The Relationship of Carbohydrate Metabolism to Protein Metabolism IV. The Effect of Substituting Fat for Dietary Carbohydrate

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Abstract
When fat is exchanged isocalorically for carbohydrate in the diet of the rat, urinary nitrogen output increases for a few days and then returns to its former level. The transitory nature of the response may account for the failure of some investigators to observe any difference in nitrogen balance between groups of rats receiving equicaloric diets of differing fat and carbohydrate content. Exchange of fat for carbohydrate in protein-containing meals leads to a much greater increase in nitrogen output than occurs after a similar exchange in meals devoid of protein. Thus the main consequence of replacing the carbohydrate of a mixed diet by fat is a deterioration in the utilization of dietary protein. The change in nitrogen balance resulting from substitution of fat for carbohydrate is of similar magnitude, whether the fat is fed with the protein of the diet or apart from it. This indicates that the phenomenon is essentially due to removal of carbohydrate from the protein-containing meal and not to an adverse effect of feeding fat with protein. There is a linear relationship between the amount of carbohydrate fed along with the protein of the diet and the nitrogen balance of the rat.