The Influence of Dietary Carbohydrate on the Course of Protein Metabolism. With Summary

Thomson, William S. T (1954) The Influence of Dietary Carbohydrate on the Course of Protein Metabolism. With Summary. PhD thesis, University of Glasgow.

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A study of the relevant literature reveals that carbohydrate has a beneficial action on the course of "exogenous" and "endogenous" metabolism of protein, for which energy in the form of fat is not a substitute. The objects of the experiments to be described were first, to attempt to determine whether the effects of carbohydrate on the metabolism of dietary and body protein are similar in mechanism and secondly, to gain some insight into the mode of action of this phenomenon. Experiments were carried out with adult rats, in which the increase in nitrogen output which results when carbohydrate fed with protein is isocalorically substituted by fat, was compared with the change in nitrogen balance which occurs when carbohydrate is transferred from a protein-containing to a protein-free meal. In view of their similar magnitude and transient nature, it was concluded that these two superficially distinct actions of carbohydrate on the exogenous metabolism of protein had probably the same underlying mechanism, which is primarily dependent on the simultaneous feeding of carbohydrate with dietary protein. The effect of glucose ingestion on the digestibility and rate of absorption of casein was next investigated, as the intestine is a feasible site of the interaction which occurs between carbohydrate and protein fed in the same meal. Faecal nitrogen determinations revealed that glucose did not alter the digestibility of the protein. In addition although glucose did cause a marked delay in the absorption of the casein, the sparing effect of glucose could not be attributed to this effect, as fat ingestion also resulted in a similar delay but without giving rise to nitrogen retention. Experiments were also conducted in which the nitrogen content of the viscera were estimated after the simultaneous feeding of carbohydrate and protein. No significant fraction of the nitrogen which was retained could be accounted for in the viscera and it was suggested that the most probable site of deposition of the retained nitrogen was muscle. Studies were also made on the effect of glucose and fat administration on the plasma amino acid levels in fasting human subjects and rats. Glucose ingestion caused a 12% reduction in the plasma amino nitrogen which was maximal at 1 hour, whereas fat ingestion resulted in a much more gradual fall, to the extent of 4% of the initial level and of doubtful significance. Similar experiments on rats confirmed the differences in action of glucose and fat. In the fasting human subject, the level of seven essential amino acids (tryptophan, histidine, leucine, isoleucine, threonine, arginine and valine) fell after glucose ingestion to varying extents. If these depressions were arranged in the form of a ratio with tryptophan as unity and compared with Rose's estimate of the human requirements of these amino acids arranged in a similar way, then a close relationship was observed between the values. It was concluded that the sparing action of glucose was probably due to a stimulation of protein synthesis. Also as the action of glucose on the plasma amino acid level is maximal at one hour and as the level of amino acids in the portal vein will also be maximal shortly after feeding protein, then it was suggested that the action of glucose on the endogenous metabolism of protein was similar to its action on the exogenous metabolism. The administration of carbohydrate to fasting alloxan diabetic rats, resulted in a very slight decrease in the plasma leucine level compared to that occurring with normal rats. This result was considered to favour the view that insulin is necessary for the sparing action of carbohydrate in the fasting rat. The effect of glucose and fat on the incorporation of S-methionine into tissue proteins was investigated both in vivo and in vitro. A study was made of the effect of added glucose, succinate, octanoate and acetate on the 35 uptake of 35S-methionine by rat diaphragm and rat liver slices in vitro. Glucose increased the incorporation by 9-16% and succinate by 59%. Octanoate had little effect on the uptake, but acetate had a definite depressant action. It was concluded that glucose and succinate increased the amount of A.T.P. available for the process of protein synthesis, whereas octanoate and acetate did not. With the in vivo studies it was found that neither glucose nor fat had any effect on the uptake of 35S-methionine by the intestine and liver. On the other hand it was observed that glucose but not fat greatly increased the incorporation of methionine by skeletal muscle. Two hours after feeding glucose the isotope concentration was slightly lower than that of the control animal, but at the fourth hour, there occurred a marked increase in the uptake which had completely disappeared again by the sixth hour. It was suggested that glucose promotes the synthesis of a peptide or protein which was isolated in some manner from the main mass of muscle protein. It has been concluded that the action of carbohydrate on the endogenous and exogenous utilisation of protein is due to one underlying mechanism, namely an increase in protein deposition in muscle.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Physiology
Date of Award: 1954
Depositing User: Enlighten Team
Unique ID: glathesis:1954-79106
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 11:42
Last Modified: 05 Mar 2020 11:42

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