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Further Studies on the Effect of Dietary Carbohydrate and Fat on Protein Metabolism in Rats

Laboratory of Nutritional Biochemistry, Department of Agricultural Chemistry, Nagoya University, Chikusa, Nagoya, Japan

A systematic experiment has been carried out in attempting to elucidate the biochemical mechanism underlying "protein sparing" action of dietary carbohydrate and fat. A more simplified experimental condition was employed in the present investigation in comparison with that in our previous experimental design; the effect of dietary carbohydrate and fat on the body protein metabolism was examined in the absence of any protein in the diet. Feeding of either carbohydrate or fat caused a marked decrease in the level of urinary nitrogen and urea. The ingestion of either carbohydrate or fat by rats resulted in a reduction of the rate of ureogenesis in the rat liver with the concomitant decrease in the activities of hepatic amino acid-catabolizing enzymes, e.g., threonine dehydratase, tryptophan pyrrolase, arginase, glutamic-pyruvic transaminase, glutamic-oxalacetic transaminase and glutamic dehydrogenase (except in fat-fed animals). The feeding of either carbohydrate or fat did not result in changes in the level of free amino nitrogen in blood, indicating that the supply of amino acid may not exert an important role in the ureogenesis in liver. The overall results indicate that the reduction of the activities of hepatic amino acid-catabolizing enzymes elicited by the feeding of either carbohydrate or fat may govern protein metabolism in the rat body.

KEY WORDS: • fat • carbohydrate • protein metabolism

Manuscript received 12 August 1971.