggest that it is possible for essential amino acids to be formed endogenously, albeit at low levels and relatively inefficiently. Although at that time there was little direct evidence to justify this conclusion, the first human data were becoming available (Giordano et al 1968), and were used by Kies as a possible explanation of the phenomenon in her later papers (Korslund et al 1977;Kies & Fox, 1978).…”

Section: Older Concepts O F Requirementsmentioning

confidence: 99%

Salvage of urea-nitrogen and protein requirements

Jackson

1995

Proc. Nutr. Soc.

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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.

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“…The nature of this relationship has never been adequately explained. A good example of the effect is seen in the study carried out in nine healthy young men, given three diets based on opaque-2 maize as the main source of protein (Kies & Fox, 1978). For the basal diet maize provided 4 g N/d (equivalent to 25 g proteinld, which is less than the requirement level).…”

Section: Essential a M I N O A C I D S A N D Total Nitrogen Intakementioning

confidence: 99%

Chronic malnutrition: protein metabolism

Jackson

1993

Proc. Nutr. Soc.

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Urea as a Dietary Supplement for Humans

Cited by 13 publications

References 21 publications

The mysteries of nitrogen balance

The mysteries of nitrogen balance

Salvage of urea-nitrogen and protein requirements

Chronic malnutrition: protein metabolism

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